Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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their host-specificity and particular tissue tropism, an

individual class of virus will only grow in certain

types of tissue culture and some viruses grow very

poorly or not at all in vitro. In practice, very few

methods exist for the detection of viruses in water.

Examination for enteroviruses (specifically the polio

and Coxsackieviruses) is undertaken most frequently

as the methods for these viruses are simple and well

established. These viruses are used as indicators of the

potential presence of other viruses in much the same

way as Escherichia coli is used as an indicator of the

potential presence of pathogenic bacteria.

0011 The procedure used for the detection of entero-

viruses in water involves concentration of the viruses

from a large volume of water (usually a minimum of

10 l) on to an electropositive membrane, followed by

elution using a small volume of a proteinaceous solu-

tion such as beef extract. The concentrated material is

used to inoculate cultured cells, which are then incu-

bated at 37



C for several days. Viruses are detected as

holes or plaques in the cell sheet which are created by

the virus killing the cells in a small area. Some viruses

are not able to cause this so-called cytopathic effect,

although they may be able to replicate to some extent

within the cell. These viruses are generally detected

using enzyme-linked or fluorescently labeled anti-

bodies. More recently, molecular methods based

upon the polymerase chain reaction (PCR) have

been introduced and these are usually applied to

tissue cultures a few days after addition of the sample,

to insure that only infectious viruses are detected.

(See Immunoassays: Radioimmunoassay and Enzyme

Immunoassay.)

Parasite Problems

0012 In recent years Cryptosporidium parvum has become

recognized as a major pathogen of human beings and

some domestic animals. It causes a self-limiting diar-

rhea in immunocompetent individuals, but may cause

mortality in immunocompromised hosts. One of the

modes of transmission is through consumption of

contaminated water and several waterborne out-

breaks have been reported. The organism survives in

water as an environmentally robust oocyst containing

four sporozoites. The organism has been reported in

drinking water and it survives chlorination during

normal water treatment. (See Parasites: Occurrence

and Detection.)

Examination for Specific Groups

0013 Much of the microbiological work within the water

industry involves the use of indicator organisms as

opposed to specific groups of bacteria or viruses. The

organisms most frequently sought in this connection

are coliform and E. coli. The term coliform has been

given several definitions, but in general, a working

definition is that it produces P-galactosidase and

grows at 37



C on laboratory media containing

bile salts. The definition of E. coli is of course

much clearer, it being a species in its own right.

0014Both E. coli and coliforms are cultured in the same

way. Water is passed through a membrane filter and

placed on the surface of a pad soaked in medium

containing proteinaceous nutrients, lactose, bile salts

(or similar compounds), and a pH indicator. The

combination of lactose and an indicator allows the

identification of lactose fermenters (a trait common

to coliforms and hence to E. coli), whilst the bile salts

inhibit the growth of nonintestinal bacteria. Now-

adays bile salts as such are not generally used, and

they have been replaced by purified detergents such as

Teepol or sodium lauryl sulfate. The membranes are

incubated at 37



C for coliforms or 44



C for E. coli

and incubation is continued for at least 18 h. The

incubation often includes a 4-h period at 30



Cto

allow organisms that may be slightly damaged to

adapt to their new environment. After incubation,

the membranes are examined and lactose-fermenting

colonies are counted on membranes incubated at



C and 44



C. These counts are termed ‘presump-

tive coliforms’ and ‘presumptive E. coli,’ respectively.

Confirmation of the presumptive counts was based

on production of acid and gas from lactose, and

indole from tryptophan at their respective tempera-

tures (37



C and 44



C). However, it has been known

for some time that these confirmation tests are not

100% effective because of differences between strains

within the groups (e.g., anaerogenic strains). It is

becoming a more common practice to identify the

isolates by means of more detailed physiological

tests, often using one of the commercially available

batteries of tests for identification of the Entero-

bacteriaceae. This permits a much more accurate

‘confirmed’ result.

0015The use of simpler tests for the detection of coli-

forms and E. coli is becoming more common and in

particular defined substrate technology is extremely

popular. These tests rely on the detection of two

enzymes, b-d-galactosidase (for coliforms) and b-

d-glucuronidase (for E. coli) in a medium which con-

tains little carbon and energy source other than the

substrate for the two enzymes. Enzyme activity is

detected by a color change (for coliforms) and the

production of fluorescence (for E. coli).

0016Other groups of indicator organisms which are

sometimes used include the enterococci and Clostri-

dium perfringens. These organisms are generally

more resistant to chlorine than the coliform group

WATER SUPPLIES/Microbiological Analysis 6119

and also tend to survive longer in the water. They may

be used to identify waters which have been subjected

to less recent fecal contamination. Isolation of these

groups of organisms generally involves membrane

filtration, followed by incubation on an appropriate

selective medium. Specific enteric pathogens such as

Salmonella spp. and Campylobacter are rarely looked

for in potable water. However, when they are sought,

techniques similar to those employed in food micro-

biology are used. (See Campylobacter: Detection;

Clostridium: Detection of Clostridium perfringens.)

0017 The two major enteric pathogens that are looked

for specifically in water are the protozoan parasites

Giardia and Cryptosporidium. Examination for these

organisms involves a very different procedure to those

used for bacteria and viruses. Large volumes of water

(typically 100–1000 l) are filtered through cartridge

filters. The filter matrix is then ‘washed’ in a dilute

detergent solution in an effort to release as much of

the retained material as possible. The washings from

the filter are then concentrated by centrifugation,

purified by immunomagnetic separation, dried on to

microscope slides, and stained by immunofluores-

cently-labeled antibodies. Examination of water

samples from ‘at-risk’ water treatment plants is now

a regulatory requirement in the UK and a specific

method has been published which must be used

for examination of these regulated samples. The

process involves concentration of large volumes of

water, separation of Cryptosporidium oocysts from

background debris by immunomagnetic separation,

and epifluorescence microscopy. A similar method

(USEPA (United States Environmental Protection

Agency) method 1623) has been recommended for

use in the USA.

See also: Campylobacter: Detection; Clostridium:

Detection of Clostridium perfringens; Immunoassays:

Radioimmunoassay and Enzyme Immunoassay;

Parasites: Occurrence and Detection; Viruses

Further Reading

Backer H (1997) Water disinfection strategies for. Crypto-

sporidium. Wilderness and Environmental Medicine

8: 96–100.

Cho HB, Lee SH, Cho JC and Kim SJ (2000) Detection of

adenoviruses in tap water and river water by reverse

transcription multiplex PCR. Canadian Journal of

Microbiology 46: 417–424.

Craum GF and Frost FJ (2002) Possible information bias

in a waterborne outbreak investigation. International

Journal of Environmental Health Research 12: 5–15.

Gofti L, Zmirou D, Seigle Murandi F, Hartemann P and

Potelon JL (1999) Waterborne microbiological risk

assessment: a state of the art and perspectives. Ve

rite

Epide

miologie et Sante

Publique 47: 61–73.

Karanis P and Kimura A (2002) Evaluation of three floccu-

lation methods for the purification of Cryptosporidium

parvum oocystis from water samples. Letters in Applied

Microbiology 34: 444–449.

Weaning See Infant Foods: Milk Formulas; Weaning Foods; Infants: Nutritional Requirements; Breast- and

Bottle-feeding; Weaning; Feeding Problems

WEIGHING

R Schulthess, Mettler-Toledo AG, Greifensee,

Switzerland

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 Well-organized and effective quality assurance in the

food industry is a must for all companies, whatever

their size. A quality assurance system has to meet

demands from both the legislative and the industrial

economic standpoint. Scales and weighing systems

represent important tools to enable such quality

assurance systems to fulfill these requirements.

The Scale as a Control and Management

Tool

0002The degree of automation of all types of production

processes must be constantly increased to withstand

the growing pressure from the competition. At the

6120 WEIGHING

same time, the demands on quality are increasing,

tolerance limits are becoming narrower, and inte-

grated control processes are assuming more and

more importance. This makes the use of ‘intelligent’

(computerized) scales and weighing systems as tools

in quality assurance an important factor in produc-

tion. (See Quality Assurance and Quality Control.)

0003 Sometimes, only a few grams or even fractions of

a gram decide the quality and the profit and loss

in mass production. Even minor weight deviations,

e.g., losses incurred through the overfilling of solid or

liquid foods, can cost a great deal of money. Difficul-

ties must also be anticipated with underfilling. In

addition to customers, consumer and other protec-

tion associations, which react extremely sensitively

to underfillings, each country has its own statutory

regulations which, particularly in the food-processing

industry, require a weight control.

The Scale: Electronics as Standard

0004 In the last two decades, balance and scale construc-

tion has undergone a fundamental changeover from

mechanics to electronics. As a result, electronic bal-

ances and scales are now standard equipment. They

offer the user incomparably more convenience than

their mechanical counterparts, and thus make pos-

sible extensive rationalization of operations. Elec-

tronic scales, however, are also intelligent scales,

which offer a wide variety of possibilities to process

the weight values further.

0005 Thus weight determinations in different units are

currently performed on one and the same scale; per-

centage weighings and formula weighing (net total

determination) are part and parcel of the daily work

routine. Furthermore, electronic scales are system-

compatible, i.e., they can be incorporated in auto-

mated operational routines in a wide variety of ways.

The Measurement Technology: Decisive

Factor for the Performance Class of a

Scale

Electromagnetic Force Compensation: Synonym

for Extremely High Accuracy

0006 At the start of the 1970s, the balance and scale manu-

facturer Mettler (Switzerland) used the principle of

electromagnetic force compensation commercially

for the first time. The measurement principle is

shown in Figure 1. With this measurement technol-

ogy, the force action on the weighing platform is

compensated by means of a coil in a magnetic field.

The current needed for this is proportional to the

weight of the loaded sample. It is transformed into a

digital signal that can be processed by a powerful

microprocessor. This principle allows an extremely

high resolution greater than 300 000 certified points

(e) and the maximum possible precision, even in

surroundings not conducive to highly precise work.

0007The most conspicuous feature of these scales is the

drastic reduction in the number of controls. A single

key is all that is needed to initiate all functions, such

as on–off switching of the scale, setting the display to

Ω

(a)

(b)

(c)

fig0001Figure 1 (a) The principle of frequency-modulated force meas-

urement. (b) The principle of strain-gage technology. (c) The

principle of electromagnetic force compensation. Reproduced

from Weighing, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

WEIGHING 6121

zero, taring a vessel on the scale, and selection of

additional functions from the menu (Figure 2).

Strain-Gage Technology for the Harsh Industrial

Environment

0008 Weighing stations in a harsh industrial environment

must be characterized by ruggedness and endurance.

With scales employing strain-gage technology, the

strain gauge, an electrical resistive film, is mounted

on an elastically deformable body and thus allows

leverless measurement of loads weighing up to several

tonnes. This technology, with its various designs, e.g.,

extremely flat platforms for wheeled access, and par-

ticularly flat ring-load cells, offers weighing results of

unvarying accuracy in the lower resolution range at

3000 e (Figure 1).

Frequency-Modulated Force Measurement

– Precise and Economically Attractive

0009 The combination of extremely simple mechanics

(monobloc) with state-of-the-art sensor technology

(vibrating string) allows a very good multirange

resolution. A value of 3 3000 e assures flexible use

and offers advantages in a wide range of applications,

especially in situations where the industrial ruggedness

of the sensor (IP67-type protection) is essential. The

frequency-modulated force measurement is distin-

guished by a very high accuracy and, at the same time,

by its attractive price: performance ratio (Figure 1).

0010 All three basic weighing technologies described

above are used in commercial instruments. The user

thus has available a balance or scale product line up

to 6 t that is geared to practically all conceivable

applications.

Automatic Calibration

0011 To ensure that 100 g is actually measured as 100 g the

world over, reference weights (so-called calibrated

mass weights) are used with a mass based on the

standard stored in Paris. With traditional, mechanical

two-pan scales, each weighing was of necessity a

comparison with the loaded reference weights. With

electronic scales, however, a comparative measure-

ment is performed only when the scale is adjusted

on-site with a known weight. This process is called

calibration.

Accurate Results – Even Under Unfavorable

Ambient Conditions

0012Anyone who performs weighings wants more than

merely accurate results. They should also appear rap-

idly on the display and be stable. However, rapidity

and stability are properties which are mutually incon-

sistent. For example, an analytical balance with

0.1 mg read ability reacts sensitively to loaded

weights, but vibrations of the support and draughts

can also influence the displayed weight values. The

effects of building oscillations, open windows, and

air-conditioning units with their powerful air cur-

rents must therefore be effectively suppressed. The

measured values were summed over an adjustable

time, the so-called integration time, and shown as a

mean value. However, this averaging method is

associated with a relatively long stabilization time.

0013Modern technological developments allow better

solutions. The built-in processor system performs a

trend analysis so that the actual weight value can be

displayed rapidly despite a less than ideal location.

Display Behavior Meeting Application Demands

0014To match the requirements of the application, the

behavior of the display can be adjusted with the so-

called weighing process adapter: the last decimal

place in the display is switched off during rapid

weight changes until the weight has gradually stabil-

ized. This prevents the operator being distracted and

quickly becoming fatigued by rapidly changing

figures that are completely superfluous at the time.

0015In fine addition, on the other hand, the display

should follow the weight increase as rapidly as

possible and show the weight continuously with its

complete resolution.

0016Yet another, completely different behavior is

demanded from the scale in check weighings. The

weight result should be displayed as quickly as pos-

sible, and this is allowed for by filters. Intermediate

values are of no interest, and hence the display shows

all decimal places only when the actual weight has

been determined.

Digital or Analog?

0017Digital numbers are still indispensable for the repre-

sentation of extremely accurate measured values, but

fig0002 Figure 2 Modern electronic balance with a single control bar.

Reproduced from Weighing, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ

(eds), 1993, Academic Press.

6122 WEIGHING

digital numbers are not geared to the human senses.

Our feeling for changing quantities is virtually non-

existent.

0018 Even today digital gages, especially speedometers

in cars, have gained little acceptance as it takes too

long to read and acquire the measured value. For this

reason dynamic analog displays have been developed.

Similar to a clock with hands, this angular informa-

tion supplements the digital values of the display

(Figure 3).

Single Range, Dual Range, DeltaRange, MultiRange

0019 The, at first sight, confusing multiplicity of terms

hides a number of different advantages, depending

on the scale type, which prove their worth in the

appropriate application.

0020 In principle, everything could be weighed with the

single-range scale. However, owing to the often very

high resolution, such scales take a relatively long time

to achieve stability and are expensive to manufacture.

Selection of the correct scale leads to the best and

most favorably priced solution. The concepts can be

characterized as follows.

0021Single-range scale provide the same readability

over the entire weighing range, e.g., 0–200 g to an

accuracy of 0.1 mg.

0022Dual-range scales have a range with a finer read-

ability, e.g., 0–50 g to an accuracy of 0.01 mg for the

weighing of light samples, and for the weighing of

heavy samples a range of, for example, 0–200 g with a

coarser readability of 0.1 mg. Switching between the

ranges can be performed manually or automatically

as a function of the loaded sample.

0023DeltaRange scales have a more accurate fine range

which covers part of the entire weighing range and

can be shifted at will. If this fine range is exceeded, the

scale automatically switches over to the coarser

range. By pressing the tare key, however, the start of

the fine range can be reset to any position over the

entire weighing range. This device is extremely useful

primarily when several components have to be

weighed very accurately into a relatively heavy

tare vessel. After every taring, the fine range is again

available.

0024Finally, multirange scales change the readability in

a variable manner. Thus the numerical increments of

the display increase from one to two to five incre-

ments with increasing load. For example, the multi-

range KA32s weighing platform has a range of 0–6kg

to an accuracy of 1 g, followed by 6–12 kg to 2 g, and

12–32 kg to 5 g.

Electronic Data-Processing (EDP) Attachment

0025The time-tested laboratory notebook can still be

found next to the majority of scales. However, anyone

who has discovered how simple it is to print out

weight values or transfer them to a computer will

need a lot of persuading to relinquish such con-

veniences.

0026Two interfaces with different standards and func-

tions are steadily gaining acceptance as worldwide

standards. RS232C is the most widespread standard

for attachment to personal computers and printers. It

allows data communication in two directions: to the

computer and from the computer to the scale. Cur-

rent loop (CL) is a standard interface that allows

distances of up to 1000 m to be covered without any

problems, which is not possible with an RS232C

connection. This enables several scales and peripheral

units, even those at different sites, to be linked and

controlled and monitored by a central computer.

Matched to Harsh Surroundings

0027In the food sector, scales and weighing systems are

often used in harsh ambient surroundings: wet condi-

tions and dust, acids and salt, and other corrosive

fig0003 Figure 3 DeltaTrac analog and digital display. Reproduced

from Weighing, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

WEIGHING 6123

substances can influence their functional dependabil-

ity. Despite the harsh conditions, they must offer

absolute reliability.

0028 The use of electrolytically polished chrome–nickel

steel and other high-grade materials ensures oper-

ational reliability even in adverse surroundings.

0029 A high degree of protection (e.g., IP67) assures

protection of the sensitive instruments and particu-

larly that of the electronics against the ingress of

water, moisture, and dust.

The Hygiene Factor

0030 In the food-processing industry, hygiene is one of the

central problems facing a plant. All production equip-

ment must be appropriately designed to ensure strict

compliance with the hygiene regulations. Thus the

construction of the scales and weighing systems de-

scribed above also takes into account the stringent

requirements with regard to cleanliness. The instru-

ments can be thoroughly cleaned once or several

times a day in accordance with the regulations with-

out any problems and without being damaged,

whether this involves hosing down, the use of power-

ful cleaning agents, or other treatments. (See Cleaning

Procedures in the Factory: Overall Approach.)

Increase in Efficiency Linked with Cost Savings

0031 Modern material flow concepts can no longer be

imagined without electronic scales and weighing

systems. As integratable components, they fulfill a

wide variety of functions over the entire goods flow

from the incoming goods via production, storage,

quality assurance, research and development, up to

the outgoing goods. Thanks to its system capability,

the scale can be integrated without any problems in

fully automated and EDP-controlled production and

flow processes.

0032 The high accuracy, the extreme user-friendliness,

and the highly diverse application capabilities make

modern scales and weighing systems an essential

factor to increase efficiency and optimize quality

and costs.

The Selection of the Right Material Flow

Scale

0033 What are the essential points to consider in the use of

scales in material flow? Basically, a distinction must

be made between features specific to the scale and

those specific to the application.

Bar Codes in Weighing Technology

0034 The use of the bar code technique is now making

headlong progress in general industry, and additional

bar codes, superficially known as industry codes,

have been developed to cover these needs. The main

use lies in conveying and handling, as well as ware-

housing; here the material flow is tracked. In all

phases of the transport process, attempts are being

made to identify automatically the goods to be trans-

ported or stored i.e., by electrooptical reading of the

bar code symbols on the packaging, transport con-

tainers, accompanying documents, etc., and to trans-

fer the data to process or normal computers and

perform automatic crediting and debiting by way of

a permanent inventory.

0035Consideration of the scale as a sensor in material

flow assumes increasing significance if the following

simple application is pictured: read in the identifica-

tion data of the weighing sample or sample to be

counted directly at the weighing terminal from the

bar-coded medium and, supplemented with the

weighing result, transmit to an external data receiver

via a data interface; alternatively, print out weighing

results in bar code form for the rapid and reliable

acquisition by bar code reading at the destination.

0036The features specific to the scale include properties

such as industrial compatibility, weighing range and

accuracy, user-friendliness, and system capability.

The application-related characteristics depend on

the location and the actual application. A number of

companies offer a whole series of software packages

with standard applications, such as weight controls

(plus or minus checks), automatic weighing in the

material flow, totalization, formula weighing, dis-

pensing, and filling process control. The requirements

of the specific departments must also be taken into

consideration.

0037The entire inventory management of the complete

chain stands or falls by the precise acquisition of

weights and batches in the incoming goods depart-

ment. Check weighings and data acquisition are of

prime importance, whether they involve stationary or

mobile, line-independent scales.

0038In production, the broad spectrum of applications

proves its full worth. Here, the production per-

formance has to be supported by efficient control

systems. Time-tested standard applications provide

assurance in checking and classifying, and in check

weighing in the material flow. The use of EDP allows

the weighing data to be processed further, and the

function of the scale as a sensor to maintain quality

and comply with quantity can be utilized to the full.

In addition to high quality, the goal is always an

increase in efficiency, whenever possible linked with

a reduction in costs.

0039It is impossible to imagine modern warehouses and

warehouse management without electronic scales and

weighing systems. The precise acquisition of weight

6124 WEIGHING

data, piece counts, etc., provides a reliable basis for

stock planning and logistics.

0040 In the outgoing goods department – the last link in

the production chain – scales again make a contribu-

tion by ensuring that with precise check weighings the

preceding efforts in the entire material flow are not

for nothing. Particularly in the food industry, rapid

flows in this department are decisive to assure quality

and freshness of the products, but without endanger-

ing the economic efficiency of the production.

From the Production to the Front

0041 What is right for production should be proper for

commerce. Even at the front, in the retail trade, the

picture has changed drastically in the past few

years. Scales with price-marking systems are today

the order of the day, as is the linkage to form complete

network systems in supermarkets. Similar to the situ-

ation in production, the scale network distributed

over the selling space is centrally monitored and

controlled. In the field of price marking and label

design, all conditions required by law and economic

efficiency can be met: bar code, layout, article data,

up to the design of advertisements for the business in

question.

Electronics also on the Road

0042 In the era of the EC and the increase in the exchange

of goods by road, it is essential to acquire complete

delivery and dispatch data. Several vehicle scales

employ the principle of strain-gage technology and

have the same acquisition and processing possibilities

of the weighing data as all other scales.

Filling Process Control in the Food

Industry

0043 One of the most important application areas of

weighing systems in the food industry involves filling

process controls. For here, as in all types of filling

processes, the tolerance limits must be defined so that

neither unnecessary losses owing to safety overfillings

nor any violations of the legal regulations owing to

underfilling occur.

0044 The following example will illustrate this point. In

the Hamburg factory of Tchibo, a major coffee-

roasting company, every year 32 000 t of freshly

roasted coffee beans are weighed to the nearest bean

and immediately packed. Approximately 580 Tchibo

branches and 15 000 fresh product depot partners

assure the customer of the proverbial Tchibo fresh-

ness, in other words top-class quality and an unvary-

ing fill quantity.

0045The filling machines run in a three-shift operation

with a capacity of 100–120 packages per min and a

net weight of 500 g. This high filling rate notwith-

standing, the weight of every package must be exactly

right. There are two contrasting reasons for this.

First, the legislative authorities or the weights and

measures office checks that the prepackaged goods

regulations are complied with and the average weight

is not less than the nominal fill quantity. Second,

constant overfilling ‘on the side of safety’ would

be hugely uneconomical with the amount that is

processed. In order to handle these two problems,

Tchibo has installed a filling process control system

which operates on the basis of random sampling. At

the 13 packaging lines, per day 3900 random samples

are taken and processed by the system using statistical

methods.

0046Use of this filling process control system ensures

that not only are the legally specified tolerances

complied with, but also approximately 40 kg of

coffee is saved every day. Scaled up to the annual

production, this means a saving of around 10 t.

Assuming the price of green or raw coffee is $1000

per t and taking into account the tax on coffee

in Germany, at present around $2 per kg, this

results in a sum of approximately $30 000 per year.

This figure shows that a well-functioning control

system pays for itself many times over after a short

time.

The Filling Machine – A Complex System

0047As a rule, a filling machine is a complex entity. Vari-

ous influences, which can be controlled only after a

great deal of effort, lead to random fluctuations in the

weight or volume of the packages. The magnitude of

these fluctuations depends on the filling principle, the

condition of the filling machine, on the packaged

product, and numerous other factors.

0048Independent of these fluctuations, in the guise of the

nominal fill quantity, a lower tolerance limit is

more or less rigidly fixed by appropriate statutory

regulations. Simple compliance with this lower

limit makes a control of the filling process necessary;

even with apparently large safety everfillings, a check

must be made to ensure that they are actually large

enough.

0049On the other hand, from the industrial economic

viewpoint, the interest of the filling plant is centered

on the upper limit. The efforts are thus always aimed

at keeping the shaded area shown in Figure 4 as small

as possible.

0050The distance between the upper and lower limit is

to a large extent a question of the control outlay and

is therefore decisive for the selection of a suitable

WEIGHING 6125

control system. Decisive criteria in this case are the

type and speed of the controls as well as the desired

informative nature of the control results, rather than

the number of packages to be checked.

The Control System – An Adjustment System

0051 A control system should not only be used to check

weights or volumes of packaged products, it must

also be possible to influence the filling process

actively with the aim of ensuring compliance with

the statutory limit values in filling at all times with

minimal material costs. This results in a control loop

(Figure 5), which in practice is by no means easy to

achieve.

0052 As every filling process is subject to random influ-

ences, it goes without saying that the control and

adjustment tasks must be solved by statistical

methods. However, an important fact should always

be noted even with the statistical weight check:

nobody – and this applies however extensive the

technical effort – can predict the weight of the next

package.

0053 In contrast to a widely held view, 100% inspection

does not result in any improvement in this respect.

With a check weigher integrated in the filling line,

although every single package is checked, statistical

methods must still be employed for control of the

filling machine.

Selection of the Suitable Organizational

Form

0054 In the selection of the suitable organizational form, a

fundamental choice has to be made right at the start:

the options are statistical filling process control, or

100% inspection, i.e., a check on every package with

a check weigher.

0055The following description of the two types of con-

trol shows that each has certain advantages. It will be

apparent that under certain conditions it can also be

eminently practical to select a mixed organizational

form employing both random sampling and 100%

inspection.

100% Inspection

0056Basically, it is wrong to assume that only 100% in-

spection can provide the necessary assurance of strict

compliance with the statutory regulations. In particu-

lar, it is the presence of a lowest tolerance limit which

contributes to this erroneous belief; the regulations

state quite clearly that to ensure the lowest tolerance

limit 100% inspection is not obligatory and a random

sample check is admissible.

0057However, there are a number of cases in which

100% inspection has advantages compared with

random sampling:

0058in a filling situation where frequently only half-

filled or even empty packages are produced and

there is no subsequent visual inspection

0059if the produced packages have to be classified and/

or sorted

0060if a complete operational data acquisition or evalu-

ation (production efficiency, throughput, etc.) is

required

0061if the product or the filling situation experiences

extreme density fluctuations

The automatic feed of the packages to a check

weigher allows a measured value acquisition without

additional personnel costs. However, with a relatively

large number of filling machines, the capital invest-

ment required for 100% inspection is considerable, as

each filling line must be equipped with a check

weigher (Figure 6).

Economy in operation

Mean fill weight

Consumer protection

Nominal

fill

quantity

fig0004 Figure 4 In the interest of the filling plant, the magnitude of the

overfilling (shaded area) must be kept as small as possible.

Reproduced from Weighing, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ

(eds), 1993, Academic Press.

Measured value

acquisition

Control

evaluation

Filling

Machine

adjustment

fig0005Figure 5 Control loop for active influencing of the filling pro-

cess. Reproduced from Weighing, Encyclopaedia of Food Science,

Food Technology and Nutrition, Macrae R, Robinson RK and Sadler

MJ (eds), 1993, Academic Press.

6126 WEIGHING

Statistical Filling Process Control

0062 In contrast to 100% inspection, the statistical filling

process control has the following advantages:

0063 The most important advantage of the statistical

control is without doubt the appreciably lower

capital investment required in comparison with

100% inspection. The disadvantage of the higher

outlay, owing to the need for additional control

personnel, is offset to some extent by selection of a

suitable organizational form. Hence, in many

plants, it is the task of the machine operators

to undertake a check of their own machines by

periodic sampling through a line inspection.

0064 If a measuring station is assigned several ma-

chines, the inspection can be made with a so-called

continuous inspection. Special control personnel

make rounds through the production, collect

samples, and then check the packages at the meas-

uring station. This form of organization is selected

primarily in plants in which the filling process

control is performed by a department independ-

ent of the production – as a rule by the quality

assurance.

0065Basically, the management of even large measure-

ment series in a statistical filling process control

poses no problems since the operating convenience

and efficiency of modern electronic scales allow

several thousand weighings in the course of a

working day (Figure 7).

0066In summary, the statistical filling process control

proves advantageous in those filling situations where

a large number of filling stations must be monitored

with low capital investment. In contrast, 100% in-

spection is advantageous in all cases where large

batches of a single product have to be subjected to

an automated filling control at a few filling stations

and hence the ability of a modern check weigher to

check several hundred packages per minute is of

prime importance.

Selection of the Correct Control System

0067What criteria are used by the filling plant to select the

control system that is optimum for its specific require-

ments? The cost-conscious plant will decide on the

control system that brings it the greatest profit, i.e.,

the smallest possible overfilling that ensures no viola-

tions of the laws. With such a minimum configur-

ation, the filling plant may well comply with the

statutory regulations, but it sometimes gives away

material worth a considerable amount through over-

filling.

0068From the industrial economics viewpoint, however,

a filling control is also indispensable even if there

fig0006 Figure 6 Check weigher (Leisi, Switzerland). Reproduced from

Weighing, Encyclopaedia of Food Science, Food Technology and Nu-

trition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Aca-

demic Press.

fig0007Figure 7 Sampling operation (weighing/Leisi). Reproduced

from Weighing, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

WEIGHING 6127

were no regulations, for the correct selection and use of

a control system can be profitable. In the final analysis

the success of the filling process control depends to a

large extent on the control personnel and machine

operators. Only when the degree of automation has

been increased will this dependence be reduced.

Filling Process Control in Practice: The

SQC52 Quality Control System

0069 The SQC52 quality control system is discussed here

in some detail as a representative of the broad spec-

trum of systems on offer.

0070 This system uses random sampling to supply all

information decisive for the optimum adjustment of

the filling machine and for the legally required

records and evaluations.

0071 The operating philosophy of the SQC52 system has

been taken over from the window technique used

successfully in EDP. The small number of operating

keys and the easily surveyed graphics display contrib-

ute decisively to the simple and convenient operation

(Figure 8).

0072 All information is displayed in the form of

windows. The status field provides general informa-

tion on the operating mode; the dialogue window is

used to support the operating sequences. It indicates

when additional details are required or information or

error messages have to be displayed for the operator.

Each operating mode contains a number of working

screens, which can be selected using the function keys.

Dependability – Thanks to Proven Menu Technique

0073 For the control operation, all necessary data are avail-

able as a working window. In the working screen

product definition, the user defines in a simple manner

all necessary product parameters for the products to

be checked. Other windows handle the parameter

selection (selection of the possible weighing modes),

the interface configuration, the memory allocation

and instrument settings. The individual windows are

selected using cursor keys. A help key is used to call

up help texts with information on the respective entry

or selection fields.

Graphical Evaluation, Informative Records

0074The sampling operations can be shown on the display

as a mean-value trace together with the deviations

with time in the filling process. The last 10 samples

in each case are shown in the correct order as so-

called box plots with mean value, standard deviation,

greatest and smallest mean value.

0075The histogram reproduces a set of batch statistics

with the distribution of all measured values. This

allows a qualitative report on the filling plant and

the production. On the basis of the statistical values

calculated by the system, the machine can be adjusted

manually (adjustment message) or automatically

(Figure 9).

0076The SQC52 system allows comprehensive docu-

mentation of the samples in four detailed sets of

statistics. All parameters, such as date, time, mean

value, standard deviation and tolerance violations,

as well as all requisite adjustments, are recorded and

printed out. The system takes into account most na-

tional legal regulations (tolerance system) including

pharmacopoeia (an official list with regulations for

preparation, quality, and application of drugs).

The SQC52 System in Operation in Leisi

0077Leisi is a large Swiss bakery and producer of different

types of ready-made pastry. It uses an SQC52 system

fig0008 Figure 8 SQC52 display. Reproduced from Weighing, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R,

Robinson RK and Sadler MJ (eds), 1993, Academic Press.

6128 WEIGHING