the highdegree of assurance that the process is beingdelivered on a consistent
basis.
35.3 Current approachesto monitoring
Monitoringis the periodic or continuousdeterminationof the amount of specific
material or substances present in a system, the condition of a surface or a process
parametersuchas flowrate,temperatureor pressure. Fouling,cleaning and
disinfectiondatamaybe produced by a number of directand indirect methods
suchas:
∑ generationand analysisof process plantdata;
∑ controlledstudiesand analysisat laboratoryor pilotplant scale;
∑ mathematicalmodels.
It is welldocumentedin the literaturethat fouling is complexand variable,and
occurs through a varietyof physicalor chemicalmechanisms. It is rarely
uniform or evenlydistributedand mayvarysignificantlyevenon a day-to-day
basis. Monitoring the build-up of foulingis importantto identify whenthe plant
has reacheda condition where cleaning and disinfectionis required. All cleaning
and disinfection processes requirea designatedend-point and this maybe either
defined by protocolsbasedon trials,or, ideally,indicatedby sensors, whichcan
monitor the progressof the cycleand determine whencleanlinessis reachedor
disinfectionis complete.
Monitoringsystems or sensorscan be classifiedas integrated, localisedor
indirect, depending on the methodapplied (Hasting 2002 ):
∑ Integrated measurements are taken overthe complete system or a specific
part of it, for example,heat transfer coefficientor pressuredropwithina heat
exchanger.The sensitivityof theseaveragedapproachesmaybe limitedand
thereare oftensizeablemeasurementerrors. Sometechniques, suchas heat
transfer coefficient, maynot be applicablefor cleaning when the systemis
operated isothermally.
∑ Localisedmeasurements are made at specificpoints withinthe system.
However, identifyingrepresentative locationsor relating the information
generatedto the complete processequipmentare key issues.
∑ Indirect techniquesuse the measurement of key parameters suchas detergent
concentration,temperatureand valveopeningto provideinformation.The
results maybe subject to interpretation, particularlywherethe responsemay
indicate morethanone eventis occurring.
Sensors mayalso be classified as invasive if the processequipmenthas to be
modified to allowcontactof the sensingelement withthe process fluid,for
example fitting a pressuregaugeinto a line.A non-invasive sensoris one that
doesnot requirethe modificationof the equipmentfor installationof the sensor,
for example the use of ultrasonicsensorson the outsideof a pipe, to measure
574 Handbookof hygiene controlin the foodindustry