10. Temperature measurement
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This chapter gives advice on how to measure the temperatures in and around fish, which influence its keeping qualities and handling and processing requirements.
Temperature is the most important factor controlling the rate at which fish go bad. For instance, as mentioned in Chapter 1, cod will remain edible for up to 15 days at a temperature of 0°C, whereas it may be unfit to eat after only 6 days at 5°C. Therefore it is important to know the temperature of the fish to a reasonable degree of accuracy.
To many people a thermometer means only the familiar mercury-in-glass type. This relies on the expansion and contraction of mercury to indicate the temperature on a calibrated scale. However, this type of glass thermometer is unsuitable for measuring the temperature of fish. This is because of the danger of breakage, the slowness of response to temperature change and the comparatively large temperature-sensitive bulb which does not allow spot measurements to be taken for example, at the centre of a small fish. Glass thermometers protected by a metal casing are suitable for temperature control checks in a number of processes but, again, they should not be used where breakage might lead to dangerous contamination of the fish.
Changes in temperature also change other properties of materials, such as electrical resistance, and thermometers can be constructed based on measurement of these changes. Generally, they are now constructed as compact, hand-held units which give a direct digital readout of temperature and utilise probes, safe to use with fish.
A brief description follows of various types of sensor which are commonly used for fish temperature measurement and also for temperature control during fish handling and stowage.
Thermocouples. When two lengths of wire made from different materials are joined at both ends to form a closed circuit, any difference in temperature between the two junctions will cause a small voltage to be generated. This tiny voltage is related to the difference in temperature. Thus, if one junction is kept at a fixed temperature, it is possible to measure directly the change in temperature at the other. The voltage is usually measured by an instrument called a potentiometer which can be constructed to give the temperature directly. The thermocouple is made in the form of a twin-core insulated cable using wire to suit the particular requirement. The temperature sensitive junction is a minute welded or soldered junction giving a rapid response, and can be incorporated in a hand probe suitable for direct insertion into fish. The wire of a thermocouple can be any length without altering the calibration of the instrument. It can be used therefore for remote reading of temperature only if compensated voltage, currentless equipment is used. Instruments for use with thermo-couples can be made to show either a single reading on a dial or a number of readings in succession by means of switches. Alternatively, the temperature can be recorded on a chart. Thermocouples made for temperature measurement of fish and fish processes are usually made from copper-constantan (type T), but care should be taken to ensure that the material specification is within the range of the instrument's calibration.
Resistance thermometer probe. This instrument depends on the fact that the electrical resistance of a metal changes with temperature. Fine wire of a suitable material is wound into a minute coil and this may be constructed in the form of a probe suitable for inserting in fish. The probe is connected by a flexible cable to a portable instrument which gives a reading on a temperature scale. Some of these instruments are not equipped with compensation for cable resistance so the length of the cable is important. This type of instrument may not be used indiscriminately with varying lengths of cable.
Thermistor probe. The electrical resistance of some semiconductors exhibits large changes with temperature variation. This property is applied in thermistor thermometers and, for most purposes, this type of instrument can be used in a similar manner to a thermocouple. However, unlike thermocouples, thermistor junctions cannot be made readily by the user. The instrument and temperature sensitive probes have to be matched carefully.
Dial thermometer. This type of thermometer is designed to give a permanent visual indication of temperature. It may be used to indicate the temperature of chillrooms, fishrooms, brine tanks, RSW systems and for many other similar applications. The instrument consists of a liquid-filled bulb, connected by a narrow tube to a dial which indicates temperature. Similar instruments rely on the expansion of a gas within the bulb or changes in the vapour pressure of a liquid to operate the dial. These instruments are available with a range of tube lengths not normally greater than 5 to 10 m. The degree of accuracy and rapidity of response, although a good deal less than the probe type instruments described above, can be selected to suit the application. Stainless steel and other noncorrosive materials should be used to construct the temperature sensitive elements for fish processing applications.
Circular chart recorder. The simplest form of this instrument works on the same principle as the dial thermometer but, instead of a temperature indicator, the temperature sensitive element is mechanically coupled to a pen which continuously records the temperature on a chart. Charts are normally suited for 24 hour or 7 day operation and this type of instrument is often used for process control.
Temperature measurement of fish
It is important to know the temperature of the warmest fish in a batch since the quality as a whole may sometimes be dependent on this data. The warmest fish may be at the centre or the outside, depending on whether the fish are being cooled or warmed at the time of measurement. It is advisable in any event to take a number of readings at random. For example, in a stack of boxed fish, boxes from the centre, the outside, the top and the bottom of the stack may be selected, and the temperature of individual fish within each box taken. Thermometers which respond slowly are not suitable for measuring fish temperatures. A thermometer with a large sensitive element would also be unsuitable since often the temperature at a precise position in the fish or package is required. Many probe type thermometers are suitable and these should be inserted in the fish so that the temperature sensitive element at the end of the probe is located at the point to be measured with at least 75 to 100 mm of the probe in the fish, if possible (Fig. 30).
Fig. 30. Insertion of thermometer in fish
This eliminates any error introduced by conduction of heat along the probe. An instrument for this purpose should have an accuracy of +/-0.5°C. The scale should be graduated in not less than half degree divisions.
The following guide rules should be remembered when measuring fish temperature:
1. Always measure the most significant temperature; that is
check those fish which are slowest to cool or quickest to warm,
or at the highest temperature.
2. Penetrate the fish with as great a length of the thermometer as possible to avoid errors due to conduction of heat.
3. Measure the temperature quickly with little or no handling of the fish.
4. Use an instrument that responds quickly to temperature changes and which reads to within one quarter of a degree of the true temperature.
5. Use an instrument with a small temperature sensitive element.
Measurement of process temperatures
Keeping a continuous record of temperature at all stages of processing is good practice and should be encouraged. If the plant is big enough, a network of thermometers with a continuous chart recorder or data logger may be considered, otherwise circular chart or dial thermometers may be used.
An instrument used for indicating chillroom temperatures should be capable of detecting small changes in the temperature fairly quickly and the detector bulb should be placed to indicate temperature fluctuations caused by such things as opening of the door. It should not be so near the door or cooling grids that it registers a temperature unrepresentative of the chillroom as a whole. In a large room, at least two thermometers should be fitted, particularly where there is likely to be uneven temperature distribution due to the position of the coolers and doors. If there is any doubt about the positioning of room thermometers, a temperature survey of the room should be made to ascertain the temperature pattern. Then the thermometers should be placed in positions which give representative temperatures.
When temperatures are measured in the fishroom of a fishing vessel, the requirements are much the same as for chillrooms. In both cases, when a position has been chosen to give a temperature representative of the room, this should also be the location of the temperature sensitive element of the refrigeration system's thermostatic control. The scale markings on the indicator should be in divisions of not more than one degree and the instrument indication should not deviate by more than one degree from the true temperature.
Preferably, other process temperatures should be recorded, since at least the changing of a chart ensures periodic checking of the temperature history. The instrument should be robust and its accuracy should be commensurate with the requirements of the process. Apart from these general rules, the instrument should be specially selected for the particular application.
In some modern processing plants, temperature monitoring and control often forms part of a totally integrated system of plant monitoring and control which may be interfaced to a computer programmed for display, alarm settings and analyses.
Calibration of thermometers
All thermometers need to be checked at frequent intervals as a routine. Invariably each instrument has some means of resetting if it should be in error. The most suitable method is to check the instrument over its whole range against a certified standard thermometer, but a single check at only one point may be satisfactory. Ice made from water of domestic quality will melt at 0°C and, for most thermometers used for chilling operations, a single check at this temperature will be acceptable. At least a bucketful of a mixture of ice and water should be used with the ice finely crushed; the water added should be clean tap water. The ice-water mixture should contain a high proportion of ice once the temperature has stabilised at 0°C, and the mixture should be vigorously stirred while temperatures are being checked.
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