Why do we need to measure temperature?
What do we use to measure temperature?
Temperature measurement of wet fish
Temperature measurement of fish during freezing
Temperature measurement of frozen fish
Practical points in measuring fish temperature
Measurement of cold store temperature
Temperature measurement of air in blast freezers
Temperature measurement in smoking kilns
Instrument selection
Calibration of thermometers
APPENDIX
Temperature is the most important factor controlling the rate at which fish go bad. For example, cod will remain edible for up to 15 days at 0°C, the temperature of melting ice, whereas it may be unfit to eat after only six days at 5°C.
White fish frozen quickly and then stored at a controlled temperature of - 30°C will keep in good condition for nine months or more; the same fish kept at a temperature of - 10°C will not remain in good condition for much longer than a month.
Freezing times for fish products are best determined by observing the change in temperature of the warmest part of the fish throughout freezing; miscalculation of freezing time can result in either inadequate cooling of the product or reduction in output of the freezer.
Knowledge of the temperature of the smoke in a mechanical kiln helps the operator to produce smoked fish of consistently good quality and to make full use of the capacity of the kiln.
Many other examples could be quoted that would demonstrate the importance of measuring temperatures in and around fish, from the time when the fish are first harvested until they reach the fishmonger's slab or frozen food cabinet. Constant vigilance to maintain suitable temperatures throughout handling, storage and processing is essential if quality is to be safeguarded and plant used efficiently.
The straightforward answer is a thermometer, but to many people this means only the familiar mercury-in-glass thermometer which relies on the expansion of the mercury to indicate the temperature on a graduated scale (Appendix, item 1). Several other kinds of thermometer that depend on the effect of change of temperature on other properties of materials, such as electrical resistance, are mentioned in this Note and briefly described in the Appendix. Temperature measuring systems may consist of two parts, the sensor which responds to the temperature, and the indicator which displays the temperature and which may be distant from the sensor.
Fish at 0°C, the temperature of melting ice, will keep in better condition than fish at higher temperatures; therefore in any batch of fish it is important to know the temperature of the warmest fish, since the quality of the batch as a whole may be judged by the condition of these. The warmest fish may be at the centre or on the outside, depending on whether the fish are at the time being cooled or warmed. It is advisable always to take a number of readings at random; for example, in a stack of boxed fish, boxes from the centre, outside, top and bottom of the stack may need to be selected, and temperatures taken of individual fish within each box.
INSERTION OF THERMOMETER IN FISH
A very convenient instrument for measuring the temperature of wet fish is a hand-held digital thermometer with a probe type sensor (Appendix, item 4). It is portable and accurate, and has a rapid response which allows readings to be taken quickly. Many instruments are available with thin sharpened probes which can be easily inserted into unfrozen fish.
The temperature-sensitive element at the end of the probe should be located at the point to be measured, with at least 75 to 100 mm of the probe inserted in the fish, as in the diagram. This reduces any error introduced by conduction of heat along the probe. Special care is required when measuring the temperature of fillets or small products in a chill cabinet. The product should not be removed from the cabinet and should be handled as little as possible to avoid warming it up.
An instrument used to measure the temperature of wet fish should be accurate to within 0.5°C; the readout is likely to be in divisions of 0.1°C.
Codes of practice and regulations for quick freezing and cold storage of fish often recommend specific freezing rates and maximum temperatures during storage and distribution. The design of a freezer and the capacity of refrigeration machinery have to be carefully selected to match the desired rate of freezing and final temperature of a particular product. Accurate measurement of freezing time is essential in order to confirm the performance of newly installed plant, and to check periodically the proper operation of existing plant.
The most suitable sensor for measuring freezing times is the thermocouple (Appendix, item 3). The thickness of the thermocouple wire can be chosen to suit the product being frozen and, since it is comparatively cheap and expendable, the wire can be cut off after freezing, leaving a short, but readily visible, length in the fish which should be recovered when the fish is thawed.
Since the freezing time of a product is the time taken for the whole of the fish to reach the desired temperature, it is essential that the temperature be measured at the point in the fish that reaches this temperature last. This point, sometimes called the thermal centre, does not necessarily coincide with the centre of a package or the middle of the thickest part of a fish; this will be so only when freezing takes place uniformly from all sides.
In the example shown here the apparent freezing time to -20°C can vary from less than one hour to 2½ hours, depending on where in the fish the temperature is measured. The shape of a temperature-time curve measured at the thermal centre is characterised by a pause at a steady temperature somewhere between 0 and -2°C, followed by a rapid decrease to near the temperature of the freezer.
The thermocouple should be inserted in the fish so that the temperature-sensitive point is in the part that will freeze last and so that as great a length of wire as possible is likely to be at the same, or nearly the same, temperature. This ensures that there is little error due to conduction of heat along the wire and also that, if the wire is pulled slightly out of position during the loading operation, the temperature-sensitive point of the thermocouple will remain in a part of the fish that freezes last.
Small items such as shrimps are individually too small to ensure that a sufficient length of the thermocouple is in the fish. In this case a number of shrimps should be threaded on to the thermocouple behind the temperature-sensitive junction which is located at the centre of the last shrimp.
It is not a simple matter to predict the location of the last part of the fish to freeze, nor is it easy to place accurately the end of the thermocouple at this point in the fish. Because of this uncertainty it is advisable to use a number of thermocouples on each occasion and, after examining the temperature-time curves, to reject the results from misplaced thermocouples.
The freezing times given by the correctly placed ones will be comparable, but not necessarily identical. There may be differences in freezing rate due to small air spaces in the pack, or to variation in degree of contact with the freezing medium; the position of the fish in the freezer and a number of other factors may also affect the result. Because of these variations, the freezing time of a product under specified conditions can be determined only after several tests with thermocouples in carefully selected places. The choice of positions in an air blast freezer, for example, would include fish nearest to, and furthest from, the incoming cold air, fish close to the tunnel walls and at top and bottom of the load, and at any other point where there is a likelihood of fish freezing faster or slower than the average. In a plate freezer, thermocouples would be inserted in fish close to comers and edges of the plates, as well as at the centre of the plate area, to make sure that any lack of uniformity of freezing becomes known. Once the performance of a freezer with a particular product has been established, subsequent periodic checks need not be so comprehensive, since only past and present performances are being compared.
Position of sensor in fish
CORRECT METHOD OF LOCATING THERMOCOUPLES FOR FREEZING
The temperature of the fish when leaving the freezer should be the same as the intended storage temperature. Since the outside of the fish will be at, or near to, the temperature of the freezer, the temperature at the centre of the fish should be lowered enough to ensure that the average temperature of the product is at the intended storage temperature. Allowance should be made to offset warming of the fish due to delays between freezer and cold store.
It is sometimes necessary to check the temperature of frozen fish during handling, transport or cold storage but difficulties arise because of the hardness of the product. Since a probe type thermometer cannot be inserted into white fish that has a temperature lower than -3°C it will be necessary to employ different techniques from those recommended for wet fish.
If thermocouples have already been used to check the freezing rate of the fish, the ends of the wires remaining in the frozen product can be reconnected to a suitable instrument to measure temperatures at any time during storage or transport. The fish or packages containing thermocouples should be located at points in the store or vehicle where temperatures are most critical, or where they are representative of the bulk of the product.
Where there is no thermocouple frozen into the product temperature measurement can be particularly difficult, and errors of more than 20°C can result from inadequate techniques. Different products and circumstances require different solutions and some methods that have proved useful are described.
Measurement near surface using slow response instrument in a wide hole - up to 20°C error
Measurement at centre using quick response instrument in a neat hole - 0.5°C error
The temperature of large blocks of frozen fish can be measured by drilling a hole in the block and inserting a suitable thermometer with a probe type sensor in accordance with the following procedure.
Remove the fish from the cold store to a place at normal surrounding temperature and drill a neat hole in the side of the block just large enough to take the probe. The hole should preferably be at least 100 mm deep, to avoid errors due to conduction of heat.
Insert the probe or thermocouple and read the temperature continuously until the lowest reading is reached and the temperature starts to rise again. The lowest temperature observed will be within 0.5°C of the true temperature of storage; any error will be due mostly to warming of the block during the operation which should not take more than two or three minutes. The drilling of the hole has no measurable effect on the temperature of the fish since the heat introduced is quickly dissipated.
Measurements outside the store at least ensure that any error is a positive one, but with smaller blocks even a short exposure outside the store will result in a substantial temperature rise. In these cases, the hole may be drilled within the store some time before the block is removed for measurement.
Fish such as individually frozen shellfish are often stored loosely in bins or containers before further processing. In this case a sensor placed at the centre of the container, and not necessarily in a fish, will give a fairly accurate fish temperature. This technique may also be used on such products leaving a continuous freezer if they are collected in a suitable container.
Accurate measurement of the temperature of products in retail frozen food cabinets requires particular care. Small items such as fish fingers and breaded shellfish meats warm up quickly when removed from the cabinet or handled: drilling a hole, even with a precooled drill, will cause errors unless this can be done without removing the package from its position in the cabinet. If the product is loosely packed, it is easier and quicker to insert the sensor into the centre of the package, with minimum handling and without moving the package from its original position. The temperature of stacked packets may be measured by inserting a thin probe between packets, without disturbance, and allowing sufficient time for constant temperature to be reached. Provided a rapid-response sensor is used, the temperature of individually frozen fillets, or similar items, can be measured by pressing two fillets together with the sensor between them.
1. Always measure the most significant temperature; check those fish that are slowest to cool, quickest to warm, or are at the highest temperature.
2. As great a length as possible of the probe should penetrate the fish to avoid errors due to conduction of heat.
3. Measure the temperatures quickly with little or no handling of the fish.
4. Use an instrument that responds quickly to temperature changes and that reads to within 0.5°C of the true temperature.
5. Use an instrument with a small temperature-sensitive element.
The keeping quality of fish in cold storage depends not only on the temperature of the cold store but also on the steadiness of that temperature. Fluctuations in temperature encourage undesirable changes in the stored product. It is therefore important that a continuous record be kept of the cold store temperature. In large plants a network of sensors with continuous recording would be best for checking the temperature of the cold store air and other temperatures throughout the plant; in small plants a circular chart recorder with a temperature-sensitive bulb is sufficient (Appendix, item 6).
The instrument should be capable of detecting small changes in temperature fairly quickly and the bulb should be so placed that it will transmit temperature fluctuations due to opening of the store door. It should not, however, be so near the door or cooling grids that it registers a temperature unrepresentative of the cold store as a whole. Instruments are available for either 7-day or 24-hour operation: if there is no other regular inspection, the daily changing of a 24-hour chart will ensure that the cold store temperature is checked at least once a day. The scale markings on the chart should be in divisions of not more than 1°C and the instrument should record to within 0.5°C of the true temperature.
Temperature measurement in air blast freezers is like that in cold stores, but because of the greater fluctuations in air temperature the instrument must respond more quickly. The measurements should be made at a point between the cooler and the fish. The instrument should be readable from the loading bay, so that operators can make sure the temperature is right before loading fish. The scale markings should be in intervals of not more than 1°C and the instrument accurate to within 0.5°C.
The temperature of a kiln is controlled or programmed either automatically or manually; in either case it is essential that there is a visual indication of the temperature. It is also essential that the kiln is checked after installation, to determine temperature distribution under fully operative conditions, and periodically thereafter, to maintain a consistent quality of product.
A network of sensors, probably thermocouples, should be used to test the kiln. The thermocouples should be located across sections of the kiln upstream, downstream and between trolleys so that the temperature distribution and temperature variation with time throughout the kiln can be determined. Temperatures should be recorded during a complete cycle of operations by taking a series of readings at frequent intervals on an indicating instrument or by using a continuous strip chart recorder or data logger (Appendix, items 7 and 8). The instrument used should have scale divisions of not more than 1°C and be accurate to within 0.5°C.
Visual indication of the temperature in the kiln in normal operation should be by a dial thermometer or digital display: a permanent record is supplied by a circular chart recorder using a 24-hour chart or by a data logger. The sensor of the instrument should be shielded from direct radiation and located upstream of the first trolley at a point that gives a representative value of the temperature of the air at that cross-section. If the kiln is a large one with booster heaters between trolleys it will be advisable to measure in addition the temperature immediately downstream of the booster heaters. The temperature indicator should be positioned where it can be viewed by the operator when loading the kiln. The indicating thermometer used should have scale divisions of not more than 1°C and should read to within 0.5°C of the true temperature. The instrument should respond to a brief opening of the doors of the kiln.
Thermometers have a number of characteristics which have to be considered when selecting an instrument for a particular use. In order to make the correct choice the user should be familiar with the following terms.
Accuracy
Accuracy is a measure of how close the measured temperature is to the true temperature. This stated accuracy, however, can be achieved only if the instrument is correctly used.
Accuracy is expressed either as ±x°C where x is the maximum deviation above or below the true temperature, or as a percentage of the temperature range of the instrument. Thus, 1% accuracy for an instrument with a temperature range of 100°C is the same as an accuracy of ± 1°C.
Although the temperature scale divisions of an instrument may reflect the likely accuracy of the instrument, the true accuracy should be determined by examining the specification of the instrument or by calibration, as described later.
Resolution
The resolution of an instrument is the minimum change in temperature which produces an effective response. Scale division markings should again reflect this resolution since there is little sense in having smaller divisions that suggest a better performance than the instrument can achieve.
Range
The range of temperature that can be measured should match the needs of the user. In general, the narrower the range, the better the accuracy and resolution will be.
Response
The response of an instrument is a measure of how quickly the instrument indicates the temperature to be measured. Generally, the smaller the sensor and the better the contact made by the thermometer the quicker is the response.
Operational limits
Instruments can be used only in locations where the temperature of the environment, the ambient temperature, is within certain limits, usually specified by the manufacturer; for example few hand-held instruments can be used within a cold store. When the temperature is outside these limits, a sensor connected by cable to a remote indicator must be used. The risk of condensation after removing an instrument from a cold environment such as a cold store should be borne in mind, even when the temperature is within the recommended operational limit.
All thermometers need to be checked at frequent intervals as a routine. The most suitable way is to check an instrument over its whole range against a certificated thermometer (Appendix, item 1) but a single check at only one point, using melting ice, may be satisfactory if the instrument is used to measure temperatures in this region only. At least a bucketful of a mixture of ice and water should be used, the ice being made from fresh water and finely crushed; the water should be clean tap water. The ice-water mixture should contain at least as much ice as water and be stirred thoroughly while the temperature readings are being taken. This mixture will provide a standard temperature of 0°C.
Types of thermometer
Displaying and recording temperature
1. Mercury-in-glass thermometers
This consists of a narrow-bore glass tube with a reservoir of mercury which expands and contracts with changes in temperature. The glass tube is graduated with a temperature scale and the temperature at the bulb of the thermometer is indicated by the level of mercury in the tube. The most familiar example of this type is the clinical thermometer. Glass thermometers are unsuitable for measuring the temperature of fish because of the danger of breakage, the slowness of response to temperature change and the comparatively large size of bulb. They can be used only down to -39°C, the freezing point of mercury.
A mercury-in-glass thermometer protected by a metal casing is suitable for temperature checks on a number of processes but again should not be used where breakage might lead to dangerous contamination of the fish.
Mercury-in-glass thermometers that have been calibrated against standard temperatures are available and are supplied with a certificate showing any errors over the whole temperature range. These certificated thermometers can be used to check the calibration of other temperature-measuring instruments.
2. Alcohol-in-glass thermometers
These work on the same principle as mercury-in-glass thermometers; the alcohol is usually coloured to make it more easily seen. They have most of the disadvantages of mercury-in-glass thermometers for measuring the temperature of fish, but they have the advantage that they can be used to measure temperatures below - 39°C.
3. Thermocouples
When two lengths of wire made from different metals are joined at both ends to form a closed circuit, any difference in temperature between the two junctions will cause an electric current to flow; the greater the temperature difference, the greater will be the current. This arrangement, called a thermocouple, can be used to measure temperature. One junction, the measuring junction, is at the point of measurement in the sample; the other, the reference junction, is kept at a known constant temperature. The reference junction may be in melting ice, but is more commonly kept at a controlled temperature inside an instrument which reads or records the temperature of the sample.
The thermocouple is usually made in the form of a twin-core insulated cable using wire of a size to suit particular requirements. Fine wire can be used to measure the temperature of something quite small like a single shrimp, whereas stronger wire may be more suitable for measuring the temperature of large fish. Various pairs of metals are used for the two wires of a thermocouple, depending mainly on the temperature range required, but, for all temperatures likely to be met with in the temperature measurement of fish and fish processes, thermocouples of copper and constantan (a copper-nickel alloy) are most suitable.
The wires in a thermocouple can be of any length without substantially altering the calibration of the system and, if necessary, the thermocouple can be threaded into a robust metal tube to form a probe. Electrical interference, giving rise to errors, can be reduced by twisting both wires together throughout their length: specialist advice may be needed in some cases. The temperature-sensitive junction, which is made by welding or soldering the two metals together, can be made very small, and need not be larger than the head of a pin. Because this junction is so small, the response to change in temperature is almost instantaneous.
4. Hand-held digital thermometers
This type of instrument is a compact hand-held unit with a probe type thermometer. The probe is usually attached by a cable for remote use but the probe may also be an integral part of the unit.
Digital thermometers use a variety of measuring systems based on thermocouples, resistance elements, thermistors or diodes. The probes are usually detachable and more than one can be supplied. Needle type probes are supplied for the penetration of surfaces and flat ended probes for the measurement of surface temperatures. The latter type of probe must be used with caution, being subject to error by conduction along the probe.
Instruments are available to cover a wide range of temperatures and some makes have switching facilities for range changing. For most purposes digital thermometers indicate the temperature in 1°C or 0.1°C increments but this does not necessarily indicate the true accuracy or resolution of the instrument.
Most digital thermometers for handheld measurements are battery operated and are ideally suited for temperature spot checks and for other field work.
Some digital thermometers are dual purpose instruments suitable for making additional measurements such as humidity, voltage, air velocity and pH.
5. Dial thermometers
This type of thermometer is designed to give a continuous visual indication of temperature, in chill rooms or cold stores, for example. One type consists of a liquid-filled bulb, which forms the temperature-sensitive element, connected by a narrow tube to a dial which indicates temperature. The liquid in the bulb expands and contracts with changes in temperature, and the movement is transmitted along the narrow tube to a curved, thin-walled, tube called a Bourdon tube; change of volume of the liquid alters the curvature of the Bourdon tube which is linked mechanically to a pointer on the dial. Similar types of instrument rely on expansion of a gas or changes in vapour pressure of a liquid to operate the Bourdon tube; alternatively, a bimetallic strip may rotate the pointer directly.
Unlike many other instruments, dial thermometers do not require a power source, such as a battery, for their operation.
6. Circular chart thermometers
These instruments work on the same principle as dial thermometers but the Bourdon tube, instead of being connected to a pointer, is linked to a pen which continuously records the temperature on a circular chart. Charts are nominally suitable for 24-hour or 7-day operation. Instruments are obtainable that have both recorder and indicator mechanisms.
Some of the sensors described can give a direct reading of temperature. Sensors that provide an electrical signal, such as thermocouples, thermistors and resistance elements can be connected to a separate, perhaps distant, system for displaying and recording temperatures.
7. Strip chart recorders
This type of instrument provides a means for recording a number of temperatures, usually as colour coded traces on a graduated strip chart. On most models chart speed and recording frequency can be altered to suit individual requirements.
Small robust recorders using bimetallic sensors are available; they do not need batteries and can operate within cold stores. Although their response is poor, they can give an indication of long term trends.
8. Data loggers
A data logger accepts an electrical input from a sensor and stores the information, sometimes as a numerical print-out on paper or, more commonly, electronically, for later interpretation and display. An immediate digital display of the information being recorded is often present. Data from a number of sources can be accepted simultaneously together with such information as identification of source and time of reading.
9. Computerised temperature monitoring
The electrical output from a sensor, or sensors, or the stored information from a data logger can be fed into a computer by a suitable interface: the information can be stored, visually displayed, analysed or printed on demand.
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Further information Further information can be obtained by contacting: The Director Tel. Aberdeen (0224) 877071 This Note is one of a new series, prepared by staff at Torry Research Station; the principal author this Note was J G M Smith. New Notes are: 91 Sensory assessment of fish quality Most Notes in the original series (numbered 1 to 90) are still in print, but are being replaced: a list is available, free of charge. Copies of all Notes are obtainable, for a small handling charge, from the above address. BL. 5811 |
