Thermocouple Surface Pyrometers
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WELFARE PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
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ITG SUBJECT: THERMOCOUPLE SURFACE PYROMETERS
This ITG has been written to familiarize Investigators with a temperature measuring device used by food processors utilizing the French manufactured Steriflamme flame sterilizer.
The Steriflamme is a high temperature short-time process where cans of food, after being steam exhausted and closed are first preheated in steam, and then further heated by means of a direct contact with flames, while rotating. Part 128b(h) of the Low Acid Canned Food GMPs requires that the surface temperature of at least one container from each flame sterilizer conveyor channel be measured periodically. This measurement should be made and recorded at the end of the holding period at intervals of sufficient frequency to ensure that temperatures specified in the scheduled process are maintained.
A band or ribbon type surface pyrometer is used by processors utilizing the Steriflamme to monitor container surface temperatures. A typical band type surface pyrometer is shown in Figure 1. This instrument is manufactured by the Pyrometer Instrument Co., Inc., Bergenfield, N.J. and is used with their type 301A detachable head. The band stretching between the guides of the detachable head (Fig. 1A) is the thermocouple hot or measuring junction. This unit incorporates cold junction compensation (which will be explained later) and meter movement dampening to prevent excessive meter pointer swing. This instrument has a 0-300 F temperature range and a stated accuracy of ± 1% of full scale or ± 3 F.
Contact pyrometers made by other manufacturers may be encountered but most are of similiar construction as the unit shown in Figure 1.
When using the surface pyrometer to measure temperatures of cans taken from the Steriflamme, a can is taken from the holding section by hand using an asbestos glove and placed on a level surface. The band or hot junction of the thermocouple is placed firmly on the wall of the can (Fig. 1B) until the indicator needle indicates a steady reading, usually in two or three seconds. The can is then rolled quickly back and forth under the band so that the cylindrical surface of the can in contact with the headspace is reheated by the product and the resulting temperature is read.
As mentioned before, a thermocouple is the temperature sensing element of the contact pyrometer shown in Figure 1. A thermocouple consists of two dissimiliar metal wires welded together at one end. When this junction is heated a voltage is developed at the free ends proportional to the temperature difference between the free ends and the welded junction. When constructing thermocouple pyrometers, the two free ends are connected by copper wires (cold junction) to a millivoltmeter which measures the voltage created and indicates this voltage in terms of temperature. For direct temperature indication, the cold junction must be accurately maintained at a known reference temperature. This must be done so that the temperature change at the hot junction only will be indicated.
For portable applications, it is not practical to maintain the thermocouple cold junction at a constant temperature. So, some means must be provided to bring the voltage developed under ambient conditions, at the cold junction, to a value equal to that which would be generated with the cold junction maintained at a standard temperature, usually 32 F (cold junction compensation). In most portable contact pyrometers, this is accomplished by a temperature sensitive resistor (thermistor) and associated circuitry, which senses the variations in cold junction temperature caused by ambient conditions, and automatically provides the necessary compensation voltage by means of a voltage drop produced across the resistor. Thus, theoretically, the instrument temperature indication becomes independent of cold junction temperature variations.
Even though cold junction compensation may be employed, most thermocouple pyrometers will display some indicator drift with changes in surrounding temperature. An accurate temperature reference source should be available and a calibration procedure established so that the pyrometer can be calibrated before each use. Calibration should also be performed in the area in which the pyrometer is to be used to minimize temperature change errors. The pyrometer should also be protected from abuse due to rough handling.
- Flame Sterilization, Specialist Courses for the Food Industry, No.2, November 19, 1971. Printed by CSIRO, Melbourne, Australia.
Figure 1: PYROMETER INSTRUMENT COMPANY MODEL 30 SURFACE, Fig 1A, 1B
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