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FAQ on Temperature
All thermometers work on the principle of detecting changes in the physical properties of things as temperature changes. Dial thermometers depend upon the expansion and contraction of metal. Electronic thermometers, like thermistor RTD and thermocouple thermometers use the effects of heat and cold on the electrical resistance that reacts to changes in temperature along predictable curves. With thermistors, resistance decreases with temperature and with RTDs, resistance increases. Both thermistors and RTDs may have a higher degree of accuracy than thermocouples, but their range is limited by comparison and they are generally not as fast.
Thermocouples work on the principle of an electronic circuit is generated when connected to two different metals across a span with a temperature difference. The voltage of the generated circuit changes with variations in temperature in predictable ways. Common thermocouples weld together nickel and chromium (called Type K), copper and constantan (Type T) or iron and constantan (Type J) and place the weld at the very tip of the thermometer probe.
Infrared thermometers are useful for checking surface temperature and they measure the emission of infrared radiation. Infrared thermometers are very fast, producing a reading in a fraction of a second, or the time it takes for the thermometer's processor to perform its calculations. Their speed and relative ease of use have made infrared thermometers invaluable public safety tools in the food service industry, manufacturing, HVAC, labs and countless other industrial applications.
While other thermometers measure the affect of heat and cold on sound waves, photoluminescence, fluorescence, magnetism, gamma rays, and many other physical phenomena.
Infrared Consideration
Infrared thermometers are great for checking surface temperature at a distance, however they are not ideal for gauging the internal temperature of an object. They provide relatively accurate temperatures without ever having to touch the object to be measured. However its accuracy and sensitivity are compromised if used incorrectly on different texture surface, distance away from target area, working environment and applications.
"Emissivity" is a measure of a material's ability to emit infrared energy. It is measured on a scale from just above 0.00 to just below 1.00. Generally, the closer a material's emissivity rating is to 1.00, the more that material tends to absorb reflected or ambient infrared energy and emit only its own infrared radiation. Most organic materials, including the byproducts of plants and animals, have an emissivity rating of 0.95.
The "spot size" of any given measurement is controlled by two variables:
1. the "distance to target ratio" or "spot ratio" of particular infrared thermometer
2. the distance between infrared thermometer and the target
Typically listed on the thermometer itself, the "distance to target ratio" (DTR) or "spot ratio" indicates the diameter of the "circle" of surface area an IR thermometer will measure at a given distance. For example, an infrared thermometer with a 10:1 ratio will measure the temperature of a 1" diameter circle of surface area from 10" away, a 2" diameter circle of surface area from 20" away, and so on.
Calibration
While adjustments are sometimes needed, "calibrating" a thermometer means testing its accuracy against a verifiable standard. If the thermometer is within the accuracy specification listed by the manufacturer or quality control agency, no adjustment should be made. When it comes to recommended frequency, there is a very wide range. Mechanical thermometers like dial thermometers should be calibrated very regularly if not daily, while digital thermometers are often only calibrated once a year.
Food Testing
Because food-borne bacteria usually land on the surface of foods first, infrared thermometers can be very useful for spot checking the holding temperatures of plated foods, serving areas, buffets and warming trays. But critical food safety temperatures (5°C and 60°C) should always be verified with an internal probe. Some infrared thermometers even come with attachable type K probes that serve this purpose accurately. To effectively gauge the doneness of foods, a penetration temperature probe is suitable to measure the inner temperature of cooked food; unlike infrared thermometer that measures surface temperatures. It is good practise to take few temperature readings at various spots.
Bacteria thrive between the temperatures of 4.5°C and 60°C. Food should not be stored between these temperatures for extended periods of time. Some leftover foods must be reheated to minimum temperatures to assure sufficient "kill rates" of bacteria or parasites. Below are the guidelines published by USDA for food holding and cooking (www.IsItDoneYet.gov).
Holding hot foods ...............................60°C or higher
Holding cold foods ..............................less than 5°C
Fridge temperature.............................4.5°C or colder
Freezer temperature ..........................-18°C to -23°C
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