Thermometer Calibration Guide - asi.k-state.edu

1 Temperature is a critical measurement for ensuring the safety Thermometer and quality of many food products. There are a variety of commer- cial temperature monitoring devices available. Whether monitor- Calibration Guide ing temperatures at receiving, throughout production or final product storage and distribution, Thermometer Calibration is essential. The validation, verification reassessment section of the Nancy C. Flores, Hazard Analysis and Critical Control Point (HACCP) system stated in the Code of Federal Regulations (9 CFR 3 ) specifies that Elizabeth Boyle, , instruments used for monitoring critical control points must be Associate Professor calibrated. The food industry is keenly aware of the critical nature of processing temperature requirements. Instrument Calibration is not only a food safety issue, but also an economic consideration since accuracy of temperature monitoring devices also affects product yields.

2 The National Institute of a bi-metal coil Thermometer Standards and Technology can detect a degree (NIST) is an agency within temperature change, while a the U. S. Department of thermistor may detect a Commerce which provides degree temperature change. certification and Calibration It is recommended that for thermometers and other process or product tempera- precision instruments. When ture monitoring equipment purchasing a Thermometer be calibrated daily, before use. check for the NIST label. New equipment must be The NIST label indicates a calibrated upon receipt and certified instrument that will before putting into service. maintain accuracy within Also, thermometers that have specified limits, for at least been dropped on the floor or one year. Each year, an NIST used frequently must be Thermometer must be recerti- calibrated more often.

3 De- fied to assure accuracy. This is pending on the instrument a service usually provided by and intended temperature use the manufacturer of the ther- range, an ice water method mometer or an NIST calibra- and boiling water method, tion laboratory. when used properly, are Calibration is the process effective Calibration methods. of standardizing a tempera- Another technique is to use ture monitoring instrument to sophisticated Calibration ensure that it will measure equipment available from within a specific temperature manufacturers. These units range in which the instrument are designed to calibrate is designed to operate. Accu- several thermometers at one racy of a Thermometer is its time within a specific tem- ability to measure tempera- perature range. Many of these ture correctly without error.

4 Units are able to interface with A Thermometer must be with- a computer to assist with re- in 2 F ( C) of the actual cord keeping. The Calibration temperature to be considered method used at your facility kansas State university an accurate device. A ther- depends on the temperature Agricultural Experiment Station and mometer's ability to detect monitoring instrument, Cooperative Extension Service subtle changes in temperature monitoring frequency and is its sensitivity. For example, intended use, and whether for 1. product or environmental monitoring. Other accept- stem indicates the area of the stem that is the tempera- able methods may also be used to calibrate tempera- ture sensing region. The temperature indicated on the ture monitoring instruments. dial is an average of temperatures along this region.

5 Calibration methods are presented for the following These thermometers must be inserted 2 to 3 inches temperature monitoring instruments: bi-metal coil ( to cm) into the geometric center of a product. Thermometer , thermocouple Thermometer , and ther- Extreme fluctuation in temperature along the sensing mistor Thermometer . Calibration of non-product region can affect the extension of the bi-metal coil, contact thermometers such as Resistance Temperature which affects Thermometer accuracy. Bi-metal ther- Detector (RTD), oven temperature recording devices mometers are sensitive to physical stress such as and infrared thermometers are also described. A major torque on the stem, or shock from dropping on the consideration when using any Thermometer is the floor. These stresses can affect the tension of the bi- consistency of the product and temperature distribu- metal coil, which necessitate Calibration of the ther- tion throughout the product.

6 For instance, to measure mometer before reuse. the internal temperature of a hamburger patty Bi-metal coil Thermometer Calibration method 1. 8 inch (3 mm) thick would require using an instru- Calibration for use in hot processes and products ment having a thin probe that can detect temperature (boiling water method): quickly and accurately. Measuring the internal tem- perature of a bone-in ham could be done adequately using a thick probe having a 2- to 3-inch ( to cm) The items needed for calibrating thermometers used in temperature sensing region. The product type or the hot processes are: environment to be monitored will influence the One pint (500 ml) PyrexTM or heat tolerant con- selection of the most effective temperature monitoring tainer instrument to use. Distilled water Heat source such as a hot plate Reference Thermometers NIST reference Thermometer Laboratory and industry supply catalogs market NIST reference thermometers.

7 Local county extension agents or high school science teachers may have these Once these supplies are assembled, the following catalogs. A temperature monitoring device such as a steps are used to calibrate bi-metal coil thermometers mercury-in-glass Thermometer or a thermistor can be for use in hot processes. used as a reference Thermometer as long as the device 1. Heat distilled water to a specific reference tempera- is NIST certified and the annual Calibration and ture. This reference temperature can be 212 F. certification is maintained through an NIST laboratory. (100 C) described as a rolling boil at sea level altitude. The boiling point of water changes with Bi-metal Coil Thermometer altitude. To find the correct boiling point for your Common types of bi-metal coil thermometers are area, refer to Figure 1, Boiling point of water with dial and instant read digital thermometers.

8 Contained relationship to altitude. An alternative reference within the stem of the Thermometer is a coil made of temperature for example is the target end tempera- two different metals bonded together to a temperature ture of a product. Specifically, if the intended use of indicator. This type of Thermometer detects tempera- the Thermometer is to measure the final internal tures from the tip of the stem to a point 2 to 21 2 inches temperature of a cooked uncured turkey roll at the ( to cm) above the tip. An indentation in the end of thermal processing, then a target tempera- ture of 160 F (71 C) should be used for the reference temperature. Advantages of bi-metal coil Disadvantages of bi-metal 2. Once the distilled water has reached thermometers thermometers the reference temperature, place the Inexpensive, readily available Temperature averaged over 2- Thermometer to be calibrated in the Easy to calibrate inch ( cm) area of probe hot water bath.

9 Immerse the stem to a Used for final end point (sensing region) minimum depth of 21 2 inches ( cm). temperature check Lose Calibration with physical Place the NIST reference Thermometer shock in the water bath at the same time, to Cannot be used over a wide the same depth, as the bimetal coil range of temperature Thermometer . Make sure that neither measurement Thermometer comes in direct contact Variable product temperature with the bottom of the heated con- in sensing region affects tainer or each other. measurement. 2. Figure 1. Boiling point of water with relationship to altitude. From the Complete Guide to Home Canning, Agriculture Information Bulletin No. 539, USDA. Reviewed 1994. 3. After one minute, compare the bi-metal coil ther- on the back of the faceplate with a number or letter mometer reading to the NIST reference thermom- indicating the temperature range of its intended use.

10 Eter. Record the readings of the two instruments on Thermometer Calibration for use in cold process and a record-keeping form such as a Thermometer products (ice water method). Calibration log. A similar Calibration procedure can be used for 4. Correct the indicator needle of the bi-metal coil thermometers intended to be used in cold processes Thermometer . Adjust the spring on the coil by and products. turning the hex nut directly behind the faceplate of the Thermometer . Digital instant read thermometers may have a Calibration button to adjust the tem- Materials necessary to calibrate a Thermometer for cold perature reading. If the Thermometer cannot be uses are: physically calibrated and the accuracy of the unit is Sturdy 1 pint (500 ml) container, more than 2 F ( C) then the unit should not be Distilled water used.