WHAT EVERY CONSUMER NEEDS TO KNOW ABOUT FOOD THERMOMETERS
 
Copyright 1997
O. Peter Snyder, Jr., Ph.D.
Hospitality Institute of Technology and Management
St. Paul, Minnesota
DRAFT September 29, 1997

INTRODUCTION

This paper describes the general characteristics of thermometers that consumers and retail food industry personnel commonly use to measure food temperatures. Each thermometer has its own particular food use, as described. For each type of thermometer, the specific food temperature range, time to respond, and other measuring characteristics are listed.
There is one characteristic that is common to all of these thermometers. There will be a sensing zone at the tip, which can be the flat end of the temperature probe, a bulb that might be 1/2 inch long, or a bimetallic coil that might be 2 1/2 inches long. In all cases, the temperature-measuring zone must be inserted into the food at least 5 times, and preferably 10 times, the diameter of the thermometer (probe). If it is not inserted far enough into the food, the part of the thermometer probe that is not in the food could cool by heat loss up the stem of the thermometer.
This means that if a thermometer probe is 0.040 inch in diameter, it must be inserted at least 0.2 inch into the food and preferably, 0.4 inch into the food. If the probe is 1/8 inch in diameter, which is quite common, the thermometer should be inserted 1 1/4 inches beyond the temperature-measuring zone in order to minimize the effects of the upper part of the stem on the temperature-measuring zone.
Depending on the measuring system, the error can be quite significant. For example, in food such as a hamburger, where the temperature might be 140F, if a thermometer with a 1/8-inch stem is inserted only 1/2 inch, there could be a 20F variation in the temperature reading vs. the actual temperature because of the cooling effects of the uninserted portion of the probe. The thermometer would read 120ºF for this 140ºF hamburger. Please take into consideration the proper insertion of all of these thermometers when evaluating which device is right for you.

GLASS THERMOMETERS

Glass thermometers have been available for a long time. They are accurate and economical. The most common products are filled with mercury and nitrogen gas above the mercury. There are also products that use spirits instead of the mercury. This is less dangerous, because it is non-toxic in case of glass breakage.
Some of the applications for these thermometers include laboratories, dishwashers (used as a maximum registering), and fever thermometers. The accuracy is typically ± 1 scale division. Calibration is accomplished by returning the thermometer to a standards lab for verification against a NIST thermometer.
Spirit-filled types of glass thermometers are used as meat (+/-4% accuracy) and candy / deep-fry thermometers (+/-10F accuracy) as well as other miscellaneous applications. There is no calibration.
The temperature-sensing portion of the thermometer is the bulb at the end. It measures the temperature of the volume surrounding the bulb. In a stirred liquid, it can be a very accurate measurement. In a viscous food, one does not know if the hottest or coldest spot in the food is actually measured.

 
MELTING-POINT INDICATOR, NON-REVERSIBLE TEMPERATURE MONITORS
Melting-point-type temperature monitor. A thin cardboard "stick" with a temperature-sensitive melting dot. To be used after cooking food to see if a desired temperature has been reached. (Trans World Services, Inc. is the only known current manufacturer.)

Food temperature measurement range and accuracy: type 1) 160F+/-1F; type 2) 142 to 144F+/-1F
Time to respond to food temperature: 5 seconds
Size: 2 1/2 inches long x 5/8 inch wide x 1/32 inch thick
Sensor: Irreversible, temperature-sensitive dot that melts and changes from white to black
Calibration: Preset
Use: When food is believed to be at 160F or 142 to 144F, depending on which type of unit is used, push the specific cardboard temperature indicator into the food so that the dot is in the middle (coldest spot) of the food. Wait 5 seconds. Pull out. If the food temperature is greater than the melting point of the dot, the dot changes color. If not, continue to cook. The cardboard thermometer can be used a second time if it has not changed color in the same food when the food has cooked a little longer.

BIMETALLIC, METAL STEM, COIL THERMOMETERS
Metal sheath bimetallic coil meat thermometer. Used to monitor the temperature of meat cooking in an oven by easy reading through the oven window.

Food temperature measurement range: 130 to 185F with 2F increments

Response time for a change from 75 to 150F when put in food: 30 seconds
Accuracy: +/-2F
Resolution: 2F
Dial diameter: 1 inch (or larger)
Stem lengths: 3 and 5 inches
Stem diameter: 0.250 inch
Sensor: Bimetallic coil
Length (wound): 1.2 inch + 0.750 inch plug length, to total approximately 2 inches.
Measurement zone: Will give an average temperature of the food around the thermometer from the tip 2 inches up the stem.
Calibration: Factory set
Use: Put into a food, wait, and take a reading. However, it is difficult to know if the temperature of the spot or area of the food is the hottest / coldest part of the food. This is due to the slow response of the unit. There is no time to do multiple probes of the food.

 Metal sheath bimetallic coil candy / jelly / deep-fry thermometer. Used to monitor the temperature of oil in deep fryers and also the endpoint cooking temperature of jellies and candy.

Food temperature measurement range: 200 to 400F with 5F increments
Response time for a change from 75 to 350F when put in food: 2 minutes. (Some manufacturers now claim that new, thinner-stemmed models a have much faster response time.)
Accuracy: +/-5F
Resolution: 5F
Dial diameter: 2 inches
Stem length: 5 inches
Stem diameter: 0.250 inch
Sensor: Bimetallic coil
Length (wound): 0.83 inch + 0.47 inch plug length, to total approximately 1.3 inches.
Measurement zone: Will give an average temperature of the liquid from the tip 1.5 inches up the stem.
Calibration: Factory set
Use: Put into the liquid, wait, and take a reading. The unit measures the temperature of the liquid surrounding the bimetallic coil. The liquid must be stirred to assure that the measurement is accurate.

Pocket, 1-inch dial, plastic-faced, bimetallic coil thermometer with sheath and clip. Used for many applications including food service and heating, ventilation, air conditioning, and refrigeration maintenance. Fits in a shirt pocket.

Food temperature measurement range: 0 to 220F with 2F increments (comes in other ranges)
Response time for a change from 75 to 150F in water: 25 seconds (some new models may be faster)
Response time for a change from 75 to 35F in air: 1 minute 20 seconds
Accuracy: +/-2F
Resolution: 2F
Dial diameter: 1 inch (or larger if desired)
Stem length: 5 inches
Stem diameter: 0.140 inch
Sensor: Bimetallic coil
Length (wound): 1.250 inches (or longer) + clearance for the coil at both ends
Measurement zone: If there is a dimple about 2 inches up the stem from the tip, the minimum submersion length is to the dimple. If there is no dimple, the minimum submersion length of the stem is 3 inches. The measured temperature is the average between the hottest and coldest spots of the food surrounding the sensing area from the tip to the dimple or 3 inches up the stem.
Calibration: Should be checked for calibration each day and each time it is dropped. If it fogs up inside the face, throw it away, because the hermetic seal is poor. Calibrate in a slush ice bath. This is crushed ice with just enough water to come to the top of the ice. Do not use boiling water; it is not accurate. Immerse the stem so that the thermometer measurement zone is completely surrounded by slush ice. Wait 30 seconds. With a wrench, hold the nut on the back of the dial. Twist the dial so that the needle points to 32F on the dial. Be accurate. If the head of the thermometer is at all loose, throw it away. The thermometer will not stay calibrated. Check the thermometer in hot water to see that there is a reasonable reading. Sometimes, the bimetallic coil corrodes in the stem. It is assumed that if the thermometer is calibrated at 32F, it is accurate at 150F.
Use: Put into a food, wait, and take a reading of the average temperature of the food. It is impossible to know if the temperature of the spot or area of the food is the hottest / coldest part of the food, because the response time is so slow, and one does not have time to do multiple probes.

THERMISTOR THERMOMETERS
Pocket thermistor thermometer. Lightweight, small, 5 to 8 inches long x 1 inch wide x approximately 3/4 inch thick. Fits into a shirt pocket. Powered by a 1.5-volt replaceable photo-electronic battery with an average 1-year operating life.

Food temperature measurement range: -40 to 300F (500F in some models)
Response time for a change from 75 to 150ºF in water: approximately 12 seconds for a unit with a 0.140-inch stem diameter
Response time for a change from 75 to 35F in air: approximately 1 minute 20 seconds
Accuracy: +/-2F from 32 to 212F
Resolution: 0.1F
Sensor: Ceramic bead resistor in the tip, potted in a high-thermal-conductivity epoxy.
Measurement zone: The tip must be immersed 0.5 inch into the food to compensate for thermal conductivity down the stem to the tip. The measuring volume is the food, about 1/4 inch diameter, around the tip.
Calibration: Inexpensive units usually cannot be recalibrated. These units can be checked in slush ice to see if they either work or do not work. Some manufacturers are now offering more expensive thermistor-type pocket thermometers that can be recalibrated. If used to measure elevated temperatures such as hot oil, the sensor calibration can become inaccurate sooner.
Use: Put into a food, wait, and take a reading. However, it is difficult to know if the temperature of the spot or area of the food is the hottest / coldest part of the food, because the response time is so slow, and one does not have time to do multiple probes.

Table thermistor thermometer. The meter is mounted in a small box that sits on a table. The sensor is in a separate probe with a 3-to-4-foot cord. Depending on the cord material, the probe can be put into food in an oven, and the meter can be used to measure the temperature of food as it cooks. This unit is normally powered by a 9-volt battery. The operating characteristics of the sensor are the same as the pocket thermistor thermometer.

Use: Put into a food, wait, and take a reading. However, it is difficult to know if the temperature of the spot or area of the food is the hottest / coldest part of the food because of its slow response. It is quite satisfactory for many measurements in a food market where food has been at a temperature for a number of hours, and the mass has had time to come to a uniform temperature..

THERMOCOUPLE THERMOMETERS

Hand-held, pocket thermocouple thermometers are bigger than other food thermometers, at least 3/4 inch x 2 1/2 inches x 5 inches. They are powered normally by a 9-volt transistor battery with an operating life of 200 to 2,000 hours, depending on the electronics. (NOTE: There are also table meters, 110-volt meters, and many different types of probes. The following specification will only be for the hand-held, pocket thermocouple thermometer.)

Meter
Temperature range: Industrial meters can range from -350 to 2,200F. Food meters normally range from -40 to +400F. The best meter accuracy is perhaps +/-0.5F. Less expensive meters can be +/-2F.
When the probe variation is added in, the accuracy can drop to +|/-1F or worse
Resolution: 0.1F or 1.0F, depending on selection
Meter update time: Approximately 2.5 times per second

Probe

Probes can plug into / be attached to the meter directly or can be at the end of a separate 3-to-4-foot plug-in cord. If the cord is made with a high-temperature covering, the probe can be inserted into, and left in, food cooking in an oven.
Probes are normally 4 to 5 inches long. The critical part is the probe tip diameter. The smaller the diameter of the tip, the faster the response. The thermocouple wires that measure temperature are usually welded at the tip of the probe in what is called a grounded junction. While rugged probes can be 0.125 inch in diameter, for example, to measure frozen food, they are slow because of the mass of the tip. The probe tip should be no larger than 0.062 (1/16) inch in diameter. The preferred probe tip for response is 0.040 inch. Probe tips can be as small as 0.010 inch, but these are very delicate.

0.062-inch probe tip response time from 75 to 150F in water: 1.2 seconds
0.062-inch probe tip response time from 75 to 35F in air: 30 seconds
0.040-inch probe tip response time from 75 to 150F in water: 0.7 second
0.040-inch probe tip response time from 75 to 35F in air: 20 seconds

Use: This is the only thermometer capable of finding cold spots / hot spots in food. To measure the cold spots / hot spots, insert the probe directly into the food and push through the food. A typical 4-ounce hamburger takes less than 3 seconds to measure. The meter reading goes from hot, perhaps 175F, down to the cold center temperature, perhaps 145F, as the probe tip is pushed through the food, and then immediately begins to rise as it goes out of the cold center. It is possible to measure cold spot temperatures with an accuracy of +/-1F.

RTD THERMOMETERS

RTD thermometers can be very accurate, but the probes are big, and response is slow. Because of these limitations, they are not discussed. To obtain further information, contact the author.

INFRARED THERMOMETERS

Hand-held, portable infrared thermometers measure the surface temperature of food and packages of food without contact by measuring the intensity of the infrared ray emitted from the surface. These thermometers are bigger than most traditional thermometers. They are usually about 5 inches x 2 inches x 8 inches, although some units are slightly smaller. They are normally powered by a 9-volt battery with a typical operating life of 20 to 50 hours.
The temperature range of IR thermometers can be from -50 to 5,400F (-46 to 3,000C). The IR thermometers used in food applications normally range from -25 to 750F (-32 to 400C). The accuracy of infrared thermometers varies from +/-0.5% of the reading, or +/- -1C (1.5F) to 2% of the reading, or 2C (3F), whichever is greater. The resolution of infrared thermometers varies from 1F or C to 0.1F or C, depending on the unit. The typical response time is from 1/4 second to 1/2 second. In general, the more expensive infrared thermometers have better accuracy, resolution, and response times.
Because IR thermometers only measure surface temperature, they should only be used to survey the surface temperature of food products in refrigerators, freezers, etc. They must be used with caution with hot food above 120F, because there is normally evaporative cooling at the surface of the food, and the surface can be 10 to 30F colder than 1/4 inch below the surface. The reading is also very sensitive to the type of surface. Metal surfaces emit energy much differently than food surfaces and printed plastics. The normal IR monitor is typically set to measure a surface with an emissivity of 0.95, which is appropriate for food and most food plastic films.
They are very useful in food markets where all refrigerated / frozen food can be checked in a few minutes. These units are very fast, providing a reading in less than 1 second. Also, they do not waste any food, because there is no probe to penetrate the food. The one assumption made in these situations is that the center and surface temperatures of the food are the same, which is typically true in food markets, where the food has been in the cooler / freezer for a few hours.
Applications in food markets include: inspecting incoming products for proper shipping temperatures, checking for proper storage temperatures in freezers and refrigerators, checking food held in warming equipment, and checking the temperature of liquids and beverages. Because the surface emissivity changes, one cannot accurately measure metal surfaces with simple IR thermometers. Whenever one has doubt about the reading of the IR thermometer, one can check the temperature with a micro-tipped thermocouple just below the surface of the food. In a restaurant, the IR has less application, because much of the food is unwrapped, and food temperatures normally can be measured with a thermistor or thermocouple.
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