• Home
• »
• Articles
• »
• Guide to Temperature Measuring Instrumentation

Guide to Temperature Measuring Instrumentation

There are several technologies readily available to accurately measure temperature. Those technologies are: infrared, bimetal, glass column, thermistor, and thermocouple thermometers. Each type has its advantages and disadvantages. Selection of the appropriate thermometer will depend upon the required measuring range and accuracy. Cost and durability of the instrument may also be critical selection factors.

IR (Infrared) Thermometer

The infrared thermometer reads the infrared energy that all objects emit. Molecular activity increases as temperature increases—emitting infrared energy. This type of thermometer has optics that collect and focus the infrared energy onto a detector, which coverts the infrared energy into an electrical signal.

The response time (detection to display) of an IR thermometer is typically on the order of 0.5 seconds. Maximum measuring distance is a determined by the quality of the internal optics and atmospheric conditions. Humidity (H2O) and carbon dioxide (CO2) decrease thermometer performance by shortening the effective range of the instrument. Discounting effects of H2O and CO2, most IR thermometers have a maximum measuring distance of 100 feet.

Internal optics affects thermometer performance (effective measuring range). The distance to spot ratio is noted for IR thermometers in Lab Safety Supply catalogs. This ratio is the size of the temperature measuring diameter (spot) to the distance from the object being measured. For example if a thermometer has a distance to spot ratio of 10:1, the measuring diameter will be one inch when the thermometer is 10 inches from the measured object. If you wanted to measure the temperature of an area 6 inches—you would be able to measure from a distance of more than 60 inches from the object. If measuring from a greater distance, the thermometer would be displaying the temperature of the object and surrounding material, since it displays the average temperature of objects/material in the measuring diameter.

The advantages of IR thermometers are fast accurate response times and convenience, ideal for remote monitoring. The non-contact feature of IR thermometers allows for temperature measurements to be taken without touching the product being tested. This allows for safety of the worker as well as limiting contamination to the product being measured.

An IR thermometer can only measure the surface temperature of an object and not the internal temperature. Because the maximum measuring range and accuracy can be affected by atmospheric conditions (water vapor or CO2), the maximum range (generally) is limited to approximately 100 feet.

Bimetal Thermometer

Coil spring technology is used in Bimetal dial thermometers. The temperature sensing device is composed of two different metals that are fastened or welded together into a helical coil. One of the metals has low temperature sensitivity, while the other metal has sensitivity to high temperature. This sensor combination is connected to the pointer/needle indicator on the thermometer face.

When taking a measurement, the metals respond in a different way which causes the metal sensing coil to curl or uncurl—this action moves the pointer/needle across the face of the instrument.

Bimetal thermometers are easy to use, can be recalibrated at low cost, and do not need a power source (batteries) to operate the thermometer.

This thermometer needs to be inserted past the top of the coil and the response time (speed of display) is not as fast as other types of temperature-measuring devices.

Glass/Plastic Column Thermometer

As originally invented by the physicist Daniel Fahrenheit, a glass column type of thermometer consists of a mercury-filled tube that operates on the principles of expansion and contraction. The thermometer (with calibration marks) is read by comparing the height of the mercury (or other fluid) to the calibration marks. The use of a bulb that contains most of the indicator fluid increases the sensitivity of the instrument. As the indicator fluid expands or contracts (as a response to temperature) the effect is amplified as the indicator flows from the bulb into the narrower tube. Above the level of the indicator, the thermometer is filled with nitrogen or it may contain a vacuum (air at less then atmospheric pressure). The glass tube can be calibrated to the Fahrenheit or Celsius scales (or both). Mercury-filled thermometers cannot be used to measure colder temperatures since the freezing point of mercury is -38°F.

Since many countries, facilities and have banned mercury, the indicator in many current column type thermometers is alcohol (ethanol, toluene) or other type of fluid. Mercury filled thermometers are still used by Meteorologists, however. Glass column thermometers can be very inexpensive, or very expensive—the cost will depend upon quality of the glass and the indicator fluid.

Most glass/plastic column thermometers are cost effective (low cost), relatively easy to calibrate, and very easy to use. The disadvantages are that glass is breakable (use a thermometer with a plastic column) and the measuring fluid can become separated. The fluid column can be recombined using cooling/heating methodology.

Digital (Thermistor) Thermometer

This type of thermometer technology uses a "thermistor" to measure temperature. The term thermistor is a combination of the terms thermal and resistor. A thermistor is a small semiconductor with electrical resistance proportional to temperature, i.e. resistance changes as temperature changes. Resistance is measured by passing small direct current through the semiconductor and measuring the voltage drop. Thermistor thermometers are compact, durable and accurate.

There are two types of thermistor thermometer technologies. They include positive temperature coefficient of resistance (PTC) and negative temperature coefficient of resistance (NTC). NTC's have temperatures that inversely vary with resistance—resistance decreases as temperature increases. NTC-type thermistors are quite common. For PTC's, resistance increases as temperature increases (and resistance decreases as temperature decreases).

The advantages of a thermistor thermometer include accuracy, durability and compact size. Generally, they are less costly than a thermocouple type thermometer. However, the temperature response/display time is not as fast as other types of thermometers and they should not be used to measure extreme temperatures. Depending upon the design, some thermisters have a measuring range of about 0 to 100°C.

Digital (Thermocouple) Thermometer

A thermocouple is a thermoelectric device used to measure temperature by generating electricity when the tip of the probe encounters heat. The thermocouple is composed of two, different types of wires, joined at one end. The wires are connected to a device that measures voltage. There is a potential difference generated between the wires. As the wires are cooled or heated, the voltage generated by the heating or cooling is correlated to temperature.

Thermocouple thermometers feature fast response times, high degree of accuracy and are capable of being used with a wide variety (type and temperature range) of probes. The most-common types of thermocouple probes are designated by letters, corresponding to the designed measuring ranges.Since thermocouple digital thermometers feature fast response times and are capable of being used with a wide variety of probes, this type of thermometer is frequently used in industrial and food service applications.

The advantages of thermocouple thermometers are fast response time and accuracy. Due to the available probes they can be used in high-temperature applications.

However, thermocouple thermometers (and probes) are more expensive than bimetal or thermistor digital thermometers.

Commonly Asked Questions

Please Note: The information contained in this publication is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the cited regulation or consult with an attorney.