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Temperature Measurement Devices

Introduction

The measurement of the relative warmth or coolness of an object, or an object's environment, is key to the successful operation of many industrial or scientific processes, devices and systems. To determine an accurate measurement of temperature, there are several choices of instruments which utilize different types of temperature scales. This document will provide some definitions and descriptions that may help you better evaluate the best device and scale for your temperature measurement needs.

Temperature Scales

Fahrenheit: In 1714 German physicist, Gabriel Fahrenheit, created a thermometer with mercury inside of a glass tube. Fahrenheit chose mercury because it remained liquid over a wide range of temperatures, expanded linearly to changes in temperature, and did not adhere to glass. This type of thermometer was initially calibrated in a mixture of ice, water, and sea salt which was designated as zero. An ice and water mixture was designated as 30. Using these points as base points, the boiling point of water was measured at 212. To aid in the ease of use, the freezing point of water was adjusted to 32 so that an even 180 separated the two points.

Celsius: Swedish astronomer, Anders Celsius, created a scale in 1742 which designated the number 100 as the freezing point of water and zero as its boiling point. Carolus Linneaus created a reverse scale in which zero designated the freezing point of water and 100 designated the boiling point. Both centigrade scales had 100 divisions between the two points. In 1948 a new scale was created based on the triple point of water. The temperature in which water, water vapor and ice coexist (0.01 degrees C). The second point of the scale is set at the boiling point of water at standard atmospheric pressure (99.975 degrees C). Each degree Celsius compares directly to a degree on the ideal gas scale. To convert from Fahrenheit to Celsius, use the following formula: F=1.8(degreesC)+32.

Kelvin: The Kelvin scale is based, not on the physical properties of water but on the properties of an ideal gas. More specifically, all gases which can be represented by the formula, PV=constant(T). Where P=pressure, V=volume, T=temperature. The zero on this temperature scale is the point at which the pressure of an ideal gas is zero. The other point is designated as the triple point of water which is set at 273.16. In order to convert from Celsius to Kelvin the following formula can be used: K=(degrees C) + 273.

Other Temperature Measuring Innovations

William Siemens created a thermometer based on the resistance of a metal instead of a gas or water. Platinum provides a fairly linear change in resistance over a wide range of temperature from -260 degrees C to 1235 degrees C. Platinum has the added benefit of not oxidizing at high temperatures.

T. Seebeck invented the thermocouple thermometer. When wires of different metals are connected at one end and as the temperature is increased, electrical current will flow from one end to the other. The current or amperage generated can be quantitatively measured and directly related to temperature. Nickel-chromium and nickel-aluminum, platinum and platinum/rhodium are examples of metal pairs used.

Commonly Used Thermometers

Liquid-Filled Thermometers: There are several types of liquid filled glass thermometers. Mercury, spirit-filled, or biodegradable liquids are the most common. They can be produced in a variety of accuracy levels depending on the needs of the end-user. They come in two basic formats: partial immersion and total immersion.

Partial immersion thermometers are designed for areas that have limited space including small samples, laboratory ovens and shallow baths. Since only a small portion of the thermometer is immersed, it is subject to outside temperature factors. Total immersion thermometers are inserted into the entire measurable portion of the object or material where it is not affected by external sources. Total immersion thermometers are generally more accurate but do require more space to obtain readings. Various temperature ranges are available.

Application of liquid-filled thermometers is limited to laboratory, medical, or other uses that allow the instrument to be handled delicately. Glass thermometers are fragile and can be broken easily. Many manufacturers have designed protective covers to help prevent damage. These thermometers are relatively low in cost.

Dial Thermometers: Constructed of corrosion resistant metal or plastic the dial thermometer is a durable alternative to glass thermometers which readily break when dropped. Dial thermometers have a probe that is inserted into the material being measured with a dial that is read from above.

These devices measure temperature with bi-metal elements or coils which have two different metal strips that expand and contract at different rates. Various temperature ranges are available.

Applications for dial thermometers include situations that require a more durable, rugged temperature measuring device. The dial face of the instrument is very easy to read even while the device is inserted into a liquid or mounted to a surface. These thermometers are also relatively low in cost.

Thermocouple Thermometers: These electronic thermometers can read in various temperature ranges depending on the type of thermocouple probe that is used. Most common probes are specified by a letter designation such as J, K, T or E depending on the temperature range they can read. Various shaped probes are also available for measuring surfaces, penetrating materials, rollers, and air/gases temperature. Many of these thermometers are capable of storing data, converting between temperature scales, averaging readings, sounding alarms or providing printouts of data.

Applications suitable for thermocouple thermometers are many, due to the versatility of the instrument. They can be used to measure various mediums such as liquids, gases and solids of various dimensions. Thermocouples can be purchased with multiple probes that have different temperature and accuracy ranges. The devices are fairly rugged and have easy to read liquid crystal displays. These thermometers are mid-ranged priced instruments.

Infrared Thermometers: These electronic thermometers can read temperatures without coming in contact with an object. Infrared thermometers collect energy that has been transmitted, reflected or emitted from an object and focuses it on a detector which converts the energy to a specific temperature. Infrared thermometers can be obtained for different temperature ranges and scales. Distance plays a factor on the size of object being measured. At close ranges a small area less than an inch can be measured. As the distance increases the measurement area also proportionately increases. Thermometers are in the mid- to high-priced range depending on the model.

Application of the infrared thermometers is especially suited for moving objects, hard to reach objects, hazardous materials, or to avoid contamination. They have easy to read liquid crystal displays and their lightweight portable design makes them especially suited to maintenance applications.

Commonly Asked Questions

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