_thumb.jpg)
Guide to Thermometers
Knowing the temperature of an object or environment can be crucial in many professions. There are many different types of thermometers with various designs and levels of accuracy. Check out PE Facts to find out which thermometer is perfect for your needs!
Knowing the relative warmth or coolness of an object or environment is very important information in many industrial or scientific processes, devices and systems. To determine an accurate measurement of temperature, there are numerous instruments to choose from, each with various 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: Gabriel Fahrenheit, a German physicist, invented a thermometer with mercury inside of a glass tube in 1714. He chose mercury because it remains liquid over a wide range of temperatures, reacts to changing temperatures by expanding linearly, and does not stick to glass. This type of thermometer was initially calibrated to zero in a mixture of ice, water and sea salt. An ice and water mixture was designated at 30. Referring to these two points, the boiling point was found to be at 212. To make it easier, the freezing point of water was changed to 32 so that an even 180 degrees separated the two points.
Celsius: Anders Celsius, a Swedish astronomer, established a scale which designated 100 as the freezing point and 0 as the boiling point in 1742. Carolus Linneaus created an opposite scale in which zero is the freezing point and 100 is the boiling point. Both scales have 100 degrees between boiling and freezing points. A new scale was created in 1948 based on the triple point of water; the temperature at 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 relates directly to a degree on the ideal gas scale. The formula to convert from Fahrenheit to Celsius is: F=1.8(degrees C) +32.
Kelvin: The Kelvin scale is based on the properties of an ideal gas rather than the physical properties of water. 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. The formula to convert from Celsius to Kelvin is: K= (degrees C) + 273.
Other Temperature Measuring Innovations
William Siemens made a thermometer based on the resistance of a metal instead of a gas or water. Platinum provides a somewhat linear change in resistance over a wide range of temperature from -260 degrees C to 1235 degrees C. Platinum has the additional quality of not oxidizing at high temperatures.
T. Seebeck created the thermocouple thermometer. When wires of different metals are connected at one end and the temperature is increased, electrical current will flow from one to the other. The current or amperage produced can be quantitatively measured and directly correlated to the temperature. Nickel-chromium and nickel-aluminum, platinum and platinum/rhodium are examples of metal pairs used.
Liquid-Filled Thermometers: There are various types of liquid filled thermometers. The most commonly used are Mercury, spirit-filled, or biodegradable liquids. They can have various levels of accuracy, depending on the needs of the user. They come in two general formats: partial immersion and total immersion.
Partial immersion thermometers are intended for areas with limited space such as laboratory ovens or shallow baths. Since only a small part of the thermometer is immersed, it may be affected by the outside temperature. Total immersion thermometers are completely inserted into the material being measured, so it is not affected by external factors. Total immersion thermometers are typically more precise but they require more space to obtain readings. Various temperature ranges are available.
The use of liquid filled thermometers is restricted to situations that allow the instrument to be handled carefully such as in a laboratory or for medical uses. Glass thermometers are quite fragile. Many come with protective covers to help avoid breaking. These thermometers are reasonably low in cost.
Dial Thermometers: Dial thermometers can be made of corrosion resistant metal or plastic, making them durable alternatives to glass thermometers. Dial thermometers have a probe that is inserted into the material being measured with a dial that is read from above. These instruments measure the temperature with bi-metal elements or coils which have two different metal pieces that expand and contract at different rates. They are available in various temperature ranges.
Dial thermometers can be used in situations that require a more durable measuring device. The dial face is very easy to read, even while the instrument is inserted in a liquid or mounted on a surface. These thermometers are also fairly low in cost.
Thermocouple Thermometers: These electronic thermometers can read in a variety of temperature ranges depending on the type of thermocouple probe that is used. Depending on the range it can read, it is classified with a letter such as J, K, T or E. They are also available in various shapes to help measure air/gas temperature, rollers, and penetrate materials. These thermometers can store data, convert between different temperature scales, have sounding alarms, can average readings, and even print out data.
There are many uses for thermocouple thermometers due to its versatility. They can measure gases, liquids and even solids of various dimensions. Thermocouples can be purchased with multiple probes that provide various levels of accuracy and temperature ranges. These instruments are somewhat rugged and have easy to read liquid crystal displays. These thermometers are moderately priced.
Infrared Thermometers: Infrared thermometers are electronic and do not even have to come into contact with an object to measure its temperature. These thermometers collect the energy being reflected, transmitted, or emitted from an object and concentrate it on a detector that then converts it to a specific temperature. They come in various temperature ranges and scales. The size of the object being measured affects the distance at which the temperature can be read. When close to an object, an area as small as an inch can be measured. As the distance increases, the measurement area proportionally increases. These thermometers are in the mid- to high-priced range depending on the model.
Infrared thermometers are especially suited for objects in motion, hard to reach objects, hazardous materials, or to avoid contamination. They have easy to read liquid crystal displays and are portable due to their lightweight design.
Commonly Asked Questions
| Q. | Can thermometers be calibrated to meet the National Institute of Standards and Technology (NIST) accuracy standards? |
| A. | Yes, most types of thermometers have versions that can be traceable to NIST standards; however, there may be additional charges for these items. You must request the NIST certificate at the time of purchase. |
| Q. | Do spirit-filled or biodegradable liquid thermometers have advantages over mercury thermometers? |
| A. | Yes. The main advantage is avoiding mercury related hazards. Mercury is considered a hazardous waste and can be released if the thermometer is broken. It must be transported and disposed of according to local, state, and federal guidelines. Spirit-filled and biodegradable thermometers are considered non-hazardous and can be easily disposed of if broken. |
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.
