General purpose thermometers

General Purpose Thermometers. Image Credit: omega.com

Laboratory thermometers are used to measure temperatures or temperature changes with a high degree of precision. They are made of metal or glass and strengthened through thermal tempering or annealing.

 

How Laboratory Thermometers work

The way a laboratory thermometer works depends upon its type. They are generally a liquid-in-glass device, a bimetallic strip, an electronic thermistor thermometer, or infrared (IR) device.

 

Selection Criteria for Laboratory Thermometers

The GlobalSpec SpecSearch database allows industrial buys to select laboratory thermometers by type, display options, operating environment, and specialty features.

 

Type

Several types of laboratory thermometers are commonly available including;

  • Liquid-in-glass thermometers are made of sealed glass and contain a fluid, usually mercury or red alcohol, whose volume changes relative to its temperature. The liquid expands as the temperature rises, rising in the tube and indicating the temperature.
    • When choosing a liquid or gas-filled thermometer, there are several things to consider, including:
  • Material in tube - gas or liquid
  • Material of tube - glass or metal
  • Level of liquid or gas in the tube
  • Bimetallic strip thermometers include two different metals that are bonded together and expand at different rates as they warm up. Since the two metals expand to different lengths, the bimetallic strip is forced to bend or curl towards the side with a lower coefficient of thermal expansion. The movement of the strip is used to deflect a pointer over a calibrated scale which then indicate temperature to the user. Often, long bimetallic strips are wound into a coil and used with a dial.
    • Advantages include: Low cost, tough, easy to use and install, accurate over a wide range of temperatures
    • Disadvantages include: only indicating types are available, possibility of calibration change due to use or environment, and not as accurate as glass stem thermometers.

Bimetal Coil

Bimetal coil reacts to light. Image Credit: Hustvedt

  • Electronic devices such as thermistors measure changes in electrical resistance and convert them to changes in temperature.
  • Infrared thermometers are non-contact devices that convert infrared (IR) energy to an electrical signal that can be displayed in units of temperature.

Infrared Thermometer

Infrared Thermometers. Image Credit: Clark Solutions

Display Options

Thermometers have two basic display options:

Graduated scalar divisions or test points determine the accuracy with which users can read each type of device. Typically, display numbers are etched or printed on the thermometer's housing. With some devices, permanent pigmentation of graduations is used to improve readability.

Features

  • Clip-on magnifiers help users read numbers on more detailed scales.
  • Immersion
    • Full immersion thermometers can be left in the substance which is being measured.
    • Partial immersion thermometers must be held or suspended over the substance for an accurate reading
  • Sheath Material
    • Polytetrafluoroethylene (PTFE) are suitable for monitoring acids or other hazardous materials.
    • Protective metal sheath
  • No-roll cap to prevent the thermometer from rolling on the counter. Many also come with a hook so the thermometer can be suspended over the material being measured.

 

Laboratory Thermometer

Laboratory Thermometer. Image Credit:thelabwarehouse

Applications

Laboratory thermometers are used in research and scientific applications such as monitoring experiments, maintaining a sterile work environment, calibrating other laboratory instruments, and testing materials.

Calibration

Organizations such as the National Institute of Standards and Technology (NIST) and the American Society of Testing and Materials (ASTM) calibrate laboratory thermometers and provide traceability.NIST provides calibration services for a variety of laboratory thermometers that cover a temperature range from -196° C to 550° C (-321° F to 1022° F). Submitted devices must pass a preliminary examination for fineness and uniformity of graduation as well as freedom from moisture, gas bubbles, and cracks. The thermometers to be calibrated are placed in a constant temperature bath along with an NIST-calibrated standard platinum resistance thermometer (SPRT) that is traceable to the International Temperature Scale of 1990 (ITS-90). Special tests may also be conducted on industrial-grade platinum resistance thermometers, digital thermometers, and thermistors. ASTM-certified laboratory thermometers must also pass rigorous testing requirements.