Despite the move to more digital-based temperature indicating systems, Liquid-in-Glass Thermometers are still one of the most used types of temperature instruments to measure the temperature of fluids in many industries. Due to their unique design, they have excellent stability, reliability and response times for indicating a wide range of temperatures.
Physically, a liquid-in-glass thermometer consists of a glass tube with a bulb at one end that contains a thermometric liquid. The upper portion of the tube is filled with an inert gas to help prevent separation of the liquid in the tube and the opposite end is sealed. Scale markings are etched into the side of the tube to indicate various temperatures.
The glass thermometer operates by the principle that the volume of the liquid in the thermometer changes based on the temperature causing the liquid to expand or contract within the glass tube. The level of the liquid in the column corresponds to the specific temperature marked on the glass tube.
For many years, the liquid used in glass thermometers was mercury. Mercury has excellent thermometric properties and is very repeatable and accurate. Recently, bans on mercury due to the concerns over its toxicity have caused the industry to manufacture liquid-in-glass thermometers using a variety of other fluids, such as alcohol, toluene or other organic liquids.
One of the major items of confusion with the use of all glass thermometers is with the immersion depth for the thermometer. The immersion depth is a design feature used to account for the thermal characteristics of the specific liquid used in the thermometer. A glass thermometer that is not used at the proper immersion depth can have measurement errors of several degrees.
There are three types of immersion depths that are commonly used in glass thermometers: partial immersion, total immersion, and complete immersion.
Thermometers that are designed for partial immersion need to be immersed in the fluid medium to a specific depth marked on the thermometer. The mark is usually a ring located near the bulb end of the thermometer.
Thermometers that are designed for total immersion need to be immersed in the fluid medium up to the level of the indicated temperature. The meniscus of the thermometer fluid can remain slightly outside of the fluid medium to make the thermometer easier to read.
Thermometers that are designed for complete immersion need to be immersed entirely in the fluid medium These types of thermometers can only be used in transparent liquids and are not commonly available.
Many users assume that once a liquid-in-glass thermometer is manufactured, its characteristics will not change and that it will always give accurate readings. This is an incorrect assumption. Errors to the thermometer can occur as a result of temperature cycling of the bulb over time.
Every time the thermometer is heated or cooled, the size of the bulb changes slightly. Over a long period of time, these changes can affect the thermometer’s indication. This change depends on the quality of glass used in the thermometer, how often the thermometer is used, and the temperatures the thermometer is subjected to. Thermometers mainly used at higher temperatures change more rapidly.
Changes to the liquid can also occur. Discolored or oxidized mercury can lead to measurement errors and separation of the liquid is quite common in all liquid types and can result in inaccurate readings.
Regular calibration of the thermometer is the only way to ensure that those changes do not significantly affect the thermometer through its normal use and that the accuracy of the thermometer is being maintained.
Before performing the calibration of the glass thermometer, it is visually inspected with a magnifying glass to check for physical defects. The visual inspection checks for:
The calibration process for glass thermometers involves the direct comparison of the thermometer reading with a highly accurate temperature reference standard, such as a Platinum Resistance Thermometer (PRT).
The thermometer and reference standard are placed together in either a temperature-controlled liquid bath or a dry-block calibrator. It is important that the depth of the thermometer bulb and the measuring end of the reference standard are at the same depth to minimize any errors in the equilibrium at varying depths of the liquid bath.
The calibration should consist of a minimum of 3 temperature points and should be checked in the low, mid and high ranges of the temperature scale. Thermometers that have a high-temperature range may also have an auxiliary scale located below the main scale that is often used as a 0°C/ 32°F freezing point indication. If the auxiliary scale is included, that temperature point should also be calibrated.
The thermometer and reference standard should be left in the bath for an appropriate amount of time for the bath and measuring instruments to achieve temperature stabilization. This time will vary based on the control characteristics of the bath and the response times of the thermometer and reference standard.
Once stabilized, the readings of both the thermometer and reference standard are recorded and compared to determine if the thermometer meets its required specifications. That process is repeated for each of the temperature points to be calibrated.
For more information on the calibration of liquid-in-glass thermometers, contact e2b calibration.
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