What Is a Recording Thermometer?
A recording thermometer is a device that automatically measures and records changes in air temperature over time also referred to as a self-registering thermometer.
There are two types of recording thermometers, each with a different mechanism.
One type utilizes the thermal expansion of metals, such as bimetals or Bourdon tubes, where the thermal expansion drives the pen of the self-registering device.
The other type uses the change in electrical resistance with temperature and employs a thermistor to convert the resistance change into an electrical current, which is then used to record the temperature with a recording thermometer.
Uses of Recording Thermometers
Recording thermometers are generally not very sensitive and may have a few minutes of delay when recording. Despite this, they can provide valuable information such as the time of maximum and minimum temperatures and the temperature change at any given time.
For this reason, recording thermometers are used in general weather observation, educational institutions, air conditioning systems, hospitals, warehouses, and other situations where temperature control is necessary.
Additionally, they find application in museums and art galleries to protect exhibits and are used in various settings such as chemical and food storage rooms in warehouses, agriculture, production plants for precision machinery like semiconductors and LSIs, and environmental laboratories.
Principle of Recording Thermometers
Bimetal thermometers consist of a sensor made from two pieces of metal with different coefficients of thermal expansion bonded together. Temperature changes cause warping of the metal, which varies with temperature and is used to measure temperature changes.
The Bourdon tube is a flat oval metal tube with a flat cross-section, sealed at one end and wound into a nearly circular shape, with one end fixed in place. As the temperature rises, the alcohol or ether sealed inside the tube expands, displacing the unfixed end of the tube, thus measuring temperature changes.
Recording thermometers that employ bimetals or Bourdon tubes use leverage to amplify the warping of the bimetals or the displacement of the Bourdon tube caused by temperature changes. This amplification is transmitted to a pen in the recording device, which records the data on chart paper (recording paper) wound around a drum rotating in a clockwork mechanism.
Thermistors are electronic components whose resistance changes with temperature variations. Thermistor thermometers measure resistance by passing a small amount of electric current through the sensor part’s metal, converting it into a temperature value. Since the measurement is conducted electrically, a digital display is also possible, and temperature changes can be recorded in a digital data logger.