Liquid or gas-filled thermometers use a gas or liquid as the temperature-sensing element. The liquid expands as the temperature rises in the tube and indicates the temperature. The basic design for this type of thermometer is a small-bore glass tube with a thin-walled glass bulb at the lower end. Typically, the liquid that fills is mercury or alcohol. As heat is transferred into the substance, it expands and pushes the column of liquid or gas higher up the capillary, which indicates the temperature.
Material in tube: gas or liquid — Mercury is commonly used to fill thermometers due to its thermal expansion characteristics. Red alcohol spirit fill is a fill choice that is safer than mercury.
Material of tube: metal tubes which are best for more rugged applications.
Level of liquid or gas in the tube — The appropriate volume of liquid or gas in the tube depends on the expansion coefficient of the substance used, the area of the capillary and the range of temperature required. The governing formula is:
h = Volume / Area = b * Vo (T – To) / A
Bimetal or bi-metallic thermometers use a bimetal spring as the temperature-sensing element. This coil spring is made of two different types of metals that are welded or fastened together. Choices include copper, steel and brass – as long as one metal has low heat sensitivity and the other metal has high heat sensitivity. Whenever the welded strip is heated, the two metals change length based on their individual rates of thermal expansion. 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 indicates temperature to the user.
Advantages of bimetal thermometers include their low cost, durability, ease of installation and use, and accuracy over a wide range of temperatures.
Important specifications for industrial thermometers include display options, dimensions, stem material, stem adjustments and operating environment.
Thermometers display options include:
- Celsius or Centigrade display, display range, and scale divisions
- Both Fahrenheit and Celsius options
- Max/min readings
- Glass or plastic viewing windows
Display range is the minimum and maximum values of temperature that can be displayed. Scale division is the smallest division of degrees that can be displayed. It is sometimes referred to as resolution in digital instruments.
- Thermometer 0…+60 °C
- Thermometer -40…60 °C