How to select a Bimetal Thermometer?


  • Application? Continuous process flow, static tank, or laboratory testing etc.
  • Environmental conditions? Ambient temperature, humidity, or corrosive atmosphere (salt spray etc.)
  • Process conditions? Pressure, temperature, media viscosity, rate of flow, and possible vibration
  • Wetted material of thermometer compatible with measured medium?
  • Window material?


  • Dial size?
  • Location of fitting connection for maximum readability when thermometer is installed in system– Back, adjustable angle, bottom, top, right side or left side connection location?
  • Fitting thread size and type?


  • Pipe size or depth of tank?
  • Immersion length required?
  • Stem length?
  • Stem diameter?


  • Operating temperature of process? High and low
  • Overall temperature range required? High and low
  • Over-Range possibilities? Minimum and maximum
  • Accuracy required? ±1%               or ±1/2% full scale         
  • External reset required?
  • Scale – °F, °C or dual?
  • Bimetal thermometers should not be used continuously over 800°F (450°C)

Picture: model AA-575R – 5″ diameter head, Adjustable Angle, 1/2″ NPT, 6” Stem Length, 0.250” Stem Diameter, Overall temperature range 20-240°F, 316SS wetted parts, dry, window material glass, ±1% full scale accuracy.


  • Thermowell?
  • Silicone filling?
  • Minimum and/or maximum indicating pointers?
  • Custom or special dial marking or colors?
  • Sliding compression fittings– male or female?
  • Thread size adapters?
  • Project tagging?


  • Use of a thermowell is recommended for pressure, corrosive fluid or high velocity applications.
  • Use of a thermowell allows removal of thermometer for calibration check or interchange of instruments without shutting down the process.
  • Install thermometers in locations that minimize injury or damage in the event of an accidental breach of stem/thermowell combination.
  • Cases may be silicone filled for additional dampening of extreme vibration, or to assure consistent performance in low process temperature/high environmental humidity applications.
  • Silicone filled thermometers are limited for use with process temperatures ranging from a minimum of -50°F (-45°C) to a maximum of 500°F (260°C).
  • Use of silicone fill should be avoided where strong oxidizing agents such as oxygen, chlorine, nitric acid, and hydrogen peroxide are present.
  • Bimetal thermometers should not be exposed continuously to process temperatures over 800° F (425° C) to avoid damaging the bimetal element.
  • For 21⁄2″ stem industrial thermometers:
  • The connection nut should be immersed or in contact with the process media for most accurate reading.
  • When used with limited space thermowells some reading error may be noticed depending on process and ambient temperatures
  • Operating conditions – It is recommended the temperature of the thermometer head not exceed:
  • 200°F (93°C) for glass lenses
  • 150°F (65°C) for plastic lenses
  • 150°F (65°C) for silicone filled thermometers
  • Optional lens materials: Acrylic plastic
  • Fair chemical resistance but low temperature limit Polycarbonate plastic
  • Higher temperature limit than acrylic but poor chemical resistance Shatterproof laminated safety glass
  • High corrosion resistance
  • Plastic laminate between two pieces of glass will hold fragments in place if broken but laminate will start to deteriorate above 200°F Tempered glass
  • Improved to withstand a broad range of temperature changes
  • More impact resistant than regular glass

Tel-Tru Manufacturing Company manufactures and distributes bimetal thermometers, digital thermometers, temperature and pressure transmitters, pressure gauges, and accessory products, that sense and monitor temperature and pressure in sanitary, industrial, OEM, HVAC, and food service processes and equipment worldwide.

ACCESS Professional Viet Nam Limited Company is distributor of Tel-Tru Manufacturing Company manufactures USA.

Keywords: #Tel-Tru # thermometers # pressure gauges # temperature and pressure transmitters # digital thermometers

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