see also:

• electromagnetic radiation
• thermal imaging cameras

• there are a number of types of thermometers and each has their own usage
• temperature can be detected via a number of mechanisms:
  • expansion of a liquid
    • eg. traditional mercury or alcohol thermometers
  • detection of radiated electromagnetic radiation
    • peak wavelength radiated from an object in microns = 2897.2/(temperature in Kelvin)
    • these allow non-contact sensing and are used in a range of manners such as:
      • clinical use to estimate body temperature (usually derived from skin or middle ear measurements)
      • industrial uses
      • thermal imaging cameras
  • resistance temperature detector (RTD)
    • this is a type of thermistor which works on the principle that the electrical resistance of metal changes with temperature
    • this is the most common sensor used in LCD digital thermometers to measure ambient air temperatures
    • most have an accuracy of +/- 1deg C although some can be calibrated for better accuracy
    • precision is usually 0.1degC
  • thermistor probes
    • works on the principle that the electrical resistance of metal changes with temperature
    • as for the RTD but uses an external probe which can be placed inside food, liquids, etc
  • thermocouple probes
    • see https://en.wikipedia.org/wiki/Thermocouple for more details
    • made up of two wires of different metals that, when joined together, create an electrical junction. When the temperature changes at the junction it creates a voltage that can be interpreted to read the temperature (the Seebeck effect).
    • there is a wide range of available, each with different characteristics, such as temperature range and their robustness.
    • they are mainly used to measure high temperatures
    • reaction times are generally well under 100msec
    • common types include J, K, L and T which refer to the type of materials used and therefore the temperature they are usable at.
      • type J is iron and copper-nickel and is usable from around -40°C to 750°C
      • Type K is chromel-alumel and the most generally used type with a range of −200 °C to +1350 °C
      • type T is copper and copper-nickel alloy often used for food to work over range -250°C to +400°C and may give accuracy down to ±0.2°C whereas a thermistor probe for same usage may only have a range of -40°C to +120°C and an accuracy of ±0.4ºC. Being copper, it has greater thermal conductivity than other types so requires extra care as heat will be transmitted along the probe to a greater degree.

• operating temperature range
• probe or no probe
• usage design
• accuracy
• ability to calibrate against a more highly accurate thermometer to give more accuracy
• precision
• sampling rate
• time to equilibrate to temperature being measured
• weathersealing
• size
• robustness
• cost
• power supply
• other sensor functions
  • eg. hygrometer to measure RH
• ability to link wirelessly with other sensors
  • this is useful to record both indoor and outdoor temperatures and to link with weather stations for example
• ability to link with smartphones via Bluetooth
  • allows remote access and also potentially data download
  • examples also include sensors to be used inside car fridges to avoid having to open the fridge to check the temperature, etc
• ability to record and manage data points
  • most just record current temperature, and some will record min and max for last 24hrs
  • you may prefer to use a USB data logger thermometer for a more complete timeline which can be exported to Excel (note some are not compatible with 64bit Windows!)

• examples:
  • Instrument Devices TH-22 USB Air Temperature, Relative Humidity, Dew Point Data Logger, LCD Screen
    • USB; -35 to 80°C 0.3degC accuracy, 0.1degC precision; $AU138 (TH-11 version is $AU110 but no RH); (ext. 3m probe TH-22E version is $AU154 and has range -40 to 125°C)
  • Instrument Devices TH-22ET 3 in 1 Air Temperature, Humidity & External Probe Temperature Data Logger, Calibratable
    • int (0.2degC accuracy) and 3m ext probe (0.5degC accuracy) temp measurements plus RH; $AU158
  • Lascar EL-USB-2-LCD+ Temperature & Humidity Data Logger Data Logger with Dew Point, Humidity, Temperature Sensor
    • compatible with 64bit Windows 10 but precision is only 0.5degC and costs $AU272
    • has replaceable lithium battery which lasts 1-2 yrs and has a LCD display to show current temp, RH
  • Corintech EL-WiFi-T Temperature Data Logger Data Logger with Internal Thermistor Sensor
    • WiFi real time temperature read out and data logging;
    • 0.1degC precision but no RH sensor; USB chargeable lithium polymer battery; $AU178
  • Corintech EL-WiFi-TP Temperature Data Logger Data Logger with External Thermistor Sensor for Temperature Measurement
    • as above but uses a 1m long thermocouple cable; $AU237
  • Corintech EL-WiFi-TH Temperature Data Logger Data Logger with Internal Temperature/Humidity Sensor for Temperature
    • as above but with RH sensor; $AU391;

• usually have a centre spot temperature measurement which generally have accuracy of around +/- 1.5 to 2degC
• thermal imaging resolution is usually around 160 x 120 pixels
• have the advantage that they can visually display hot and cold spots for detecting industrial problems
• examples:
  • https://www.flir.com.au/browse/industrial/handheld-thermal-cameras/
    • FLIR TG165-X MSX Thermal Camera
      • detects temperature ranges -25°C to 300°C
      • $AU660
    • Flir One Pro camera adapter for smartphones
      • attaches to smartphones
      • accuracy ±3°C
      • detects ranges -20° to 120°C and 0°C to 400°C
      • 55° x 43° field of view and can measure objects 15cm to 50m away
      • iOS version connects via lightning connector
      • android version connects via USB-C
      • $AU630