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thermometers

Introduction

  • 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
    • 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
      • 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.

General characteristics of thermometers to consider

  • 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!)

data logging thermometers

thermal imaging cameras

  • 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:
photo/thermometers.txt · Last modified: 2021/07/29 17:15 by gary1