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science:thermometers

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measuring temperature / thermometers

see also:

  • I don't sell any of these nor do I receive any remuneration if you buy them, and I have not personally reviewed all of them, they are listed here to give you perspective

Introduction

  • measuring the temperature seems easy but it can be quite complex depending upon what is being measured and how accurate and timely one needs the measurement

General characteristics of thermometers to consider

  • operating temperature range
  • probe or no probe
  • usage design
  • accuracy
    • +/- 2degC IR thermometers will probably not suit your needs in measuring temperatures 0-30degC, but are great for high temperatures when small changes or inaccuracies are less important
  • ability to calibrate against a more highly accurate thermometer to give more accuracy
  • repeatability
    • consistency of repeated measurements of same object may be important for detecting change
  • precision
    • you could have an accurate thermometer but if it only shows 1degC increments instead of 0.1degC then it may not serve your purpose
  • sampling rate
  • time to equilibrate to temperature being measured
    • some are near instantaneous (< 250msec) such as the IR thermometers, while ambient temperature ones may take 15-60min to equilibrate
  • weathersealing
  • size
  • robustness
  • cost
  • power supply
  • other sensor functions
    • eg. hygrometer to measure RH; probe temperature plus ambient; probe temp plus IR temp;
  • 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 as well as meat probes for cooking in ovens and grills
  • 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

iPhone app Bluetooth or WiFi logging thermometer/hygrometer

Cloud-based IoT WiFi systems

  • these need to connect to an internet WiFi (usually 2.4GHz only) where data is stored and then accessed via smartphone app or computer web access
  • UbiBot WS1Pro
    • indoor temp,hygro - optional additional temp probes; microUSB, AAA batteries; has large internal memory for data storage when internet not available.

Digital ambient sensor thermometers

  • these are the most common thermometers currently in use
  • they can quite accurately measure ambient temperature although many do take a long time to equilibrate to a change in temperature which may take 30-60 minutes depending upon the device
  • these use a 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

Bluetooth or WiFi standalone indoor-outdoor thermometer/hygrometers

  • do not connect to smartphones and only store limited data - no export options
  • these are all monochrome LCD displays unless specified
  • InkBird
    • InkBird IBS-TH1 Plus
      • Bluetooth; internal temp/RH sensor with display plus external temp probe;
      • iOS app can connect to multiple devices and can store max 300,000 data points; 2xAAA;
      • Amazon.com.au InkBird IBS-TH1 Plus
    • InkBird IBS-TH2
      • ? as above but no display and no probe;
    • Inkbird IBS-TH2 Plus
  • ThermoPro
  • ORIA Indoor Outdoor Thermometer with 3 Wireless Sensors
  • lots of weatherstations available are similar to this

Digital Infrared thermometers

  • these assess the infrared light that objects are radiating (or reflecting) and ascertain temperature based upon the maximal wavelength peak
  • peak wavelength radiated from an object in microns = 2897.2/(temperature in Kelvin)
  • fast - usually less than 0.5sec but accuracy is usually lowish for ambient temperatures at +/- 2degC although those especially designed for human foreheads are much more accurate in reading skin temperature but have a limited temperature range
  • they do need to be adjusted for the emissivity of the object to gain the correct readings
  • see infrared thermometers
  • see also electromagnetic radiation

Digital Infrared Thermal Imaging devices

  • these work on same principle as infrared thermometers however use a sensor capable of outputting a visual image of the object and a thermal heat map with temperatures
  • see 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:

Digital contact probe thermometers

  • these are usually special metal probes attached to a wire which transmits the electrical impulse from the probe to the thermometer device
  • they may take a few minutes to equilibrate - sometimes up to 15 minutes for accurate ambient temperatures within 1 degC
  • swimming pool solar heating systems use these probes attached to the water pipe and the solar tubing to ascertain the difference and determine whether or not to activate the solar water pump
    • the temperature they read on the water pipe does depend upon where in the pool the water is coming from - if using a pool cleaner it may be the colder bottom of the water or the warm surface water depending upon where it is at that time
    • 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.

Meat thermometers

  • these are very useful to ensure you cook your meats to the desired amount without having to constantly check on them
  • to prevent infectious diseases such as toxoplasmosis, it is generally recommended the internal temperature of beef and lamb is over 63degC although this does take it into the “medium” range rather than “medium-rare” and you need it to be over 71degC if it is ground meat or has been tenderized as these allow spread of bacteria throughout the meat (if just re-heating a frozen meat pie which has already been baked, you only need the internal meat to be around 45degC)
  • fantastic when camping with an oven on a wood stove as well as you don't need to open the oven door and have the oven cool down all of a sudden
  • perhaps the most popular and well reviewed BlueTooth/WiFi one is the Meater Plus:
    • sends internal meat and external ambient temperature readings from a probe you place inside the meat and inside the oven or grill via Bluetooth to your smartphone app and the Plus model allows the range to be further extended via your WiFi
    • the app alerts you when the desired temperature is reached and you can also set other timer alerts
    • you can also have multiple probes active at the same time
    • the “ambient” external oven/grill temperature sensor will only display a value if it is more than 5degc warmer than the “internal” sensor reading
    • it does need to be cleaned with bicarb soda and white vinegar and an abrasive sponge to remove baked food residues then washed in soap and water and dried
    • it is charged by placing back in the wood case which contains a AA battery to automatically charge it over 4hrs and this battery is said to last about 1 yr of use
  • there are less expensive digital models but these often only have the internal sensor and the software may not be as well developed, or they may require cumbersome and hazardous wires
  • you can also get cheaper analogue spring dial probes (see below) to do this as well

Analogue expanding metal spring dials

  • these are commonly used on BBQs, ovens and stoves

Analogue expanding liquid thermometers

  • these were the traditional thermometers:
    • mercury within glass
    • alcohol within glass

Stellar temperature measurement

  • measuring the temperature of stars can be done via spectral analysis of their light
science/thermometers.txt · Last modified: 2023/08/18 15:05 by gary1
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