FAQs
Temperature - FAQs
Frequently asked questions relating to temperature.
General
'Traceability' means that the measurement results can be related to a stated reference by an unbroken chain of accredited comparisons.
Temperature
- A number of factors need to be taken into account when considering sources for calibrating radiation thermometers.
- Radiation thermometers measure the thermal energy emitted by a source and relate this to its temperature by means of the Planck law of radiation. They consist of optics (generally lenses) to collect and focus the emitted energy onto a detector.
- There are a number of possible reasons for the difference, in addition to possible calibration errors.
- Surface temperature is notoriously difficult to measure...
- Liquid-in-glass thermometers can be used in varying ranges from around - 200°C to +600°C. In calibrating a liquid-in-glass thermometer one must consider the thermometer itself, the environment used for the calibration, and the standards used to provide the reference temperature.
- A blackbody source is an ideal, 'Planckian', radiator, i.e it emits thermal (visible and infrared) energy whose intensity at a given wavelength and temperature is given by the Planck Law of radiation.
- A platinum resistance thermometer (PRT) is a device which determines the temperature by measuring the electrical resistance of a piece of pure platinum wire.
- A thermistor is simply an electrical resistor whose resistance changes rapidly with temperature.
- Thermocouples are the most common sensors in industrial use due to the fact that they are small, simple, rugged and of low cost. They consist of two dissimilar conductors (wires) joined at a junction and contained in an insulator.
- A thermometer is a device which has a measurable property which changes with temperature.
- The triple point temperature of a pure substance is the unique temperature at which the solid, liquid and vapour phases of the substance co-exist in thermal equilibrium. Such triple points make ideal reference points for the calibration of thermometers.
- All objects at temperatures above absolute zero emit thermal radiation. However, for any particular wavelength and temperature the amount of thermal radiation emitted depends on the emissivity of the object's surface.
- Fundamentally, temperature is the potential for heat transfer. Thus an object at a higher temperature will lose heat to an object at a lower temperature when they are placed in contact.
- The best thermometer to use in a particular application is something that only you can decide. Your choice will depend on many factors and a selection of these is listed below.
- The ITS-90 (International Temperature Scale of 1990) defines procedures by which certain practical thermometers of the required quality and precision can be calibrated in such a way that the values obtained from them can be precise and reproducible, while at the same time representing the corresponding thermodynamic temperatures as closely as possible.
- The International Temperature Scale of 1990 goes down to 0.65 K, below this, to 1 mK, is the Provisional Low Temperature Scale of 2000 (PLTS-2000).
Thermophysics FAQs
- Accurate measurement of thermal conductivity is not quite as straightforward as the simple steady-state theory would suggest.
- The key references for thermophysical properties data are as follows.
- The following list summarises the main standards documents relating to the measurement of thermal conductivity of construction materials, refractories and plastics.
- Thermophysical properties can be simply defined as material properties that vary with temperature without altering the material's chemical identity.
Humidity
- Yes - Saturated (or unsaturated) salt solutions, and certain other chemicals, can be used to generate an environment of a particular relative humidity in an enclosed space.
- Dew point (or dew-point temperature) is the temperature at which dew, or condensation, forms, on cooling a gas. Where the condensate is ice, this is known as frost point.