Non-contact thermometry FAQs

Back

Click on the E-number for an answer...                     

E1.  What should the emissivity setting be when measuring furnace temperature?

E2.  Can I use a 8-14 m IR-thermometer for surface temperature measurements in a furnace at elevated temperatures?

E3.  What is the best way to measure the temperature of food items with an IR-thermometer in a freezing room at -18C?

E4.  Can I measure the gas temperature inside a combustor with good precision using an IR-thermometer?

E5.  Which surfaces behave like a grey body?

E6.  What is the emissivity of painted metal surfaces and how does it depend on layer thickness?

E7.  How can I correct the emissivity setting of my IR-thermometer looking through a window?

E8.  How does a radiation thermometer work?

E9.  What is emissivity?

E10.  Why is emissivity important?

E11.  I am using a radiation thermometer to measure the temperature of a sample, but why am I getting different results compared with using a thermocouple immersed in the sample?

E12.  What is the maximum distance I can make measurements from the target?

E13.  Can I use an anodised aluminium plate or a tungsten ribbon lamp, rather than a blackbody source, to calibrate my radiation thermometer?

E14.  What is spectral range? Why is it important?

E15.  How close do I have to be to an object to take its temperature?

E16.  How do infrared thermometers work?

E17.  Some IR thermometers use lasers. Do they take temperature readings? Are they dangerous?

E18.  Why can't I see infrared?

E19.  I need a technique for measuring surface temperature of metal parts that is quick, economical, portable (hand held) and has an uncertainty of between 0.1 and 0.2 C What measurement technique should I use?

E20.  Can I use an anodised aluminium plate or a tungsten ribbon lamp, rather than a blackbody source, to calibrate my radiation thermometer?

E21.  What is a blackbody source and what is it used for?

E22.  How does a radiation thermometer work?


E1.  What should the emissivity setting be when measuring furnace temperature?

The emissivity setting of the IR-thermometer should be 1.00. The furnace can be seen as a blackbody cavity, i.e. the effective emissivity approaches 1.00 due to multiple reflections of thermal radiation inside the furnace. The corrections and errors on the measured surface temperature are small with surrounding surface at similar temperature.

Top

E2.  Can I use a 8  - 14 m IR-thermometer for surface temperature measurements in a furnace at elevated temperatures?

Normally not. Water vapor along the line of sight will absorb and emit thermal radiation in this spectral range. Use a IR-thermometer that measures at wavelengths outside gas absorption bands, e.g. a 3.9 m IR-thermometer.

E3.  What is the best way to measure the temperature of food items with an IR-thermometer in a freezing room at -18C?

Use a 8  - 14 m IR-thermometer with emissivity set to 1.00. Using an emissivity setting of 0.95 and the IR-thermometer temperature as a measure of the ambient reflected thermal radiation would lead to large errors despite a surface emissivity close to 0.95. Avoid large temperature changes of the IR-thermometer during measurements, e.g. store the IR-thermometer dry at 5 C rather than in the sun.

Top

E4.  Can I measure the gas temperature inside a combustor with good precision using an IR-thermometer?

The reading of IR-thermometers are not affected by the gas flow velocity as for contact sensors and the response time is fast. IR-thermometers looking at radiation from particles or the 4.3 m hot-band of carbon dioxide can usually be used. Two-colour pyrometers based on emitted radiation from particles require high concentration of particle and long path length, i.e. the gas temperature measurement is influenced by the temperature of the opposite wall with too few particles in the field of view. The IR-thermometer based on radiation from a single band of carbon dioxide works well if the temperature profile along the field of view is flat and with a content of 1-12% carbon dioxide of the fluegas (low carbon dioxide concentration requires long path length).

Top

E5.  Which surfaces behave like a grey body?

In practice none!  The emissivity is independent of wavelength for a grey body. The emissivity of all real surfaces changes with wavelength, although some surfaces are close to a grey body. The ideal gray body concept is useful for simplified equations in heat transfer calculations.

E6.  What is the emissivity of painted metal surfaces and how does it depend on layer thickness?

The emissivity of most diffuse painted surfaces is approximately 0.95 in the spectral range 8  - 14 m, regardless of the visual colour. The emissivity of painted surfaces usually approaches the emissivity of the paint for a sufficient layer thickness, i.e. usually 2 or 3 thin coats. The emissivity of painted surfaces usually increases with temperature due to broadening of the absorption bands of the chemical components in the paint. A metal surface painted with a thin layer (1 coat) might change emissivity from 0.95 at 500 C to 0.4 at 50 C.

Top

E7.  How can I correct the emissivity setting of my IR-thermometer looking through a window?

Three effects must be considered: absorption, reflection or scattering of light/thermal radiation. The ideal window has only low reflection losses of light, i.e. the window material has a small refractive index. The best way to compensate for the effect of the window is to calibrate the IR-thermometer with the window in the same geometry and conditions as in the application, e.g. decrease the emissivity setting of the IR-thermometer until the same reading is obtained looking through the window as without the window. Be aware of things becoming fairly complex when the window is hot and absorbs in the spectral range of the IR thermometer.

Top

E8.  How does a radiation thermometer work?

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. The signal from the detector can either be measured directly, or it can be converted to a temperature using a system of electronics. Filters are usually used to define the wavelength or wavelength band over which the emitted energy is measured. Many types of radiation thermometer are available for different applications. For measuring high temperatures a thermometer should be chosen that operates at a short wavelength, where the rate of change of emitted radiation with temperature is very high. However, for low temperature applications where the amount of emitted radiation is low, a broad-band device operating at longer wavelengths is required. (Further info)

Top

E9.  What is emissivity?

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. Emissivity is defined as the ratio of the energy radiated from a material's surface to that radiated from a blackbody (a perfect emitter) at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter). The emissivity of a surface depends not only on the material but also on the nature of the surface. For example, a clean and polished metal surface will have a low emissivity, whereas a roughened and oxidised metal surface will have a high emissivity. The emissivity also depends on the temperature of the surface as well as wavelength and angle. (Further info)

Top

E10.  Why is emissivity important?

Knowledge of surface emissivity is important both for accurate non-contact temperature measurement and for heat transfer calculations. Radiation thermometers detect the thermal radiation emitted by a surface. They are generally calibrated using blackbody reference sources that have an emissivity as close to 1 as makes no practical difference. When viewing 'real' more reflective surfaces, with a lower emissivity, less radiation will be received by the thermometer than from a blackbody at the same temperature and so the surface will appear colder than it is unless the thermometer reading is adjusted to take into account the material surface emissivity. Unfortunately, because the emissivity of a material surface depends on many chemical and physical properties it is often difficult to estimate. It must either be measured or modified in some way, for example by coating the surface with high emissivity black paint, to provide a known emissivity value. The NPL provides a service for measuring the emissivity of samples (for further information see the NPL website) which is used by customers when they need valid surface temperature measurements or heat transfer calculations. (Further info)

Top

E11.  I am using a radiation thermometer to measure the temperature of a sample, but I am getting different results compared with using a thermocouple immersed in the sample?

There are a number of possible reasons for the difference, in addition to possible calibration errors. Firstly, the thermocouple might not be in good thermal contact with the surface of the sample, or there might be temperature gradients within the sample. If this is the case then the thermocouple and radiation thermometer will not be measuring the same temperature. Alternatively, if the emissivity of the sample is low, or not precisely known, the temperature measured by the radiation thermometer will not represent the true temperature of the sample, again leading to differences. Also, if the sample is small, it might not be fully filling the field-of-view of the radiation thermometer, and the radiation thermometer temperature will therefore be low compared to that of the thermocouple. (Further info)

Top

E12.  What is the maximum distance I can make measurements from the target?

This is a function of the optics in your thermometer. Use the distance-to-size ratio and the diameter of your target to determine the maximum distance you can be from the target. Most IR thermometers have a maximum measuring distance of approximately 100 feet (30 metres), depending on atmospheric conditions. (Further info)

Top

E13.  Can I use an anodised aluminium plate or a tungsten ribbon lamp, rather than a blackbody source, to calibrate my radiation thermometer??

A number of factors need to be taken into account when considering sources for calibrating radiation thermometers: Firstly, the calibration source needs to have a high and accurately known emissivity, to ensure that measured temperature will accurately reflect the true temperature of the surface. Secondly, the source needs to be large enough to fill the optical field-of-view of the radiation thermometer, since under-filling the field-of-view will result in measurement errors. Thirdly, the temperature of the source needs to be measured by some means, for example by using a contact sensor inserted close to the radiating surface. A blackbody source with an aperture of a suitable size meets these requirements and should therefore usually be used to calibrate radiation thermometers. The use of an anodised aluminium plate is not generally recommended for checking or calibrating radiation thermometers. Firstly, the emissivity of anodised aluminium is quite low and depends on the thickness of the anodised layer. Many radiation thermometers operate at wavelengths in the infrared, and in this region the emissivity can be anywhere between 0.4 and 0.9. The actual value must be known if measurement errors are to be avoided. Secondly, because the emissivity is low, the radiation measured by the thermometer will be a combination of radiation emitted by the plate and radiation reflected from the plate from other objects in the room. This will, again, lead to potentially significant errors in the reading. Thirdly, there must be some other means, such as a contact probe, for determining the temperature of the plate. If this probe is not in good thermal contact with the plate, or if there are temperature gradients across the plate, it will not give a good indication of the true plate temperature. It will therefore not be possible to accurately check the calibration of the radiation thermometer. (Further info)

Top

E14.  What is spectral range? Why is it important?

The infrared spectral range is 0.7 to 1000 m, the range for wavelength in which infrared radiation is transmitted. For cost reasons, IR thermometers generally operate under 20 m. Many commercial IR thermometers have a spectral response in the region of 8-20 m. This range is used because it is minimally affected by CO2 and H2O in the atmosphere. With longer, lower-energy wavelengths, the accuracy decreases with increased distances due to the affects of the atmosphere (humidity).  

Top

E15.  How close do I have to be to an object to take its temperature?

Distance does not affect the measurement. However, infrared sensors measure the energy from a circular spot on the target, and the size of that spot is a function of the distance between the sensor and target. The farther away from the target the sensor is, the larger the spot. Consequently, distance is only limited by the size of the object you want to measure. (Further info)

E16.  How do infrared thermometers work? 

IR thermometers capture the invisible infrared energy naturally emitted from all objects warmer than absolute zero (0 degrees Kelvin). Infrared radiation is part of the electromagnetic spectrum that includes radio waves, microwaves, visible light, ultraviolet, gamma, and X-rays. Any object emits energy somewhere within that range. Infrared falls between the visible light of the spectrum and radio waves. Infrared wavelengths are usually expressed in microns with the infrared spectrum extending from 0.7 microns to 1000 microns. In practice, the 0.7 to 14 micron band is used for IR temperature measurement. (Further info)

Top

E17.  Some IR thermometers use lasers. Do they take temperature readings? Are they dangerous?

No on both questions. Lasers are used only for aiming or sighting. The lasers are low voltage units and are not dangerous. Note, however, that all lasers have government regulated labels on them stating power ratings and any necessary safety measures (usually "do not stare into beam"). (Further info)

E18.  Why can't I see infrared?

Human eyes are designed for visible light, but two species are known to detect IR: some rattlesnakes and beetles. Even though IR is not visible to the human eye, your skin can sense IR. When beside a campfire, you can feel the warmth of heat radiated from the fire. (Further info)

Top

E19.  I need a technique for measuring surface temperature of metal parts that is quick, economical, portable (hand held) and has an uncertainty of between 0.1 and 0.2 C What measurement technique should I use?

Surface temperature is notoriously difficult to measure. There is a discontinuity between a hot or cold object and its environment and the  resulting heat flow can be substantial. A sensor or probe attached to the surface will register its own temperature, and the problem is to get it into proper contact with the object without changing the temperature to be measured. In principle this is impossible!

However, there are surface probes made for the purpose. What is best depends on the circumstances. A fine thermocouple wire laid along the surface for some distance should make reasonable contact, but this is not always practicable. A pointed probe like a skewer is more practical but unlikely to read correctly. Probes with a contact pad may be effective, provided they do not mask the surface unduly and so change the heat flow and temperature. You should make enquiries to some of the many sensor supply companies and discuss what may be most appropriate for your purpose.

All this presupposes that the surface temperature is significantly different from its surroundings, i.e. a significant discontinuity exists and heat flow is occurring. In these circumstances it is not reasonable to expect uncertainties as small as 0.1 or 0.2 C. However, if the temperature is not too high or low, things get much easier - the smaller the temperature difference, the smaller are the heat flows and the measurement interference.

In general, the best that can be done is to attach sensors in intimate contact with the object where the temperature can conveniently be measured, covering only a small area, and using a heat-sink compound, if appropriate. The objective is to maximise contact while minimising interference. The supplier will advise about how to do this, and about the intrinsic uncertainty of his sensors, but the accuracy achieved in the measurement situation will depend on the circumstances. A few C might be all that can be achieved.

Beyond that, one supplier - Isothermal Technology - offers a 'true surface' temperature probe, which includes feedback heating to compensate for heat losses along the probe, which then more closely approaches the 'true' surface temperature. This should improve the results where a 'rough and ready' method is not acceptable.

Another alternative is to use a non-contact technique - radiation thermometry. This is non-invasive and so avoids the problem of interference, but it brings its own difficulties: what is the emissivity (a radiative property) of the surface, is there any contribution from background radiation reflected from the surface, etc. Radiation thermometers can operate down to ambient or even lower temperatures, though ambient radiation then becomes problematic. (Further info.)

Top

E20.  Can I use an anodised aluminium plate or a tungsten ribbon lamp, rather than a blackbody source, to calibrate my radiation thermometer?

Firstly, the calibration source needs to have a high and accurately known emissivity to ensure that measured temperature will accurately reflect the true temperature of the surface. Secondly, the source needs to be large enough to fill the optical field-of-view of the radiation thermometer, since underfilling the field-of-view will result in measurement errors. Thirdly, the temperature of the source needs to be measured by some means, for example by using a contact sensor inserted close to the radiating surface. A blackbody source with an aperture of a suitable size meets these requirements and should therefore usually be used to calibrate radiation thermometers. The use of an anodised aluminium plate is not generally recommended for checking or calibrating radiation thermometers. Firstly, the emissivity of anodised aluminium is quite low and depends on the thickness of the anodised layer. Many radiation thermometers operate at wavelengths in the infrared, and in this region the emissivity can be anywhere between 0.9 and 0.4. The actual value must be known if measurement errors are to be avoided. Secondly, because the emissivity is low, the radiation measured by the thermometer will be a combination of radiation emitted by the plate and radiation reflected from the plate from other objects in the room. This will, again, lead to potentially significant errors in the reading. Thirdly, there must be some other means, such as a contact probe, for determining the temperature of the plate. If this probe is not in good thermal contact with the plate, or if there are temperature gradients across the plate, it will not give a good indication of the true plate temperature. It will therefore not be possible to accurately check the calibration of the radiation thermometer. Tungsten ribbon lamps can be used for calibrating some types of radiation thermometer. However, they suffer from a number of potential drawbacks. Firstly, the filament is very narrow: typically 1.5 mm, although lamps with 3 mm wide filaments are also available. This is generally too small to fill the field-of-view of most modern, commercial radiation thermometers and will lead to significant measurement errors. In addition, the emissivity of the tungsten filament is low (about 0.4), and varies with both temperature and wavelength, and the transmission of the glass envelope also varies with wavelength. Since the lamp is calibrated in terms of radiance (apparent, or brightness) temperature at a precise wavelength (usually around 650 nm - 660 nm) these effects can be neglected provided that it is only used to calibrate radiation thermometers that operate at that wavelength. However, radiation thermometers which do not operate over such a narrow and precise wavelength band cannot be calibrated using lamps. (Further info)

Top

E21.  What is a blackbody source and what is it used for?

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. Blackbody sources, whose temperatures are known or can be measured, are therefore extremely useful standards for the calibration of radiation thermometers. An ideal blackbody source is a completely enclosed cavity held at a uniform temperature. The radiation field inside the cavity depends only on the temperature, and not on any physical property (size, shape, construction material). It completely absorbs and emits all radiation and has an emissivity of 1. For practical purposes, in order to view the radiation field inside the cavity, it is necessary to introduce a hole or aperture. Since this means a departure from the 'ideal' situation it is not possible to have a practical blackbody cavity with an emissivity of 1. However, by careful design cavities can be made with emissivities that closely approach 1. (Further info)

Top

E22.  How does a radiation thermometer work?

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. The signal from the detector can either be measured directly, or it can be converted to a temperature using a system of electronics. Filters are usually used to define the wavelength or wavelength band over which the emitted energy is measured. Many types of radiation thermometer are available for different applications. For measuring high temperatures a thermometer should be chosen that operates at a short wavelength, where the rate of change of emitted radiation with temperature is very high. However, for low temperature applications where the amount of emitted radiation is low, a broad-band device operating at longer wavelengths is required. (Further info)

Top


                                                                            .
.
.
.
.
.

.

.

.

 

.


Back