Remember
the truism that all sensor have errors in their readings - all the time. One key
secret to high quality measurement results is to have confidence in the error
estimates. Neglecting to make a careful error analysis can result in error much
larger than the assumed values.
It is worth noting that all competent error
analyses start with the uncertainties assigned to the traceable calibration of
the sensor itself. Without traceable calibration, one is forced to make assumptions.
(You know what the word ass|u|me means, we hope.)
Without traceable measurements, the numerical values of results will always be questionable and hardly worth the effort, and cost. It most often pays to get started on the right path to technically sound measurements by beginning with some understanding of the options involved in selecting a temperature measurement device and then in obtaining one that meets the expected conditions and standards, is calibrated and that the calibration is traceable to either a fundamental standard (e.g. the triple point of water) or a national standard. See our calibration and standards pages for more details on each aspect of sound measurement practice.
Contact
Sensors
Contact temperature sensors measure their own temperature.
One infers the temperature of the object to which the sensor is in contact
by assuming or knowing that the two are in thermal equilibrium, that is, there
is no heat flow between them.
Noncontact Sensors
Most commercial and scientific noncontact temperature sensors measure the thermal
radiant power of the Infrared or Optical radiation that they receive from a known or calculated area on its surface, or a known or calculated volume within it (in those cases where the obect is semitransparent within the measuring wavelength passbad of the sensor).
One
then infers the temperature of an object from which the radiant power is assumed
to be emitted (some may be reflected rather than emitted). Sometimes the
inference requires a correction for the spectral emissivity (NB: the two words, spectral & emissivity,
are used together in correcting IR Thermometer readings -the "emissivity", unspecified, is a big trap which even some of the suppliers of devices and calibration equipment fall into unwittingly for a variety of reason about which one can only speculate ) of the object
being measured.
Knowing how and when to apply a spectral emissivity correction
is part of the inference, too, and can introduce significant errors if not done
correctly. See our Trip down the E-missivity Trail to
help you understand that aspect a little better.
Dewpoint
Temperature
-- Humidity--
Although this area is in reality just an
application of temperature sensors and other sensors, it grew out of temperature
measurements.
Remember the old style humidity indicators that consisted
of two little glass thermometers, the wet and dry bulb thermometers with a little
look up table that told you the humidity, both absolute and relative? Have a look,
it's a very important area in terms of human comfort,
food safety and energy conservation and efficiency in
thermal processes.
Thermal Imaging
The special world of thermography and thermal images includes
the temperature-measuring kind of thermal imagers called "Radiomatic",
by those in the business, and "Quantitative" by those mostly in R&Dwith
thermal imaging. Then, too, there are those who call it "Thermology"
when it applies to measurements made on the human body and "Medical Thermography"
by still others, some even in the same business.
Users of infrared thermal
imaging have many options in cameras both with and without temperature scales
or temperature indication.
It seems really odd to have all these different
names kicking about, when they all refer to the same basic technology. The names
seem to differ only by application area. In reality, they all work because of
the same Law of Physics, called Planck's Law.
That's the same law that describes
how IR thermometers, optical pyrometers, radiation thermometers and infrared intrusion
or people detectors work (note the common trait of multiple names).
The
only thing that an IR thermal imager of any denomination really does is take the
output from an infrared detector, or plethera of detectors, and presents a 2-D
scan of the infrared intensity distribution in the field of view of an optical
system. These devices could be called by one common name. The devices that provide temperature information, probably more than any other type of device should
be called Infrared Imagers, or Thermal Infrared Imagers or, simply, Thermal Imagers.
Go to our thermal imaging section by clicking the above underlined
link and learn more than you ever thought you would want to know about the subject.
Click here
to find out about ISPoT!
The Applications
page can lead you to many well-known solutions or examples, possibly similar
to the one you are trying to solve. Why re-invent the wheel?
Two excellent
reference by Baker et al. are listed in the References
page and worth reading to get an idea of the complexities that can arise,
how to test and get around them. They are older books and while the technology
of the equipment has changed, especially the electronics, the measurement fundamentals
have not. Heat flow is heat flow and thermal radiation physics was unified theoretically by Max Planck more than 100 years ago!
A great many temperature measurement
problems are solved through a good understanding of the heat flow involved in
a specific measurement situation.
Surface temperature problems with contact
sensors are often best solved in many cases through the use of non-contact sensor.
They are in use in many industrial plants worldwide in great numbers. The above reference texts provide interesting analyses of the likely errors making contact temperature measurements of surfaces, both stationary and moving surfaces. We have not seen any recent analyses with as much detail!
Good
luck and best wishes.
If you have some interesting success, let us know and we'll help you share that with others who visit these pages.
