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Instruction Manual MM850341 ed.01a
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3.2.1
Optical system
The detector is positioned at a focus distance of a high quality lens. The lens forms an image of the source in the
plane of the detector. The size of the cone of radiation accepted from an axial point of the target and, the
irradiance at the detector, is determined by the clear aperture of the lens, while the active area of the detector
determines the field of view. The lens and detector then act as the aperture and field stop of the system
respectively.
3.2.2
Detectors
Thermopile
A thermopile contain a large number of thermocouples in a very small area. The thermocouples are connected
together and the output is the difference between the ambient temperature (cold junction) and the temperature
collected by the optical system (hot junction).
Inspite of large number of thermocouples, the signal output is very small: few
µ
V/°C. The response time of the
detector is very long because the detector must be warming up (or cooled) by the collected energy.
The waveband is determined by an additional optical filter chose by Eurotron for the specific application. This
solution permits to reduce disturbs due by atmosphere absorption and to maximise the output signal.
Photodiode
Both Silicium and InGaAs photodiode principle is completely different from thermopile: the collected energy is
transformed in a electron flow. The result is a current proportional with the incident energy.
This transformation is very fast (only few
µ
S). This time the response time of the measuring system is limited by
electronics: high resolution and low energy consumption, make it a little bit slower.
The waveband is determined by the physical characteristic of the sensor.
Ratio detectors
Special IR applications could not be solved using the standard monochromatic thermometers. A ratio detector
could be the solution.
Typical problems which will cause an incorrect reading include:
•
small objects e.g. to small to fill the target area
•
smoke, dust or steam which obscure the line of sight
•
dirty windows in the process
•
emissivity of the product changes (due to changes in alloy or surface condition).
A ratio detector is a single chip twin detector with different working wavelengths. The ratio of the two simultaneous
measurements is independent from emissivity and the result is the correct target temperature.
Every ratio thermometer has a limit as to how much signal can be lost. This is referred as the reduction ratio that
can vary up to 90% without effecting the measurement accuracy. To be also kept in consideration that the loss in
signal can come from 3 sources:
•
low emissivity of the target
•
object too small to fill the cone of vision
•
obstruction caused by steam, smoke, dirt and dirty windows
In all the above condition a ratio thermometer will operate with better accuracy than any monochromatic
thermometer. The IRtec “Rayomatic” thermometers was designed to operate with spectral bands with limited
temperature coefficient.
3.2.3
Input stage
Is the most important part of the thermometer. It should be able to interface the detector to the micro-controller.
IRtec Rayomatic series uses different kind of detectors (thermopile, photodiode, and ratio detectors) with different
operating specifications.
•
Thermopile models needs electronics with high impedance input and very low drift.
•
Photodiode models needs fast electronics.
•
Ratio models needs electronics able to drive a double detector.
For the best modularity of the measuring system, Eurotron has developed special configurable electronics to meet
all requirements.
