Infrared Sensor System for MRF 16/22
Introduction:
Infrared sensors work by measuring the amount of Infrared radiation that is emitted from the
surface of an object and then adjusting the measured reading by the emissivity value of the material.
A perfect black body material which is a material that emits Infrared perfectly from its surface has a
value of one; if the material does not emit infrared as efficiently as a black body then it will have a
lesser value of say 0.8 or 0.5. The measured value of surface temperature by the IR sensor is then
adjusted to take this factor into account.
IR sensors are designed to look at certain wave lengths of Infrared radiation. The full spectrum of
infrared does not transmit through air, certain wave lengths of infrared will transmit through air;
unaffected whilst other wave lengths do not transmit at all. It is for this reason that IR sensors are
designed to work at given wave lengths where there is good IR transmissibility.
These waves are 1.0um, 1.6um, 2.3um, 3.9 to 4.6um and 8 to 14um as can be seen in Figure 1
below
Figure 1. Infrared Spectrum
The IR sensors measure the intensity of IR radiation at these wave lengths and calibrate them to a
temperature.
The amount of IR radiation at a particular wave length for a given temperature varies. So at the
shorter wave lengths (1.0 to 1.6um) very little IR is given off at temperatures below 385°c making
accurate readings difficult ,but there is a lot of IR at temperatures above 385°C up to 1800°C.
At the longer wave lengths, say 8 to 14um , far more IR is emitted from an object from ambient up to
900°C and very little after that , so making temperate measurement difficult above 900°C.
An IR sensor cannot work from ambient up to 1800°C because the transmitability of IR through air
makes it impossible.
The Carbolite IR system:
The IR sensor has a built in LASER sight to align the sample so that the sensor focuses onto the
sample surface. The controller then can be set up to include an offset in the controller of say 5°C for
the microwave control, so if the set temperature is 1200°C for the heating elements, then the main
chamber would reach 1200°C and maintain that temperature. The surface of the sample may only
reach a maximum of 1195°C without microwave heating; with microwave heating the surface
temperature would exceed 1200°C, so the microwave power would be adjusted by a signal from the
IR sensor focused on the sample surface to maintain a surface temperature of 1205°C (1200°C + 5°C
off set), (see figure 1). However, this would not indicate what is happening inside the sample, but
the power consumption (as a %) could be data logged to give an understanding of the power
required. Testing of the sample afterwards would show the effect it had had.
1195°C
Temperature Signals
Temperature
Controller
IR Sensor
1200°C
LASER sight
Heater Control Signal
1200°C
Microwave
Control
Signal
1205°C
Heater
Elements
Thermocouple
IR Light to
Sensor
Magnetron
Microwave
Heat
Radiant
Heat
Sample
Figure 1. Control circuit schematic for a MRF furnace with a IR sensor controlling microwave power.
Note:


if the offset is set at 0°C then the temperature would be maintained at 1200°C
if the offset is set at 25°C then the temperature would be maintained at 1225°C
This gives the flexibility to be able to either monitor the surface temperature for a given microwave
power level, or control the microwave power to maintain a desired surface temperature.
This would also work on a temperature ramp rate, so as the furnace heated up from say 800 °C to
1200°C over 40 minutes, the surface temperature of the sample would not exceed the offset
temperature difference with reference to the control thermocouple
The type of tests that can be carried out:
There are 3 tests that can be performed with this arrangement:

The testing of a sample with no microwave power but IR monitoring of the surface
temperature (with data logging of the temperature).
 The testing of the sample with microwave power pre-set at a given power level for a given
time, with the surface temperature being monitored by the IR sensor (with data logging of
the power and temperature).
 The testing of a sample with microwave power being controlled to a set surface temperature
by the IR sensor (with data logging of the power and temperature).
From these 3 tests the effect the microwave has had can be determined and the microwave power
required to obtain the desired result.
Note:


The Nanodac controller/recorder does this as standard. The results can be viewed using
Excel, or alternatively Eurotherm’s iTools or a suitable data logging software package can
be used.
The data logged power will be as a percentage of the maximum power available so
conversion of this using spread sheet software such as Excel would be required.
Potential heating problems:
Under some test conditions a sample can heat up above the indicated temperature on the
temperature controller due to direct IR heating from the elements. This is a common problem in any
type of furnace, but as most furnaces do not have an IR sensor then this goes unnoticed by the user.
To overcome this problem and to give better heating control of the sample, it is recommended that
heating baffles are used to shield the small samples from the direct effect of IR emitted from the
elements. Figure 2 shows a simple heating baffle arrangement using high temperature insulation
board.
Baffle
Thermocouple
sample
Elements
Figure 2. MRF furnace with simple heat shield to protect a sample from direct IR heating from the elements.