LumaSense Technologies M67 - Manual (Page 37)

M67, M67S Manual
36
10.1.7 Stabilization
If coolant is used, it should be allowed to circulate for at least one hour before readings are taken. Coolant temperature leaving
the jacket should be kept below 105°F (41°C) and above the dew point at all times, and always within the normal ambient
compensation range of the INFRADUCER. Also, the electrical circuits should be connected for at least half an hour to allow
amplifiers to stabilize.
10.2 Current/temperature relationships for linear output
If a temperature indicator is not used (for example, when instrument readings are input to a process controller) the temperature
may be calculated from the following relationship:
T T
mA T T
R
 
 
1
2 1
4
16
( )( )
where: T
R
=Target temperature
T
1
=Beginning of INFRADUCER temperature range (at which the base current of 4mA flows)
T
2
=end of INFRADUCER temperature range (at which the maximum current of 20mA flows)
mA =milliampere current reading.
The milliampere reading for a given target temperature may be found from:
mA
T T
T T
R 



16
4 1
2 1
( )
EXAMPLES: (It is assumed that the emissivity has already been determined and set).
(1) The output of the INFRADUCER reads 11mA. The range is 200 to 1000°F.
(11 - 4 / 16) (1000 - 200) + 200 = 550°F
(2) The target temperature is known to be 650°F. (Same range as above). Using formula 2, the output current is:
(650 - 200) / (1000 - 200) (16) + 4 = 13mA
Some recorders, indicators and controllers may be calibrated 0 to 100%. Converting these to temperatures is done as follows:
T
T T
T
R


 (%) 2 1
1 100
EXAMPLE:
(3)
An indicator reads 45%. The INFRADUCER range is 200 to 1000°F.
45 / 100 (1000 - 200) + 200 = 560°F
10.3 Operating caution
Special note when operating the INFRADUCER with closed loop control systems: It is very important to understand that the
optical path is part of the feedback loop. If the field of view of the INFRADUCER is obstructed temporarily, the control loop is
opened, which could be hazardous to the process control. For instance, if a cooler object blocks the field of view, the controller
will receive an input temperature signal that represents the temperature of the blocking object. In a typical control loop, this will
cause the controller to call for more heat until the final control element reaches its full open (or highest output) position, or until
the optical path is again restored. Extreme care must therefore be taken to ensure this part of the control loop is not interrupted.
If this is impossible, two options should be considered:
(1)
limit the maximum output from the final control element to a safe value, and
(2) install high temperature alarms on an independent temperature measuring device.