You produce cryogenice fridges in your factory. You mustdesign a control system for the fridge, but, using the same sensor,must also supply the end-user with a temperature monitoring. Unlikea commercial freezer with a somewhat daunty electromechanicalthermostat, a cryo freezer is controlled in a tight band aroundminus 85oC.
Therefore, in controlling a cryogenic fridge, you need to have themost accurate information available that you can get around theoperating range of the fridge (nominally at -85oC, butin monitoring the fridge, it can run away (colder than-85oC) or defrost if faulty and reach ambienttemperature (say max 50oC to be safe).
Due to the economy of RTD sensors, it is decided by management touse a PT100 RTD sensor (range -400 oF to 1200oF) with a smart transmitter and it need to be set upcorrectly to a 4-20mA input on the PLC, controlling thefridge.
As motivated above, the measuring range should be -100oCto 50oC (which should cater for the worst ambienttemperature).
(i) Make sure the RTD is the correct choice,
(ii) set the smart transmitter up correctly (span, offset,type),
(iii) find the straight line equation (in the form y = ax+b, y inmA, x in oC) for the temperature in oC to mAoutput of the smart transmitter,
(iv) test your equation at the extremes (-85oC to50oC)
