Presure transducer 4-20ma span

[qwerty2]

I am a electrician and work in a Hospital . I have studied instrumentation but have a small amount of experience in the field and need some help in choosing a pressure transmitter for a steam boiler . The current situation is a gas steam boiler that operates at 800 Kpa the engineer has asked for the PID controller BTC9090 4-20ma input and output, to modulate the pressure over the range of 850 Kpa to 750 Kpa or better. The current pressure sensor used for the input to the controller is a GEMS 2000BGB1601A10A which has a range of 0-16 Bar and 4-20 ma with a accuracy of approx. 1% . I think this pressure sensor is not suitable .

The range of control desired is 100 Kpa so the pressure sensor will have a output of 1 ma over the 100 Kpa range . The pressure sensor's accuracy of 1% over 16 Bar would give a resolution approx. 6.25 over a range of 100 Kpa .

I thing this is not good enough for good PID control .

Is there a pressure gauge that can withstand a pressure range of approx. 0-10 bar but have a 4-20 ma output when the pressure is in the range of approx. 700Kpa to 900kpa
This would give the controller input greater resolution and hopefully better control.

The other pressure transducer that may work is one with an adjustable span , but I am not sure they exist .

I an not going to be the one to make the modifications to the steam boiler because of safety regulations etc. but I am looking for info to make sure the contractors doe it right .

Any advise on the above would be appreciated.

Best Regards
qwerty2

 

[BrianG]

Hi qwerty2. Your calculations seem to be correct in that the change in signal will be 1mA over the range 7.5 to 8.5 bar (750-850kpa), however make sure you do not confuse accuracy with resolution. Although the quoted accuracy may be 1% the resolution can be much higher if the device is linear, unless the transducer has discrete steps or jumps in its repsonse.

If you can't change the pressure transducer you may be able to expand the relatively small 1 mA signal range with a good quality instrumentation amplifier. This would allow you to compensate for the non-zero range offset and produce a useable signal for the PID controller.

 

[Grommit]

Your term "modulate" - is this just terminology or am I missing something - I would expect to control or "regulate" the pressure, modulating the firing to maintain this under varying loads - presumably this is what you are doing.

I would have thought that to modulate the fuel gas in order to maintain 800 kPa +/-50kPa would be a doddle - hardly requiring a three-term controller, P+I should be ample.
The prop gain (or prop band) will allow your 1mA variation to give a full range output if required, but the 2 or 3 term controller needs setting properly, not 'ranging'.

As you ask, pressure transmitters with adjustable range and zero are common, try Rosemount (now Emerson), model 2088-G2 or 2088-G3 - these will easily cope with 10Bar and recalibrate to much smaller ranges

Avoid SMART transmitters - £1000 communicator to replace a 50p screwdriver!