Foxboro Differential Pressure Transmitter

[PaulZ]

Hi All
We are using Foxboro Differential Pressure Transmitters IDP10 as level controls on accumulator / surge vessels on Ammonia.
We have followed the instructions supplied and what we could get off the Net but are having problems with the readout.
The problems are with the controls that sit below the liquid level of the vessels.
We are using the low entry point on the control column as minimum level and the high entry point as the maximum level.
Referring to the info in the Universal Instruction Manual we are using the instructions for a transmitter connected to a closed tank with a dry leg.
We have measured the distance between the entry points and this is our range 0% to 100%, X and multiplied by the SG of NH3.
There are 2 or 3 sight glasses on the columns and we have calculated the % level of these glasses.
We have measured from the lower entry point to the transmitter this is our Lower Range Value (LRV) Y and multiplied by the SG of NH3.
We have added X and Y to get our Upper Range Value (URV).
Once all the has been inputted the readout on the control does not correspond with the calculated levels at the sight glasses.
We have tried several different setting but with no success.
Has anyone had experience with these controls and if possible point out what we are doing wrong.
Any assistance will be greatly appreciated as supplier has not been able to help.
Regards
Paul

[Magoo]

Hi Paulz
am I missing something here but you said differential pressure controller, in a liquid column the pressures would be constant if vented top and bottom. Perhaps after scaling instrument close the top entry point so the sensor can react to pressure changes from bottom entry into column.
Sorry no experience with Foxboro system , only the Hansen magic stick system. And they are a pain to operate on ammonia.

[HVACRsaurus]

What a pain.

Some devices have a calibrate sequence where you bring the level to your minimum desired & push the button to say "this here is 0%" then bring the level to maximum & push the button to say "this here is 100%".

Not sure if yours does this, also real hassle to make control column full of liquid...

Ps, you probably already have, I'd suggest contacting Foxboro in the USA..

[HVACRsaurus]

Another thought;

Does the device know that it has one "wet" leg & one "dry" leg, ie the impulse line at bottom of column will be full of liquid whilst the impulse line at top of column will only have vapour..

Perhaps it's calculating for both legs same, possibly wet.

[PaulZ]

Magoo
These detect the pressure created by the weight of the liquid on one side of a sensor, the other side has gas at the same pressure.
HVACR
We have know levels that we can check calibration but these don't correspond to what we read on the control.
The control has a H & L connections, H = High Pressure or wet side and L= Low Pressure or dry side.
We haven't contacted Foxboro in the US yet hoping to get some answers here first, much quicker.
We have gone through all the possible ways of setting with no joy.
Regards
Paul

[HVACRsaurus]

Paul,

Can you give information regarding the readings you have?

Say, what is the controller showing when

Control column empty
Level at 1st bulls eye
Level at 2nd bulls eye
Level at 3rd bulls eye

Also, estimated opinion on expected readings relative to bulls eye position

Eg.

Control column empty expected 0% (obviously)
Level at 1st bulls eye expected 25%?
Level at 2nd bulls eye expected 50%?
Level at 3rd bulls eye expected 75%?

[RANGER1]

Paul we use a lot of this type & apparently if programming you have to install a 250 to 1000 ohm resistor in either connections to controller. They usually are programmed off line with resistor.
If using on a high pressure liquid receiver, fill vent line with oil & program as per instruction to compensate.
This prevents condensation in the line giving crazy readings.
Maybe your problem?

[RANGER1]

Paul we use a lot of this type & apparently if programming you have to install a 250 to 1000 ohm resistor in either connections to controller. They usually are programmed off line with resistor.
If using on a high pressure liquid receiver, fill vent line with oil & program as per instruction to compensate.
This prevents condensation in the line giving crazy readings.
Maybe your problem?

[RANGER1]

http://resource.invensys.com/instrum...mi_idp10-a.pdf

Page 40 250 ohm resistor

[PaulZ]

Ranger
Thanks for the link it's a different manual to the one we have. We are going to try the resistor and also we think the cable may be run next to power cables, the control cables are not shielded.
HVAC
I can't give the info you asked for as I am not in Adelaide and the project engineer will contact Foxboro in the US if we can't get it sorted by installing the resistor and sorting the control cable positioning.
Paul