Adjusting 4-20ma device

[cirtcele]

At our waste water treatment plant we have several 4-20ma pressure transmitters. My job is to check calibration of all of our level devices the brand of level transmitter we are using has no built in calibration adjustment when checking these units in a known depth of water they can be up to six inches or more off in thirty six inches of water. We are using SLC 500 with rslogix
500 the way I tried adjusting was by changing the scaled max. I not sure if this is the best way.

 

[Mickey]

Adjusting the scaled max would be a span/range adjustment. The error may be a zero error.

I would check the transmitter at both zero and its upper limt (span/range) and then do the math to correct the error(s).

 

[danw]

Some things to consider:

1) What brand/model of pressure transmitter has no calibration adjustment?

2) In relatively low pressures, like 36 inches of water column, air trapped in the impulse line can create an error. I know it runs contrary to all statements about pressure being equal in all parts of a closed system, but I've seen it happen.

That being said, you might investigate whether there is trapped air in pressure measurement line, which, if vented, could eliminate your error.

3) I doubt these are smart pressure transmitters, but smart xmtrs only have analog output trim function. The sensing part would be considered 'failed' with a 6 inch error in a 36 inch span.

4) If the analog input uses an external dropping resistor for 4-20mA inputs, any deviation from the spec'd nominal (typically 250 ohms) resistance is a direct proportional error in the reading.

5) Common mode on single ended inputs can appear as an offset with this kind of error. The analog input calibrates OK, the field pressure transmitter calibrates OK, but when connected, the signal shifts because of a common mode ground loop.

 

[cirtcele]

Span for these are 0-10 psi or 0-23 feet I'm not able to check them at 23 feet at our remote sites. The brand is Blue Ribbon

 

[Mickey]

Span for these are 0-10 psi or 0-23 feet I'm not able to check them at 23 feet at our remote sites. The brand is Blue Ribbon

Model number?

 

[cirtcele]

Bc001

 

[shooter]

always tricky, normally i use a span function to change the line in the PLC, this way i dont have to open the transmitters.
with a three way valve i can go from measurement to an external line, here is my calibration pump and meter.
i put a known low value on my set, and put the result in the PLC, same for a high value.
you are more 10% off so must be something weird, as that is very much offscale.

 

[Lemming]

At our waste water treatment plant we have several 4-20ma pressure transmitters. My job is to check calibration of all of our level devices the brand of level transmitter we are using has no built in calibration adjustment when checking these units in a known depth of water they can be up to six inches or more off in thirty six inches of water. We are using SLC 500 with rslogix
500 the way I tried adjusting was by changing the scaled max. I not sure if this is the best way.

Hydrostatics are usually fairly accurate...unless they are buried in a pile of sludge ?? If you are onsite, the zero can be checked by simply pulling them out of the water.. span is a function of where they are placed relative to the head of water above them.. You may find they just haven't been fitted accurately..

 

[cirtcele]

I place it in a container with a known water level when removed from water they show zero inches it's the incorrect level when in the calibration tube that concerns me.

 

[Lemming]

So u are finding the 6" error in the cal tube ?? Can just use the raw mA / AI counts for the given cal tube water head, and apply to the SCP at that point. The SCP is open ended, so will carry on past...
The other thing to check is is zero at atmosphere?? do a check at a couple of inches..

Is still all relative to where its hung in the actual process tank...

 

[osmanmom]

the transmitter is non built in calibration,

base on manual user BC001 for 5 psi and BC 002 for 10 psi,

 

[osmanmom]

Correction BC001.001 5 PSI and BC001.02 10 PSI

 

[shooter]

is the vent line open (check by blowing on it, and look at the meter for signal change.
are calcs oke?

 

[danw]

Condensation can build up in the vent tube and drain down the vent and collect at the bottom of the tube which is then seen and measured by 'low side' of the transmitter.

When that happens,the zero offset is negative (high side minus low side), so the indicated reading is low.

If the readings are low, condensation might be the cause.

 

[Lemming]

Condensation can build up in the vent tube and drain down the vent and collect at the bottom of the tube which is then seen and measured by 'low side' of the transmitter.

When that happens,the zero offset is negative (high side minus low side), so the indicated reading is low.

If the readings are low, condensation might be the cause.

**should** be terminated / fitted with hydroscopic filter on breather tube to prevent this...