sensor output signal preferences mV or mA?

[themroc]

I am in the process of purchasing a pressure sensor to measure differential pressure.

Can someone explain the pros and cons for the different output options. (Especially with respect to accuracy)
The sensor I intend to by has a 2 to 10 V output signal.

Constarins:
- Distance Sensor to data acquisition about 5m
- Pumps controlled by frequency inverters are in a distance of 2m to the pressure sensor, does this have an impact on the output signal to choose. Or does screened cable prevent any disturbance?

 

[JLSeagull]

The impulse tubing between the two connections may impact the accuracy nearly the same as the precision of a modern pressure transmitter. We use 4-20 mA in industry because of compatibility with measurement systems, electrical hazardous areas, weather protection, rangeability, etc. Raw mV sensors may be used for downhole drilling tools for compactness but are uncommon for plant measurements.

 

[themroc]

JGSeagull, what do you mean with impulse tubing....?
Do you have any idea which signal is more sensitive to disturbences by Frequency inverters mA or mV?

 

[JimCasey]

Millivolt signals are noise susceptible. Local emf fluctations are picked up on the signal wires, voltage loss due to resistance causes error too.

Milliamp signals do not pick up ambient noise in the same way, and current is constant at every point in the circuit.

 

[Michael2006]

The impulse tubing is the piping that connects each side of your differential pressure transmitter to the part of the process over which you're trying to measure the differential pressure, for example on the upstream and downstream sides of a filter or over an orifice plate used for flow measurement.
If your medium is a gas or vapour the impulse tubing should rise up to the differential pressure transmitter, so that any liquid or condensate can drain back to the process and not falsify the dp reading, whereas if your medium is a liquid the differential pressure transmitter should be below the process, so that the impulse tubing is always filled with the liquid medium.

 

[themroc]

Michael thanks for the info. How would you install the tubing in case you are measuring diff pressure in a vertical tube.

 

[Michael2006]

The same principle would apply in a vertical line:
If your medium is a liquid, locate the pressure transmitter below the lower pressure tapping. This keeps the impulse tubing full of the medium. If the medium is vapour/gaseous, put the transmitter above the higher pressure tapping so any liquid or condensate can drain back into the process line.

 

[JimCasey]

What Michael says is true and correct.

If the DP cell is installed with the taps over and under, there will be an opposing signal developed by the weight of the cell fill and mechanical components. If the process fluid is a liquid the net correction will be small. If the process fluid is a gas it will be the signal required to support the weight of the dp cell components.

If the dp cell is installed with its connections horizontal, then the elevation change between the two process taps will need to be subtracted from the signal. This would typically be about 3 inches of water column, x the density of the process liquid.

 

[themroc]

Thanks Jim,
I don't quiet understand. In case I mount horizontally the two tap connections at the sensor have the same hight, so I can not see why I should substract anything.
How do you derive the 3 inches of water column, The sensor I intend to use (from Druck) has a cylindrical horizontal body:
(

http://www.gesensing.com/downloads/datasheets/920_214b.pdf)

I also do not understand wy there should be an opposing signal developed by the mechanical components. The sensor will be fixed and the weight will be "carried" by the fixation?

Can you please elaborate.
Thanks

 

[danw2]

Assume the flow is upwards. One tap, the upstream tap, must be at a lower elevation than the downstream tap, right?

If your impulse tubing runs horizontally from each tap, each impulse tube will be separated by the distance in elevation between the upstream tap and the downstream tap. If filled with liquid, this difference contributes to the physical signal impinging on the DP cell. At some point the impulse tubing has to connect to the DP cell.
If you mount the DP cell horizontally so the high side impulse tube is straight horizontal from tube to DP cell, the higher elevation downstream impulse tube must have a bend in it to 'lose elevation' in order to connect to the horizontal DP cell.

That change in elevation makes a difference with liquid filled impulse lines.

If the DP measurement is 600psid, a couple inches of liquid in the impulse lines is not signficant.
If the DP measurement is 1.0 psid, (approx 28 inches of water) the 3" of elevation change is > 10% of the measurement, quite signficant.

re: derived 3" water column
I suspect that figure comes from the industry standard mounting dimensions of industrial DP transmitters, which have each 1/4" NPT port on 2 1/8" centers (54mm).

 

[JimCasey]

Dan covered the question nicely. I was speaking of the possible pitfalls of a vertical installation. AND if you tip the TX on its side, the components in the cell will displace downward and introduce a small signal that will need to be eliminated with the zero adjustment.