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Active Contributor

Join Date: Sep 2008
Location: Mount Isa
Posts: 22

### Condenser Vacuum Transmitters

08/08/2012 7:07 AM

Hi,

I have an absolute pressure transmitter at work that measures 0-760mmHg or 0-101.3kPaA basically 0 to 1 atmosphere e.g sea level.

Its on the the condenser vacuum on a steam turbine.

Should this be referenced to the atmosphere E.g. 790mmHg = 20mA and 30mmHg = 4mA due to our elevation approx 30mmHg or should the vacuum transmitter be set to 760mm = 20mA and 0mmHg = 4mA ?? and read true to sea level ???

or should we subtract the 30mmHg off the span and go from 30-760 or even 0-730 ??

we are nearly killing each other at work on this one...........

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lost.instro
Pathfinder Tags: Condenser Vacuum Measurement
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Guru

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing. Kettle's on.
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#1

### Re: Condenser Vacuum Transmitters

08/08/2012 7:21 AM

A range of 0-760mmHg would encompass the absolute air pressure at that location with some margin.

Other options would tend towards a pseudo-gauge pressure reading, and are therefore not applicable in the context of an absolute pressure transmitter used on a condenser.

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Guru

Join Date: Jun 2010
Posts: 535
#3

### Re: Condenser Vacuum Transmitters

08/08/2012 9:26 AM

ditto what PW wrote. The pressure in a surface condenser is independent from atmospheric pressure.

Power-User

Join Date: Jul 2010
Posts: 398
#2

### Re: Condenser Vacuum Transmitters

08/08/2012 8:40 AM

Does the device analyzing the pressure signal expect to see absolute or gauge pressure?

Guru

Join Date: Apr 2009
Location: USA, Florida
Posts: 955
#4

### Re: Condenser Vacuum Transmitters

08/08/2012 3:45 PM

Changing your settings based on altitude would be similar to changing them again every time a cyclone comes through, due to the inherent pressure change associated with that.

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Power-User

Join Date: Oct 2007
Location: Canning Vale Western Australia.
Posts: 140
#5

### Re: Condenser Vacuum Transmitters

08/09/2012 3:49 AM

An absolute pressure transmitter is already reference to zero. This is the reason it can give you a vacuum reading that is in "absolute" terms and not in relation to the changing outside pressure of the condenser. ie: they are not effected by elevation or external pressure.

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Power-User

Join Date: Dec 2010
Location: New Orleans, LA
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#6

### Re: Condenser Vacuum Transmitters

08/09/2012 9:06 AM

If the pressure is measured with low side to the condenser and high side to the atmosphere then you need to put the elevation compensation.

If both the high and low sides are connected to the process then you use absolute measurement where the external elevation difference does not affect the process.

Active Contributor

Join Date: Sep 2008
Location: Mount Isa
Posts: 22
#7

### Re: Condenser Vacuum Transmitters

08/18/2012 8:03 AM

Thanks everyone

we sorted it out turns out it was called to 0 to atmosphere and then had the output inverted.

so it was out by 30 all the time...

reset the instrument to factory calibration and it was all good.

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lost.instro