Understanding Absolute Pressure

M

Thread Starter

Matt

I am trying to figure out how to go about calibrating this transmitter we have. What we have is a Rosemount 1151 Absolute pressure transmitter that is measuring the condenser pressure in relation to barometric pressure. The Low side of this transmitter is sealed with no process connections. Our process connection is on the high side tied to a pressure tap on our condenser. The range of this transmitter is 0-32inHg = 4-20mA respectively. Currently the barometric pressure according to the local airport is 30.05inHg which is close to what we are reading in the control room. The problem I am having is that I have no means of drawing a vacuum of greater than -30.05 on the high side of this transmitter. It would be an easy fix if there were a process connection on the low side. Any suggestions? I have never come across a transmitter used in this application before and nobody around here seems to know much about it. They say that it has caused problems in the past with how to calibrate it.
 
R
There is no connection on the Low side because it's under vacuum, if you cracked that you would never get it back again.

Suck it down as low as you can then adjust the zero to give the mA equivalent of that pressure (not 4mA). Then break the vacuum and adjust the span to give you the mA equal to the pressure at airport.

Hope this helps
Roy
 
C
> I have no means of drawing a vacuum of greater than -30.05 on the high side of this transmitter.

Don't feel left out. No one has any means of drawing a vacuum greater than 30.05"Hg absolute when the barometric pressure is 30.05"Hg. The 30.05"Hg you reported as the airport barometric pressure means that there is 30.05"Hg pressure above absolute zero pressure. To be able to pull a vacuum lower than absolute zero would be quite remarkable.

> It would be an easy fix if there were a process connection on the low side.

The low side is sealed with as hard a vacuum as Rosemount could pull when it made the transmitter. If the low port were open, how would you pull the hard vacuum needed to be the reference?

The real question is, what do you have that you consider accurate enough to compare an absolute transmitter with?

What's the traceable accuracy uncertainty of whatever you're making a reference measurement with?

Let's suppose Mr. NIST stops by and looks at whatever gauge vacuum meter you have and says, "yeah, that's cool" and gives you a thumbs up.

Then you connect up your vacuum pump and pull, say, 29.85"Hg (gauge) on the high side as measured by your trusted Mr. NIST approved vacuum meter reference. You then check your current airport barometric pressure (absolute) which is reported as, say, 30.37"Hg.

So, the 1151 should indicate 0.52"Hg absolute: 30.37"Hg (barometric absolute, referenced to absolute zero) minus 29.85"Hg (gauge vacuum, referenced to atmosphere).

If the transmitter indicates very close to a trusted barometric reading when the high side is open to atmosphere, then the transmitter shows little absolute offset. If the transmitter shows an offset at barometric conditions, then checking the offset at the low end (by pulling a vacuum) will show you whether the error across the range is just an offset or a skew.

In either case, I'd make the correction adjustments at the indicator end, not the transmitter end.
 
To check our condenser transmitters, we normally tally them with a factory calibrated new absolute transmitter by making a parallel connection online and then we make necessary adjustment with HART Comm. I guess you can also tally with a mercury U-tube manometer, however it will give you the vacuum made against the prevailing atmospheric pressure. Since we have redundant transmitters so we scan off the point for a short while before closing transmitter isolation valve for parallel connection.

BEWARE of your process logic before attempting to scan off the point or else you may trip your machine. Bypassing protection can be very dangerous.
 
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