Scaling a Multi Unit Pressure XMTR. How?

skyfox

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Nov 2006
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Good day all,

How does one go about scaling a pressure transmitter that indicates both Vacuum and pressure? The transmitter I have has a range of Vacuum to 75 PSI. I am trying to wrap my head around scaling this with an AllenBradley 1768-IF8 AI card. Application in which this transmitter is used, exposes it to both Pressure and vacuum conditions. So, if my Scale max value is 75psi what is my scale Minimum value? Instead of psi it will be mmHg is what I am guessing. Or, is it 0 psi? Can anyone point me in the right direction?

Thanks!
 
Then you probably have a 0-75psia transmitter. A as in absolute pressure. Atmospheric pressure is 14.7psi. That's about what it will show when measuring your surrounding room.

If it would be a psig type transmitter, the range on it would say something like -10 to 75psi.

Do you have a model number? The data sheet would tell more. Alternatively, what does it say on the transmitter? Does it say psia or psig?
 
Last edited:
Hi Dirt,
Thanks for the reply. I do not have a model # at the moment. But the marking said 0-45 (not 75 my mistake) PSI (without the "A"). I am guessing "A" or absolute is implied here.
 
On the transmitters I've used, mostly leaving out the a or g (but these are speced in bars or Pa) means it's a g-type. G-types are more common, at least in my field. And in that case it's not a transmitter for vacuum at all.

But this is all guesses. We need to know what transmitter it is to be sure.
 
It is not unusual for a pressure transmitter to have a range that measures both positive and negative pressures, but I can't answer your question if you don't know the actual measurement range of your instrument.

A typical scenario would be a transmitter that measures say, -10 to +10 psig. I would expect a 4 mA output at -10 psig and a 20 mA output at +10 psig for such an instrument.

I wouldn't think that you'll ever encounter a transmitter that measures positive pressure in one unit of measurement and negative pressure in a different unit of measurement.
 
Your scale would simply be in psi either from -psig to +psig in the case of a PSIG meter or 0 to max psia in case of a PSIA meter.

if you want to show it in mmHg for the vaccumm portion you could use a multiplier on your scaled psi tag since there is a linear relationship between mmHg and psi.

or if just for display do the scaling on the graphical object
 
Your analog input scaling depends on what the tranmitter's 4-20mA scaling is.

The 4-20mA output of the pressure transmitter defines the range that you scale your input to.

If transmitter's 4-20mA is ranged 0-45psig, then an applied vacuum will read slightly less than 4.0mA until it bottoms out at 3.75 or 3.8 or whatever that unit is capable of.

If the transmitter's 4-20mA is ranged -10 to 30psig, then 4.0mA = -10 psig, 8.0mA = 0psig (atmosphere), and 20.0mA = 30 psig

Again, some pressure transmitters are capable of vacuum/pressure, others are not. Depends on the model.
 

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