# Measuring pressure with SITRANS P transmitter

J

#### Jas

Hello experts,

I have a question regarding measuring very small pressure with SITRANS 7MF4332-1D transmitter.
I have read how this transmitter works in general manual. However I'm not sure about one thing. I have read that this transmitter has a measuring span from 8.3 mbar to 250 mbar. This is below normal atmospheric pressure. Is it possible to calibrate transmitter in the following way: LRV: -50, URV 50 mbar? I have heard of this but don't know how to interpret it. I suspect that this value of -50 mbar means 50 mbars below normal atmospheric value of P = 1.013 bar.

I think that this transmitter has reference vacuum which is very small pressure (smaller than 8.3 mbar), and that it measures actual pressure. Can someone explain this in more detail?

R

#### Roy Matson

Hi,

I was unable to find the data sheet for 4332. Are you sure it's an absolute pressure transmitter as you describe?

If it were an absolute pressure transmitter the range would be in bar abs, barA or some variation. If its range is 8.3 mbar to 250 mbar it's likely a Gauge or Differential model. If it's gauge it's reasonable to expect to be able to offset the zero to -50. If it's differential as long as the difference LP - HP is < 250 mbar the zero point can be anywhere between absolute zero and several bar gauge.

Roy

D

#### David

Note that I'm not a Siemens guy, I'm only interpreting public catalog specs. I cannot locate a model 7MF4332, but there is a 7MF4333 model in the current catalog (pg 38) posted here:
http://pia.khe.siemens.com/efiles/i.../fi01/fi01_en_extract/sitransp_ds3_fi01en.pdf

It is clearly an absolute pressure transmitter.

Page 5 of the catalog pages shows a diagram, "Measuring cell for absolute pressure from differential pressure series", with a description of a typical absolute pressure transmitter with one side referenced to hard vacuum.

The measuring range of the device in question is
8.3 ... 250 mbar a (absolute)
(0.12 ... 3.63 psi a) for us Americans
This range is what we commonly call the 'vacuum range', below atmospheric pressure

The reference side is under hard vacuum, an hard as they pull vacuum and seal it.

Now to the issues.

1) You forgot the letter 'a' representing absolute pressure when stating the pressure units.
Units are correctly stated as 'mbar a' for absolute pressure. The units designation mbar (without an 'a') is assumed to be gauge pressure with reference to atmosphere.

2) Your target range of -50 to 50mbar reflects a gauge pressure measurement, where zero is atmospheric pressure, and the +50mbar is a positive pressure above atmosphere and the -50mbar is a negative pressure below atmosphere, both referenced to atmospheric pressure as zero. This is commonly called a compound range.

This transmitter can't even get to atmosphere. It's top end is 250mbar ABSOLUTE, a fair amount of vacuum, as most of us think of it.

Even if this transmitter could get to atmospheric and above, which it can't, you'd still have an error in the difference between atmospheric and wherever you 'fix' your pseudo atmosphere = 0 point.

No absolute transmitter can have a negative value for its LRV (like -50mbar) because physics does not allow for a pressure less than absolute zero.

If you actually need that compound range (it isn't clear whether it's academic or real), then you need a conventional differential pressure transmitter than can be compound ranged. It isn't clear whether Siemens gauge pressure DPs can be compound ranged, or not.

All this being said, there's an odd statement (translator dosed off?) on page 3, under "Mode of operation of the DS III HART electronics":

The pressure transmitters with spans ¡Ü 63 bar measure the input pressure compared to atmosphere, transmitters with spans ¡Ý 160 bar compared to vacuum.

a) Why HART would make any difference in measuring technique is beyond me (HART is just sending/receiving numbers)
b) Why higher measuring ranges would have a different reference point than lower measuring range isn't clear.
c) Is it referring to the difference between gauge pressure and absolute pressure? atmosphere and 'vacuum'?

D

#### David

Note that I'm not a Siemens user, I'm only interpreting public catalog specs. I cannot locate a model 7MF4332, but there is a 7MF4333 model in the current catalog (pg 38) posted here:
http://pia.khe.siemens.com/efiles/i.../fi01/fi01_en_extract/sitransp_ds3_fi01en.pdf

It is an absolute pressure transmitter.

Page 5 of the catalog pages shows a diagram, "Measuring cell for absolute pressure from differential pressure series", with a description of a typical absolute pressure transmitter with one side referenced to hard vacuum.

The measuring range of the device in question is
8.3 ... 250 mbar a (absolute)
(0.12 ... 3.63 psi a) for us Americans
This range is what we commonly call the 'vacuum range', below atmospheric pressure

Now to the issues.

1) You forgot the letter 'a' representing absolute pressure when stating the pressure units.
Units are correctly stated as 'mbar a' for absolute pressure.
The units designation mbar (without an 'a') is assumed to be gauge pressure with reference to atmosphere.

2) Your target range of -50 to 50mbar reflects a gauge pressure measurement, where zero is atmospheric pressure, and the +50mbar is a positive pressure above atmosphere and the -50mbar is a negative pressure below atmosphere, both referenced to atmospheric pressure as zero. This is commonly called a compound range.

This transmitter can't even get to atmosphere. It's top end is 250mbar ABSOLUTE, a fair amount of vacuum, as most of us think of it.

Even if this transmitter could get to atmospheric and above, which it can't, you'd still have an error in the difference between atmospheric and wherever you 'fix' your pseudo atmosphere = 0 point.

No absolute transmitter can have a negative value for its LRV (like -50mbar) because physics does not allow for a pressure less than absolute zero.

I think you need a conventional differential pressure transmitter than can be compound ranged. It isn't clear whether Siemens gauge pressure DPs can be compound ranged.

All this being said, there's an odd statement (translator dosed off?) on page 3, under "Mode of operation of the DS III HART electronics":

The pressure transmitters with spans = 63 bar measure the input pressure compared to atmosphere, transmitters with spans = 160 bar compared to vacuum.

a) Why HART would make any difference in measuring technique is beyond me (HART is just sending/receiving numbers)
b) Why a higher measuring range would have a different reference point than a lower measuring range isn't clear.
c) Is it referring to the difference between gauge pressure and absolute pressure? Atmosphere and 'vacuum'?

T

#### TiffyGeoffrey

Hello

Question:-
Has the transmitter one or two impulse tappings?

If one then the - side of transmitter is probable sealed with a vacuum and the transmitter is able to measure absolute pressure.

If you want the range of instrument to be -50mb to +50 mb you set the zero of the cell with atmospheric pressure -50 mb ie (1013-50) 963mba vac of 50mb applied to + side of cell. then set span with atmospheric +50mb ie (1013 +50) 1063mba +50mb applied to + of the cell.

The transmitters output will now be 4ma 963mba and 20ma 1063mba ie 12ma output for atmospheric pressure of 1013mb