# Rosemount Pressure Transmitter 2051TG vs 2051TA

Q

#### QUANGNGUYENVAN

Dear Member control.com
I could like to asking you the question about the 2051TG and 2051TA of rosemount pressure transmitter.

Here is the spec of them

2051T In-Line Pressure Transmitter
Model Measurement Type
G Gage
A Absolute
Code Pressure Ranges (Ranges/ Min. Span)
2051TG1/2/3/4/5
-14.7 to 30 psi/0.3 psi (-1,01 to 2,1 bar/20,7 mbar)
-14.7 to 150 psi/1.5 psi (-1,01 to 10,3 bar/103,4 mbar)
-14.7 to 800 psi/8 psi (-1,01 to 55,2 bar/0,55 bar)
-14.7 to 4000 psi/40 psi (-1,01 to 275,8 bar/2,8 bar)
-14.7 to 10000 psi/2000 psi (-1,01 to 689,5 bar/138 bar)
2051TA1/2/3/4/5
0 to 30 psia/0.3 psia (0 to 2,1 bar/20,7 mbar)
0 to 150 psia/1.5 psia (0 to 10,3 bar/103,4 mbar)
0 to 800 psia/8 psia (0 to 55,2 bar/0,55 bar)
0 to 4000 psia/40 psia (0 to 275,8 bar/2,8 bar)
0 to 10000 psia/2000 psia (0 to 689,5 bar/138 bar)

We are normally use 2051TA for vacuum pressure measurement. Our power plant, condenser vacuum pressure is using 2051TA1 (range 0mmHgA to 760mmHgA).

Refering to above spec, the 2051TG1 and 2051TA1 have same range so my question is can I using 2051TG1 instead of 2051TA1 (in case 2051TA1 failure, and I have spare part of 2051TG1 only) for vacuum pressure measurement? What is the merit and demerit?

Thank you very much.

#### Carl_E

The TG1 is a gauge pressure transmitter.
The TA1 is an absolute pressure transmitter.

Substituting a TG1 for a TA1 of equivalent range will require ranging the 4-20mA so that the receiver interprets the signal properly.

There will be a barometric pressure error because the TG1 references atmospheric (barometric) pressure which changes hour to hour, whereas the TA1 references absolute zero (or as close as manufacturing can manage) which does not change hour to hour.

The error will be the difference between whatever Rosemount considers sea level Standard pressure for 1.000 bar absolute, (probably something close to 750mmHg absolute) and whatever the barometric pressure is at measurement point.

If both TA1 and TG1 xmtrs were installed on the same pressure tap at a steady, applied vacuum of 0.500 barA, each will read (bar_) according to its reference:

Baro. 1.000A 1.040A 0.960A
TG1 -0.500G -0.540G -0.460G
TA1 0.500A 0.500A 0.500A

The error could be corrected by periodically applying offset values to correct for barometric pressure changes.

Q

#### QUANGNGUYENVAN

From looking outside both 2051TA and 2051TG look the same, I mean they have only one port process connection only. The L reference port might be already seal by itself inside by MFG.

If so, then 2051TG might not cause the error as you mentioned above.

Any merit demerit about the using 2051TG instead of 2051TA for abs pressure measurement???

Thank you very much.

#### Carl_E

Internally, the TA1 "low" side port is evacuated to a hard vacuum (absolute zero) and sealed.

Internally, the TG1 "low" side port is open to the atmosphere, but you can't see it from the outside.

The merit issue is for gauge pressure is the relative ease of calibration checks, compared to absolute.

The demerit issue is whether the error contributed by barometric pressure changes for gauge pressure values (compared to the absolute values you've been using) is tolerable for your process.

Q

#### Quangnguyenvan

Thank you very much for your clarify.

If the TG, low side open to atmospheric then I agree that will cause the error when barometric changing.

Thanks

B

#### bohdand

For condenser measurements you should use the absolute pressure transmitter. Also be very careful with your impulse line design, since it is very easy to have condensate form in the impulse lines and cause erroneous readings.

#### dwpatterson

Hi,

No you cannot use a gauge transmitter. The min zero on the gauge transmitter is 0 psig or 14.7 psia. For a vacuum application you need an absolute transmitter.

#### Carl_E

> For a vacuum application you need an absolute transmitter.

Not always, in fact most vacuum applications are gauge pressure vacuum referencing atmostpheric pressure. A condenser application might well be better served with an absolute pressure measurement reading as opposed to a gauge pressure measurement but both gauge and absolute measurements can be made in vacuum ranges.

> The min zero on the gauge transmitter is 0 psig or 14.7 psia.

The minimum LRV for the Rosemount 2051 gauge pressure transmitter (and a bunch of other brands) is -14.7 psig, definitely a vacuum value, as the Rosemount 2051 spec sheet for the 2051T G 1 model on page 11 states:

Pressure Range: -14.7 to 30 psi (-1.0 to 2.1 bar) (and others)

Q

#### QUANGNGUYENVAN

I totally agree with you Carl_E

>> For a vacuum application you need an absolute transmitter.

> Not always, in fact most vacuum applications are gauge
> pressure vacuum referencing atmostpheric pressure.

---- snip ----

J

#### Jmtyler

Dear QUANGNGUYENVAN

My company represents Rosemount in distribution as well technical support. With any questions concerns please feel free to email
[email protected]

Q

#### QUANGNGUYENVAN

Thank you very much.

My plan 1200MW, using a lot of rosemount transmitter. We meet the serious problem with Rosemount 2501. I will send your email, you might help.

Best regards

R

#### Roy Matson

The company I work for makes Acid concentrators flashing off weak acid into a vacuum. This is similar in a way to a condenser.
I just spoke to the process engineer he told me the process is effected quite significantly by an absolute change of 0.5 kPa (2"WC). That's the reason we use Absolute rather than gauge for Vacuum applications, so it's not effected by weather changes.

The engine control unit in your automobile also uses an absolute transmitter to weaken the fuel mixture as you drive up to higher altitude otherwise it would get too rich as used to happen to carbureted engines.

You certainly could use a gauge pressure transmitter as a backup with the LRV set negative URV set to zero.