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# Daft question about pressure transmitter range

## Daft question about pressure transmitter range

(OP)
Hi Gents,

I'm going to show my ignorance here, but hopefully you'll be gentle with me.

I believe that ideally a pressure gauge should be sized so that the needle is in the middle of the range when reading normally - i.e. if I have 5 barg pressure then a gauge of 0 ~ 10 barg range would be optimum, whereas say a 0 ~ 100 barg range would be inadvisable because of the inaccuracy of both the reading and the instrument itself at that low end of the scale.

So my question is, assuming the above to be correct, does the same apply with a Rosemount Pressure Transmitter? In the particular case in question, I have been supplied a 0 ~ 150 barg transmitter on a systekm with a maximum pressure of around 10 barg, and a working pressure of 2 ~ 4.5 psig.

My intention is to reject it, but I don't want to make a fool of myself (any more than I possibly am already in asking the question here!!!!!).

Any input much appreciated.

### RE: Daft question about pressure transmitter range

The uncertainty of the instrument is a percent of calibrated range. So if you calibrate the instrument to 0-30 barg, it will give you uncertainty based on 30 barg not the 150 barg.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

### RE: Daft question about pressure transmitter range

Airsporter1st,

Kind of yes and no. You need to find out exactly which transmitter has been ordered and what it said on the data sheet. Rosemount supply their transmitters in a set of pressure ranges. Normally the specific range is then set on site to decrease the uncertainty, e.g. if you have their standard range 3 (0-55 bar) you would calibrate the maximum value to above wha your max value is supposed to be, just so that if it ever exceeeded the max value, your transmitter wouldn't just "max out", so probably 12 barg in your example. This reduces the uncertainty in that range 0-12 bar. However if tou have a transmitter with an inital much bigger range, then this may not apply. Find the exact model number with all its digits or serial number and then give rosemount technical salses a call and describe your issue and whether a different ranged transmitter would be better or not. What you're asking is a fair question, but the best answer will come from the vendor.

ALways make sure that the transmitter is an indicating transmitter - the additional cost over non indicating is marginal, but the benefit when you're walking around a plant trying to work out what's going on is enormous and helps reduce the number of guages which suffer from the problem you mention over range.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

### RE: Daft question about pressure transmitter range

Actually re-reading your OP, i though you were operating at 2 to 4.5 barg on a 10 barg system, but its psig. You may find you actually need two auto switching transmitters to cover the 0-1 barg range and then the 0-10 barg range in order to give you enough accuracy and repeatability. This is fairly common on orifice metering where you really need accurate oressure data over a wide range of pressures.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

### RE: Daft question about pressure transmitter range

(OP)
Many thanks for the help so far. I guess I did not really make myself clear: I've been supplied a piece of kit which has a working pressure of 2 to 4.5 psig and a maximum of 145 psig. The system pressure is monitored by a pressure indicating transmitter which has a range of 0 - 150 barg I.e. around 2250 psig. What I am looking for is an understanding of whether the pressure transmitter will be accurate when the actual pressure measured is so low?

### RE: Daft question about pressure transmitter range

Transmitters are far more linear than Bourdon tubes were. If your transmitter is a 0.5% uncertainty (medium cost device) then your uncertainty is +/-11.25 psig at a calibrated range of 0-2250 psig. That means that any value within your expected operating range will be outside the resolution of the device with that calibrated range.

If you can recalibrate it to 0-100 psig then your uncertainty will be +/-0.5 psig which might be OK. Calibrating it for 0-25 psig would give you +/- 1/8 psig which would be better.

It has been a few years since I actually did this, and the technique could easily have changed, but back in 2000 you would put a 25 psig physical pressure into the device and crank the current to 20 mA. Then verify that 12.5 psig was 12 mA, etc. It really was that easy back then. I do not know if manufacturers have backed off that and put a switch within the device to select specific ranges (which feels like a major step backwards to me).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

### RE: Daft question about pressure transmitter range

As it stands no, it won't be accurate. Pressure transmitters can be re-ranged on site such that max output is X - in your case about 150 psi. Even for this, a working range of 2 to 4.5 psig, it will have difficulty which is why I recomend you look at two. However your current 0-2250 psig transmitter is likely to be not capable of being re-ranged sufficently low.

You can either ask the designer / vendor / supplier to provide you with details of the accuracy of the transmitter at your working range or do a bit more homework with Rosemount before you go back to them saying - I've checked with rosemount and they tell me I need a XXX-YYY model - Please supply the right part. If you have the rosemount transmitter model no then you can do a lot yourself or just ring them up, describe your process cnditions and ask them what is the right one.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

### RE: Daft question about pressure transmitter range

(OP)
Excellent! Many thanks for this, I understand now.

### RE: Daft question about pressure transmitter range

(OP)
Thanks to all who have responded.

I've hovered around the forum for a while now and even try to help out occasionally where Meccy seals are concerned, but instrumentation has always been a bit of a black art to me!!

Thanks again.

Paul.

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