Choosing Instrument Range

[Michael2009]

if my instrument calibration range for level only -32.6 mabr to - 10.7 mbar.

I choose DP transmitter A, instrument range -1000 to 1000 mbar and span are 5 to 1000 mbar.

and DP transmitter B, instrument range -100 to 100 mbar and span are 1-100 mbar.

If i choosing A. is it will effect the result??
or should i choose Tx B?

Thanks....

 

[danw2]

The rated accuracy is likely to be less for A than B, because most pressure transmitters determine accuracy as a percentage of range, not percentage of reading.

For tight ranges, the accuracy spec usually has a 'disclaimer'; accuracy = percentage of range unless range is smaller than X, then the error is Y.

How important is accuracy (comparison to a standard)for what you want to use it for?

As to whether you should choose A or B:

Are other factors equivalent?
- environmental rating,
- overpressure rating,
- ability to configure with whatever tool you now own?

Sometimes having a spare that can go into service immediately is a factor in a decision. Does your site already have multiple A range transmitters and an A spare on hand?

Do you have to update the handheld configurator tool if you buy one or the other?

Is the cost differential between A & B significant?

 

[Michael2009]

Thanks danw2..

what do you mean X???
Range smaller than X, X represent what value?? Middle value of range??

 

[danw2]

Here's the accuracy spec for a Honeywell good grade DP transmitter. It's typical.

Note in the analog section that the accuracy is 0.075% URV, unless the URV is less than 62 mbar (25 inches w.c). For spans less than 62 mbar (25 in w.c.), you have to calculate the accuracy using the arithmetic formula.

Which is reasonable. One can't have a tiny span and expect the same accuracy over that tiny span as over a large span.

Your 21.9mbar range is less than the reference value 62mbar, so you'd have to calculate the accuracy for a 21.9mbar span.

[0.025+0.05 (62/21.9)] = ±0.166 % span

So, what I meant was, accuracy is 0.075% (x) of range, unless the range is less than 62mbar (25 in w.c.), in which case accuracy is whatever the calculation comes out to be 0.166% .

 

[Michael2009]

Dear danw2

if my range regard to this transmitter.
my accuracy are 0.166% right??

unless my span are biggest than reference valuve 25 in H2O or 62 mbar. So i cant say accuracy are 0.075%?

 

[danw2]

For this given brand and model DP transmitter, the answer is yes, the accuracy at your range of ~22mbar would be calculated as 0.166% span. The accuracy is NOT 0.075% for your span, as defined by the manufacturer's calculation statement.

There is a higher (better) accuracy transmitter model than the example posted. So the numbers used apply only to the brand and model posted, not to other brands and other models, any of which would have their own accuracy calculation.

Other brands will be similar:
- good, better, best models
- limitation as to the accuracy spec when applied to small spans.

To determine ultimate stated accuracy is a shopping/research project. The bigger question is whether accuracy, meaning comparison to a known standard, is really of concern.

Because many models have excellent repeatability, but do not have as small an accuracy deviation (uncertainty) spec.

My observation is that for about 90% of industrial applications, repeatability is more important than accuracy.

 

[Michael2009]

Transmitter repeatability are mean deviation of two output of transmitter????

 

[Mass44]

Hi there,

The choice of transmitter size for a application is actually quite simple. For a piped and capability installation measure the distance between the installed transmitter in the field (or the position you plan to install the transmitter) and the top tapping point on the vessel. Say this distance is 3000mm. Now you know that you need a DP transmitter that will be able to read at least -3000 to + 3000mmH2O. Keeping in mind that ideally the max measurement should always be as close to the maximum capability of the transmitter so select a transmitter, in this example, that is just slightly bigger than 3000mm for instance something like -3200 to +3200mmH2O. The further you move away from the max measurement with your selection the less sensitive the measurement will become and if you move to far away the measurement can actually become inaccurate due to the inaccuracy tolerance that become to big. This will only start to happen if you select a transmitter with a range about three time the size of the max measurement. So the best is to stay as close to the max measurement as possible during the selection.
Since most of the time the transmitter specs are given in PSI or mBar or Kpa, just convert the 3000mmH2O into whatever units the transmitter specs are given in and you will find the transmitter that is perfect for your application.

 

[Michael2009]

Hi mass44

thanks ... noted

 

[sebastiancito]

Hi michael, i can try explain with my bad english

if my instrument calibration range for level only -32.6 mabr to - 10.7 mbar.
HERE YOUR SPAM IS 21,9 mBAR.

I choose DP transmitter A, instrument range -1000 to 1000 mbar and span are 5 to 1000 mbar.

Your max spam es 2000 mBar or 2bar.. From -1000 to 1000mbar.

and DP transmitter B, instrument range -100 to 100 mbar and span are 1-100 mbar.
Here the maX spam is 200mBAr.


If i choosing A. is it will effect the result??
or should i choose Tx B?

The max spam for the A transmiter is 2000mbar, and you like measure 21.9mbar, typicaly the rangeability is 100:1 but when you go mora than 20:1 you loss accuracy.

The max spam for the b transmiter is 200 mbar, and you like measure 21.9mbar, typicaly the rangeability is 100:1 but when you go mora than 20:1 you loss accuracy. In this case, this transmiter is put to 20:1 Spam ratio. The b opcion is the correct choice.

Any transmiter can range the zero over the atmosferic preassure, by example... You can use like zero at -100mbarGAUGE and the spam in 0mbar Gauge (1bar ABSOLUTE).

i hope that i was clean in my explanation.
BEST REGARDs!!