# Calculating Pressure Transmitter Range

K

#### Kamaraj

Hello Everyone,

I have a NaK filled Pressure Transmitter. Pressure Range is 0-70 bar.

Kindly advice me how can i check the Rangeability of this pressure transmitter. If any procedure (Formula) is available, kindly provide the details.

Thanks and Regards,
M.kamaraj

A

#### ashiq

what is the output of this transmitter?

suppose its output is 04-20 mA than zero and span would be mentioned in the configuration of TX.<pre>
0 bar 04 mA ---
17.5 bar 08 mA ---
35 bar 12 mA ---
52.5 bar 16 mA ---
70 bar 20 mA ---</pre>
if you don't want use full range, because normally we use maximum 75% range to avoid sensor damage, then change span value you require. suppose 100% is 40 bar, then output would be 08 mA on 10 bar and 12 mA on 20 bar so on.

hope it helps

K

#### Kamaraj

Hello Mr. Ashiq,

The transmitter output is 4-20mA only.

when i am Googleing i seen rangeability is ratio of max measured value to min measured value at which system has acceptable performance.

I don't know how can i relate this definition with your Answer?

Thanks and regards,

M.kamaraj

#### David_2

The definition you found is a correct definition of rangeability.

I have no idea what a NaK filled transmitter is, what its process input is or what degree of accuracy it might be capable of. In light of my ignorance, I'd say that if it's a commercial device, you can check the spec sheet for citations on turndown or rangeability or see how much the range can be changed before the accuracy changes.

For example, a Honeywell ST700 differential pressure transmitter spec sheet says it is capable of a 100:1 rangeability, meaning that the transmitter can be ranged from 0 - 4.0 in w.c. (100:1 turndown) all the way up to 0 - 400 in w.c. (1:1 turndown).

But if you read closely, you'll see that any range with a turndown equal to or less than 16:1, those ranges from 0-25 in wc to 0-400 in wc, have a best reference accuracy spec of 0.050%. There is a calculation for ranges with turndowns greater than 16:1 to determine ref accuracy.

For instance, a 20:1 turndown has a ref accuracy of 0.059%, 40:1 has a ref accuracy of 0.107% and the 100:1 turndown has a 0.200% ref accuracy.

If that level of documentation is missing or not available, you can experiment to determine what the turndown ratio is by applying various process input levels and checking the transmitter's response.

At some low point the transmitter's response will not be as accurate as the readings from higher level process input responses. The ratio of the 100% process input value to the value where the accuracy falls off is the turndown ratio.

#### David_2

A colleague informed me that an NaK transmitter is a pressure transmitter with a local fill fluid of sodium-potassium used for high temperature applications. There are a couple models that appear in a Google search that are fixed range transmitters, with zero and span adjustments, but without a range adjustment.

The concept of range rangeability does not apply - they're fixed range devices. You can't take a 0-200 bar transmitter and re-range it to 0-100 bar.

Pressure transmitter accuracy spec is based on percent of span:
For an accuracy of 0.5% with a 100 bar full scale, the uncertainty (accuracy) is plus minus 0.5 bar.

At an applied pressure of 100 bar, the reading could be 99.5 to 100.5 bar, or 0.5% of reading

At an applied pressure of 50 bar, the reading could be 49.5 to 50.5 bar, or 1% of reading.

At an applied pressure of 25 bar, the reading could be 24.5 to 25.5 bar, or 2.5% of reading.

At an applied pressure of 10 bar, the reading could be 95 to 105 bar, or 5% of reading.

You are entitled to determine at what pressure the indicated value is of little use because the uncertainty is such a large percentage of the indicated reading.

Ratioing that selected pressure to the full scale pressure would be turndown ratio for that device.

For instance, if 2.5% of reading uncertainty is the worst case uncertainty you can tolerate, then such a device would have a useable 4:1 turndown, using the numbers above (100/25 = 4:1).

K

#### Kamaraj

Dear Mr.Ashiq,

NaK filled transmitter is pressure transmitter used for high temperature Applications such as polymer processing. It uses Nak liquid (Near incompressible liquid) as transmission fluid for pressure transmission to strain gauge. Nak mainly for isolating electronics from process input.

Spec for KE2 Pressure transmitter:
Make: Gefran
Model: KE2
Range: 0-70 bar
Accuracy: 0.5% FSO

Range cannot be adjustable and it has auto zero and span function.

can you advice how can calculate turndown for this KE2 pressure transmitter since i little bit confused.

Thanks and Regards,
M.kamaraj

K

#### Kamaraj

Dear Mr.David,

what you have wrote about NaK pressure Transmitter is Correct.

My Ke2 model Pressure Range:0-70 bar

100% of full scale o/p is 70 bar with accuracy of 0.5%

Last reply you have wrote that,

> 1.At an applied pressure of 100 bar, the reading could be 99.5 to 100.5 bar, or 0.5% of reading

> 2.At an applied pressure of 50 bar, the reading could be 49.5 to 50.5 bar, or 1% of reading.

> 3.At an applied pressure of 25 bar, the reading could be 24.5 to 25.5 bar, or 2.5% of reading.

> 4.At an applied pressure of 10 bar, the reading could be 9.5 to 10.5 bar, or 5% of reading.

In first step you have took 100bar as 100% with 0.5% accuracy
In 2, 3, 4 step you have took 50, 20, 10 bar as 100% respectively.
How do you selected Accuracy? Kindly advise.

Thanks and Regards,
M.kamaraj

#### David_2

The term in "FSO" in your spec means (percent) of full scale uncertainty.

My selection of a 100 bar model and yours, a 70 bar model both have a 0.5% of full scale uncertainty.

0.5% of 100 bar = plus or minus 0.5 bar
0.5% of 70 bar = plus or minus 0.35 bar

That value applies to ALL applied pressures because accuracy is stated as percent of full scale, NOT percent of reading.

However, one can use the percent of full scale uncertainty value to see how the percent of reading error changes at different applied levels.

1. At an applied pressure of 70.0 bar, the reading could be 69.65 to 70.35 bar, or 0.35% of reading

2. At an applied pressure of 35.0 bar, the reading could be 34.65 to 35.35 bar, or 1.0% of reading.

3. At an applied pressure of 15.0 bar, the reading could be 14.65 to 15.35 bar, or 2.3% of reading.

4. At an applied pressure of 7.0 bar, the reading could be 6.65 to 7.35 bar, or 5.0% of reading.

Accuracy falls off at the lower end of the scale with devices that are spec'd percent full scale (all pressure transmitters are, these are not an exception to the rule).

K

#### Kamaraj

Dear Mr.David,

In your Last Post you wrote that

At 7 bar pressure, the reading could be 6.65 to 7.35, 5.0% of 7 bar.
It is also 0.5% of full scale output referring to 70 bar. So it is within the accuracy.

If accuracy falls at 7 bar means then turndown is 70/7, 10:1. Am i correct?

or how can i identify Accuracy Fall? kindly advice?
or all the ranges are within accuracy then how can i calculate turndown?

Thanks and Regards,
M.kamaraj

#### David_2

The word 'Turndown' is used in two ways.

One use of the word turndown is to define how far a device can be re-ranged and still maintain a cited accuracy spec. Industrial transmitters that use HART are typically designed to be re-ranged and turndown would apply to those types of pressure transmitters, like I cited for the Honeywell. Your transmitter is fixed range, so the rangeability turndown concept does not apply.

The other use of the word turndown refers to the range of readings over which the instrument produces 'useable' data within an accuracy spec. This applies to fixed range devices like the NaK pressure transmitter, and has been used historically as the 'turndown' used for flow readings from differential pressure transmitters (3:1, 4:1, maybe 5:1).

You can identify the 'accuracy fall' by using a spreadsheet or plotting out the error as percent of reading over then entire range of the transmitter.

It's up to you to determine at what uncertainty the data is not longer useful, a maximum limit at which the data is no longer useful.

If a plus/minus 5.0 percent of reading uncertainty is the maximum limit, then yes, that transmitter has a 10:1 turndown.

But, if a plus/minus 2.0 percent of reading uncertainty is the maximum limit, then that transmitter has only a 5:1 turndown. 70/14 = 5:1

Take the example of a applying 7.0 bar with a deadweight tester. The reading from the transmitter is 7.29 bar. That's within the uncertainty spec of the transmitter. But is it a useable value? I can't answer that.

The nature of instruments whose accuracy spec is percent full scale is that the uncertainty increases proportionally the further the reading gets from full scale. Low end readings have a high uncertainty. That's the nature of % FS accuracy.

K

#### Kamaraj

Thanks Mr.David & Thanks for all.

B

#### Barrett W.

B

what mean accuracy % of span? this is confuse me. for example above, if the turn down between 1:1 to 1:16 the accuracy is 0.050% of what?
if we suppose is 0.050% of span. is that mean the accuracy for turn down 1:1 to 16:1 will depend on calibrated span that's mean the accuracy of transmitter at 1:1 will be 0.05% *url and for 1:16 the accuracy is 0.05% *url/16. or the accuracy will be same at these range?

thanks

T

#### TREUBENC

>what mean accuracy % of span? this is confuse me. for
>example above, if the turn down between 1:1 to 1:16 the
>accuracy is 0.050% of what?

The accuracy of the transmitter or rather its error is expressed as a percentage of span.

M

#### muthu

how to calculate pressure transmitter uncertainty, based dkd r6-1 standard. anyone help

#### transmittershop

nice information provided above