can we directly configure square rooted DP (4 - 20mA) from DP transmitter to flow?

we have using orifice, rosemount 3051 DPT for measuring Oxygen flow. & Also using data logger for Data acquisition.
DP range: 0 - 10Bar
orifice measuring Range:0-120 m3/hr
kindly advice below conversion is correct?<pre>
DP in bar || mA || Flow in m3/hr
0 || 4 || 0
5 || 12 || 60
10 || 20 || 120</pre>
And also advice how to measure volume/mass flow(with unit) using dp?

 

Every 3051 has a parameter to convert the output of configured DP range from a linear DP output to 0-100% flow rate by extracting the square root.

The parameter is called transfer function or xfer fnctn. The setting choices are linear or square root (Sq Root).

I can't recall whether the local meter reads out in flow units or not, but in square root mode the signal will be
4.0mA = 0% flow
12.0mA = 50.0% flow
20.0 = 100% flow

Correction to your table:
81% DP = 90% of full scale flow rate
50% DP = 70.7% of full scale flow rate
25% DP = 50% of full scale flow rate

For what it's worth, you need to make sure that the differential range is configured properly. I seriously doubt that your DP is 10bard. 10 bar sounds more like the 'static' line pressure, not the DP. Most plants do not want to pay for the energy loss of the high permanent pressure drop that a 10 bard differential drop would incur. But maybe yours really is 10 bard.

 

As David correctly informed, you can do the conversion from linear output to square root output in the transmitter itself, what ever you sending to your display unit is the actual flow.

the calculation for flow conversion is as follows just to cross check your transmitter output:

(%P/100)square root X 100 = % Flow;

where %P is the percentage pressure of the differential full range.

for example if your differential pressure is 0-5 Psi for flow of 0-50 SCFM; @ 1.25 psi input, your output will be: 1.25 = 25% of full range; (25/100 ) square root = 0.5 x100 = 50% flow which is 25 SCFM

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Just be careful and make sure you are not also doing the square root function in the control system. The result is not a lot different.
Been there done that, it took me a while to figure out why my reading was slightly out.

I believe it's much more accurate to do it in the transmitter, a tiny zero offset on the 4-20mA signal would be amplified by the square root function in the control system.

 

Am sorry you are right, its not 0-10 bar ,its 0-98.3 mbar.

and i have set transfer function as sq root in DPT.

the display reads DP only. As your statement if DP is 8mA (25%) the flow will be 50% how can we justify that?

And the max range of orifice is 120 m3/hr if my DP is 20mA then is my flow equals to 120m3/hr?

kindly advice how can i convert DP to Flow?

 

As your statement whatever DP displays is Flow?

 

> And the max range of orifice is 120 m3/hr if my DP is 20mA then is my flow equals to 120m3/hr?

Ans: yes

The local display reads DP
The 4-20mA output = 0-100% flow rate = 0 - 120 m3/hr

Configure whatever your receiver (indicator/totalizer/recorder/PLC/DCS) is for 0 - 120 m3/hr and read flow rate directly.

>the display reads DP only. As your statement if DP is 8mA (25%) the flow will be 50% how can we justify that?

Ans: If having DP values on the local display is not acceptable, change the display configuration.

3051 Hart manual:
http://tinyurl.com/muwklj2

Pages 69 and 70 (pdf) describe the procedure for changing the range and units for the local display to display the flow rate.