Implementing the square root 1

[kareemao]

Dears all,

Happy new year to all,

Today I have a discussion with our engineering team in our Petrochmical plant regarding implementing the square root in the Local feild transmitter or in the DCS (( System ))

Please can any one advise what is the advantage and disadavantage for each princible, and what is the best from control point of view.

Your reply is highly appreciated.

BR

 

[danw2]

A reason for doing square root in the field transmitter is that it is functionally a flow transmitter. When the boys at ISA wrote the standard [ISA-5.1-1984 (R1992)] for tagging and ID'ing instrumentation, that standard says that the device's function defines its tag, not the device's construction. So a DP transmitter on a flow element is tagged as a flow transmitter.

If the instrument is known as a flow transmitter, why not have its output directly in flow values/units rather than pressure values/units?

My opinion is that if the process variable is flow, I'd rather view (square rooted) flow values in the local display than pressure values, but that's just me.

I'm not aware if any inaccuracy issues one way or the other. Maybe someone else does.

 

[JLSeagull]

I agree withdanw2. However no universal protocol exists. Some prefer the linear dp for calibration purposes. OTOH if the transmitter is linear with flow and the DCS screen is linear with flow then both the DCS operator and field technician see the same signal.

 

[rmw]

Doing it in the DCS would allow for minimization of spares so that any other DP transmitter of that range could easily be quickly substututed for the flow device DP X-mitter (if other such DP X-mitters existed of course). Other than that, I can't see any difference.

rmw

 

[JLSeagull]

Square root extraction is a configurable option in most dp transmitters just like in the DCS. Thus no issue likely exists with spares.