In my past experience with similar applications the Combustion Air flow (Linear in Dp Tx) used to interface to DCS I/O to be square rooted in DCS S/w and then corrected for the Temperature before linking to any PID blocks. Is it the right practice or whether to square root at first in the field and then do the Temp correction in the DCS s/w?

Pl. let me know the best practices.

 

The mathematically correct approach is to apply any temperature and pressure corrections to the differential pressure signal before applying the square root function. So it should be done in the DCS or PLC, not at the transmitter.

 

A reason for doing square root in the DP transmitter is that it is functionally a flow transmitter.

When the Standards committee at ISA wrote ISA-5.1-1984 (R1992) for tagging and ID'ing instrumentation, the standard was based on the concept that an instrument's function defines its tag, not the device's construction. Thus, a DP transmitter on a flow element is tagged as a flow transmitter, not a pressure transmitter.

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

If the process variable is flow, I'd rather view (square rooted) flow values in the local display or interpret a 4-20ma as linear flow, rather than seeing pressure values, but that's just
me.

Temperature and pressure corrections can be done by a multivariable transmitter in the transmitter itself.

 

I agree that if you are using a multivariable transmitter with temperature and pressure transmitters along with the differential pressure transmitter, it is fine to take the square root at the transmitter and give the actual flow as output.

 

>A reason for doing square root in the
>DP transmitter is that it is
>functionally a flow transmitter.

When the transmitter measures differential pressure, it is a differential pressure transmitter.

>When the Standards committee at ISA
>wrote ISA-5.1-1984 (R1992) for tagging
>and ID'ing instrumentation, the standard
>was based on the concept that an
>instrument's function defines its tag,
>not the device's construction. Thus, a
>DP transmitter on a flow element is
>tagged as a flow transmitter, not a
>pressure transmitter.

In my opinion when the instrument is measuring differential pressure, the conversion to flow shall be done in the DCS/PLC. The instrument is still a differential pressure transmitter

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

Where do you put in the size of the orifice?

>If the process variable is flow, I'd
>rather view (square rooted) flow values
>in the local display or interpret a
>4-20ma as linear flow, rather than
>seeing pressure values, but that's just
>me.

Why not use a proper flow transmitter, with temperature and pressure corrections implemented?

Best regards,
Heinrich Baumann

 

I am sorry, just reading my own post I think something misses.

On combustion systems it is very important to reliable determine the amount of combustion air available.

In case a dedicated combustion air fan is provided I would recommend measuring the current to this combustion air fan. The current gives a very reliable figure on the amount of combustion air available.

Best regards,
Heinrich Baumann