Incorrect use of FT for a PDT
Incorrect use of FT for a PDT
(OP)
thread196-122680: ISA Tag for a PLC
Hello, I would like to point out that unless a differential pressure transmitter has a square-root output, it should not be designated as 'FT', it should be 'PDT'. I know, 'FT' is still widely used with a linear output, but that doesn't make it correct...
What do you think?
Mike
Hello, I would like to point out that unless a differential pressure transmitter has a square-root output, it should not be designated as 'FT', it should be 'PDT'. I know, 'FT' is still widely used with a linear output, but that doesn't make it correct...
What do you think?
Mike





RE: Incorrect use of FT for a PDT
If you use an orifice or other head producing type element to measure flow, the primary element is a flow element. When you transmit the differential pressure it is transmitting the flow signa. You are measuring flow. In olden times, we used a 0-10 scaled square root scale or chart with a scale multiplier for flow units. Those were flow indicators or recorders. Those could have been pneumatic or even mechanical with no transmitter. The square root could be taken using a pneumatic signal converter. This was normally done for more complex control such as three-element boiler feedwater control where a linear flow was desirable for adding, multiplying etc. With the electronic dp transmitters the external computing device could be eliminated. However many do not extract the square root in the transmitter. Regardless it is a flow transmitter.
Tell Jim B. that I said hello.
RE: Incorrect use of FT for a PDT
I know it is from a primary differential producing FLow element/fitting, but the descriptor needs to represent the actual signal, not the element/fitting use.
I appreciate your thoughts on this. I assume Jim B. and I were in agreement. I'll say 'Hi' if I ever meet him.
RE: Incorrect use of FT for a PDT
RE: Incorrect use of FT for a PDT
FT-301, PT-301, TT-301, and yes, even PDT-301 (I keep wanting to write 'DP'!).
RE: Incorrect use of FT for a PDT
ISA-5.1-1984(1992) is very specific about this in Section 4.2.2. It does not matter what the construction is, the identification is made according to the function. Examples given in the standard include "a differential-pressure recorder used for flow measurement is identified by FR; a pressure indicator and pressure-activated switch connected to the output of a pneumatic level transmitter are identified by LI and LS, respectively."
I have to agree with JLSeagull that it is an FT regardless of whether or not the output needs conditioning.
xnuke
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RE: Incorrect use of FT for a PDT
RE: Incorrect use of FT for a PDT
On tank level readings when using a Pressure Transmitter to read tank head pressure to determine level, the instrument bubble is LT-xxx, not PT-xxx.
The instrument bubble should represent the function you are trying to read.
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