Need help building a dual range transmitter

[OzCalvin]

Hi,
I have a flow being measured by two transmitters. The high flow is covered by the high range transmitter FIb. The low flow is covered by the low range transmitter FIb. What would be the best way to build this in a control network database using logic so that I get one flow indication and:

1. transition from one transmitter to another transmitter is smooth.
2. bad PV of any transmitter is handled correctly (like hold last good value).

Thanks,

 

[ilan7]

Do not know how can you tell which transmitter is reading right and which is not if the reading are not close to one another.
For show the reading think the best way is to make an avarage of the readings, but put each one with a multiplier when in low readings the low transmitter output multiplier is 1, and in high flow readings is zero, and the other way with the high flow readings.

 

[bmsg]

I understand that the inputs from the two transmitters are connected to two different analog input (AI) channels in your system - say the 'low' transmitter corresponds to AI-1 and the 'high' transmitter to AI-2..
You can write a logic something like this:

1. Different scale ranges are programmed for AI-1 & AI-2 (same as the calibrated ranges of the respective transmitters).

2. Use a OR function block (or write logic) with AI-1 & AI-2 as inputs. The block will select and output the value of AI-1, until it exceeds 80% (say): thereafter it will output value of AI-2.

I mentioned 80% arbitrarily: select a value at which both transmitters read with good accuracy - then your changeover will be smooth.

I do not think you can detect a 'bad' reading with the kind of system you describe. For that, you would have to install 3 transmitters of EACH range - and work out a 2-out-of-3 logic.

 

[dcasto]

We gave up on split range transmitters. With newer pressur transmitters the error is so small. Instead of a 50 inch and a 200 inch, we switched to 150 inch single transmitter.
These were on custody transfer meters on very expensive products. A Rosemount gold series will give all the performance you'll ever need.