Temperature Loop on Fisher Provox DCS.

[Kbradio]

Have a Rosemount 3144 TT in series with a Rosemount I/P being looped powered from a Fisher Provox AI CL6859 HART smart I/O. When calibrating the loop with a "R" thermocouple simulator and get up over 75% of the range ~ 17ma the reading on Provox goes minus and have to cycle power to the loop to get it reading again. Replace all parts in the loop at least 3 times!

Has anyone seen anything like this? The reason why the I/P is in the loop, still have a pneumatic control panel that still uses 3 to 15 PSI pressure switches for interlocking.

This loop has been functioning for over 9 months without an issue, and calibrated monthly. Found this issue when doing our January monthly calibration.

Thanks
Frank.

 

[Compositepro]

You have given few details but it may be you are trying to calibrate a 4-20ma loop using a 0-20ma signal.

 

[waross]

Have you checked your simulator?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[danw2]

Does the temperature sensed by the transmitter during operation ever get to a level above whatever the 75% of span for that type R T/C is?

I'll bet it doesn't, and that the loop has been and is functional at a temperature below 75% span.

But when you 'calibrate' the temperature transmitter with a T/C simulator at 75% of span, the transmitter can no longer drive 16mA (or more) current through the combined loop resistance, maybe because it's being voltage starved. The action you're seeing is classic example of when the loop's supply voltage cannot drive the loop current through the loop resistance.

What is the input resistance of the Rosemount I/P?
What is the input resistance of the Provox AI?
How much wire between them all? a hundred feet or a couple thousand feet? What gauge wire?
What voltage does the Provox AI supply to the loop?

If the I/P is a Rosemount 3311, I believe it's a branded Fisher 846. The Fisher 846 has this to say about it's
equivalent electrical input circuit:

"The 846 is not a constant resistor in series with an inductor. It is better modeled in the loop as a 50 ohm
resistor in series with a 6 volt DC voltage drop with negligible inductance."

The 3144P TT says,
"With 250 ohms of resistance in the loop, the transmitter requires a minimum of 18.1 Vdc for communication." If the 250 ohms drops 5V, the transmitter needs a minimum 13.1V

"The power supplied to the transmitter is determined by the total loop resistance and should not drop below the lift-off voltage. The lift-off voltage is the minimum supply voltage required for any given total loop resistance. See Figure 2-13 to determine the required supply voltage.

Let's assume the Provox AI has a typical a 250 ohm input resistance. That resistance drops 4 volts at 16mA. Then you lose 6 volts plus the IR drop of 0.8 volts across 50 ohms in the I/P, plus the minimum 13.1V the transmitter needs to operate.

4 + 6 + 0.8 + 13.1 = 23.9V

Drop 0.2V in wiring resistance or a dirty connection in a junction box and your nominal 24Vdc is all used up and that's only at 75% of span trying to drive 16mA. There's another 25% of span left!

Or your dc supply might be drooping at 23.8V.

Crank the loop power supply up another 1.5 volts.

 

[waross]

Another issue may be off-spec devices. I had an alarm relay that had a higher than listed input resistance. Everything looked good on paper but the power supply could not drive the full 20 ma through the circuit. The shut-down alarm activated close to 20 ma but the loop current would not go high enough to activate the alarm. Damage was done.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kbradio]

Thank you for your responses!

Does the temperature sensed by the transmitter during operation ever get to a level above whatever the 75% of span for that type R T/C is?

In normal operation never get above 72% of span. It worked for 9 months to 20 ma. This loop is a RCRA covered loop and needs a 5 point verification every month. This month is the first time this loop won't go to 100% 20ma.

I'll bet it doesn't, and that the loop has been and is functional at a temperature below 75% span. Yes it is.

 

[danw2]

FYI, I Ran into a Bellofram 1000 I/P today. The I&M sheet says the 4-20mA input resistance is 180 ohms for the 3-15psi output range; half that, 90 ohms, if a split range 3-9 or 9-15 psi range is used: