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Member

Join Date: Apr 2016
Posts: 6

Is This Low Flow Cutoff Issue

04/02/2016 5:46 PM

why my flow transmitter some times showing bad pv while tx getting zero. transmitter is linear tx display is square root dcs also square root but dcs getting negattive bad pv.i checked all calibration after lineup getting bad pv i checked low flow cut off 10% found

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Guru

Join Date: Oct 2008
Posts: 39041
Good Answers: 1532
#1

Re: is this low flow cutoff issue

04/02/2016 11:01 PM

Have you:

Read the manual?

Gone through the trouble shooting guide and performed the necessary diagnostic steps?

Asked an associate for advice?

Powered the instrument off and back on?

Gone on-line and checked the manufacturer's website?

Called the tech help desk for assistance?

Ordered a replacement instrument?

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Power-User

Join Date: Nov 2009
Posts: 272
Good Answers: 51
#2

Re: is this low flow cutoff issue

04/03/2016 10:32 AM

1) Is the transmitter a differential pressure transmitter?

2) What is the transmitter's maximum flow rate (URV, 20mA)

3) What flow range is giving a "bad PV"?

4) Is a tx a loop powered indicator? If not, what is a tx?

5) Please give examples of what kind of numbers are a good PV?

6) Please give examples of what kind of numbers are a bad PV?

7) How do you know a good PV from a bad PV?

You can number 1), 2), 3), 4), . . . your answers.

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Member

Join Date: Apr 2016
Posts: 6
#3
In reply to #2

Re: is this low flow cutoff issue

04/04/2016 5:59 AM

1)yes Dp

2)urv 70t/h

3)below zero in DCS but that time Tx still zero showing

4)yokogawa

5&6)-1.2t/h like some times.its steam service we filled water and did zero as per standard after lined up some times

6)DCS showing bad pv

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Power-User

Join Date: Nov 2009
Posts: 272
Good Answers: 51
#4

Re: Is This Low Flow Cutoff Issue

04/04/2016 4:47 PM

If the transmitter's low flow cutoff is set to 10%, then any flow below 7t/hr would be output as 4.0mA. My statement assumes that your 10% low flow cutoff number is really 10 percent and not 10t/hr or 10 something-else units.

Presumably, a 4.0mA signal should appear in the DCS as a flow rate of 0.0t/hr, but the transmitter output could be 3.995mA and the DCS input could see that as 3.987mA. So the DCS could interpret is as the transmitter's signal as a negative value, instead of rounding it to zero.

In order to understand exactly how the DCS deals with numbers below 4.0 mA you would have to connect a certified calibrator to the DCS analog input and see what the results are because experience shows that no one who has posted on this forum has access to DCS documentation.

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