help Scaling transducer for Vaccumm

Micrlogix 1400
spectrum controls 1762sc-IF8u analog input card
Dwyer 628CR-10-GH-P1-E4-S1 0-100 PSI 4-20 Ma

We are putting transducers on each side of pumps that draw ink out of tanks and keep a supply lines pressurized. The sensor between the tank and the pump usually has a negative pressure on the gauge there and they can use that value to tell how clogged the filter is which is also behind sensor. It has a positive value at times depending how full the tank is which is above the pump.
not sure how to properly scale this because of the negative values.

Use the "SCP" instruction. Settings for the input min/max will depend on how you have set up your analog input card's data format. If the value goes below the scaled min setting it (the "SCP" instruction) will display a negative value in it's destination register. The instruction is just an equation for a straight line, it does not clamp the values to the min/max settings.


Check the specs on your transmitter, as there are probably limits on just how low the ma output will go.

One thing to keep in mind is that the transducer you are using is a gage transducer, not an absolute transducer, so it may not give you accurate readings when you get into the vacuum range.

I have used Rosemount 2088G transmitters before which are gage psi transmitters, however they can be re-ranged to -14.7 psig which is 0 psia. I don't think that is possible with the Dwyer sensors you are using.

That's a 0-100 psi Gage transducer. You're not going to measure vacuum with it.

As others have said you should get a pressure transmitter that's properly ranged for the application.

I should have added that to my previous post.

Yes, you need a pressure transmitter that will read in a vacuum range.

But, if you don't know what absolute pressure is, don't buy an absolute pressure transmitter.

Buy a gage pressure transmitter than is ranged for a vacuum range, like -15 to 0 psig or a compound (gage pressure) range like -15 to +15 psig. (one can't get -15 in reality, but close)

The typical vacuum gauge is gage pressure, referenced to atmosphere, which is what 9 out of 10 people need, but many end up buying an absolute pressure transmitter and screw it up by trying to 'zero' it without having access to a diffusion vacuum pump that can pump a hard enough vacuum to be considered 0 psia or -14.7 psig.

We called Dwyer about this and they are sending us
628-01-GH-P1-E4-S1 0-30 PSIA units. Still unclear about the min max scaling with the negative reading. What would my scaled Min be?

bdoutney said:

We called Dwyer about this and they are sending us
628-01-GH-P1-E4-S1 0-30 PSIA units. Still unclear about the min max scaling with the negative reading. What would my scaled Min be?

Click to expand...

Scaled min will be 0 PSIA.

If you want your scaled value to read PSIG then the scaled min would be -14.7

Your transmitter will never have a 4ma output, with that scaling.

0 PSIA= -14.7 PSIG (4ma)

14.7 PSIA= 0 PSIG (12ma)

30 PSIA= 15.3 PSIG (20ma)