Assume I connect a -250 in to 250 in WC rosemount 3051SCD2A.. type transmitter such that the high side is connected with process line below atmospheric pressure (vacuum) while the low side is left open to atmosphere. the transmitter reads between 1 to 3 inch. Now if I reverse the connections, shouldn't the transmitter read the same. what is recommended connection for a below atmospheric pressure application. I have experienced unstable and wrong readings when the connections are reversed, but I am not sure what is causing it. Application is air and an analog gauge in parallel with transmitter is working absolutely fine.

 

> Now if I reverse the connections, shouldn't the transmitter read the same?

Value should remain the same, but the algebraic sign should change from plus to minus or minus to plus.

Some brands require factory characterization for reverse connection to meet an accuracy spec. I don't know if Rosemount is in that category or not.

 

Value will not remain same.

When you are connecting the transmitter positive side to vacuum process, the transmitter is seeing a pressure - 1 in. to - 3 in. To get a 4-20 mA from the transmitter somebody must have done zero suppression to the transmitter to give -3 in.= 4 mA and -1 in. = 20 mA.

When you reverse the connection of the transmitter i.e low side to vacuum process, the transmitter will see +1 in. to + 3in. pressure. Therefore, the same transmitter will show over range and output > 20 mA.
However, if you recalibrate the transmitter with value +1 in. = 4 mA and +3 in. = 20 mA, the transmitter will show correct value.

Hope this helps.

 

hi

First of all it is not clear why you are experimenting?

some problem with Transmitter and trial and error

whatever, normally for vacuum applications

+ side is Atmosphere

- vacuum connection

If you reverse it value is same but will read it is negative value

pandu

 

Gents,

First of all, for this query we need to ask what is the DP sensor type used. For Rosemount 3051, typically it's a capacitance type.

I expect the we need to re-calibrate the range, because, what the sensor do is just generating small current based on the two plate distance (connected to electronic + terminal and - terminal). The electronics (i.e. signal processing) inside the transmitter that change this sensor current to a 4-20mA signal.