An engineer came across one of my blog posts and sent me a question about the configuration of a Rosemount 3051C pressure transmitter:
I’m working on a system that uses several 3051C coplanar pressure transmitters. The transmitters were installed to provide flow information via a PLC. The name plate on the transmitters indicate they have a range of 0-xxx [details removed] in H20 for a 4-20 mA signal and the PLC is supposed to convert the signal to GPM with a scaling factor. The value displayed is not representative of the actual flow rate.
According to the reference manual, the transmitter output can be configured for Square Root for flow measurements. The 3051 has two output settings: Linear and Square Root. Activate the square root output option to make analog output proportional to flow.
Is this true for both the 3051SFC and 3051C?
The 3051 SFC transmitters are also used on this system, but the PLC takes the 4-20 mA signal and multiples it before taking the square root.
When Rosemount measurement product questions like these come my way, I turn to my friends on Emerson’s Rosemount Technical Support team for answers. A member of the DP [differential pressure] flow specialist team responded:
The 3051SFC is a flow meter assembly. It uses the 3051S transmitter which has a “Scaled Variable” feature which allows us to relate the measured DP to a flow rate, then actually range the transmitter’s 4-20 mA output in terms of flow rate. This way you can forget about DP and forget about the square root function, it is all done in the 3051S transmitter, and the transmitter output represents flow rate. Because it would be scaled in terms of flow rate, the transfer function to the 4-20 ma output is left in the linear mode.
The 3051C, can be set to either linear or square root output, but it will always be ranged in term of a DP value, so if you want that range to represent flow, you would set the transfer function to square root output. The 3051C does not offer the “Scaled Variable” feature.
For your 3051SFC, I am a little confused why your PLC takes a 4-20mA signal and multiplies it by a factor then takes a square root. If the unit was configured to output flow then there is no longer a need to square root the value. The 4-20 mA output should already represent the flow scale that it is ranged for.
Could you supply us with the Serial number of the 3051SFC so that we can check our records on how the unit was configured when it left the factory? Kindly provide the full model number as well in order to facilitate our search.
If you have technical questions about your Rosemount measurement products, I highly recommend sending it to the appropriate specialist group email address for answers.
