Original post: 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.
Update: I wanted to update the post to share the answers to Kasi’s questions below in the comments.
For Level transmitter , 2088 Pressure transmitter with 1199 remote flange assembly. The tank is an open tank so we use only one flange. Question: My 4-20 mA cab set to tank level ( mm or % ) or only can be set in mmH2O ?
Answer: Your 2088 Primary Variable can be set to any of the units of measure mentioned in the Rosemount 2088 product data sheet:
If desired, % of Range can be locally shown on the LCD by using a configuration tool (Ex: 475) to turn on Percent of Range under the Display menu. (Menu Tree: Configure > Manual Setup > Display)
Also, the 2088 can be configured to read out as any variable you wish by using scaled variable. There is more information about this in the 2088 manual. There is also a good step by step tutorial for DP Level Scaled Variable shown here. The tutorial discusses setting scaled variable on a Rosemount 3051; however, the process will be the exact same for the Rosemount 2088 since the menu trees used across our Pressure portfolio almost identical. If you add the C1 option in your model string when ordering from the factory, your scaled variable units can come configured from the factory.
For Flow Measurement 2051CFC (compact flow transmitter ) with an orifice. Question: A. Based on the above explanation only scaled transmitter able to provide the direct flow unit, so this 2051CFC transmitter only can be provided DP value output?
The 2051CFC can also be configured to read out as any variable you wish by using scaled variable. There is an example of how to do DP Flow Scaled Variable in the 2051 manual on page 25. If you add the C1 option in your model string when ordering from the factory, your scaled variable units can come configured from the factory.
Question B. What is the difference between Linear output and square root output?
A linear output on a 2051CFC transmitter simply outputs the DP measurement. Since DP flow is calculated using the square root of the DP value, you can select a Square Root output to calculate DP Flow.
Can I scale in my PLC the square root output in the same way in the linear output? or any difference?
You can read either a Linear output or Square Root output into a PLC. If you choose a Square Root output on the transmitter side, make sure you don’t also do a square root calculation on the PLC side which will result in you taking the square root of the DP measurement twice.