Using the Rosemount 3051 Scaled Variable

The Rosemount 3051 DP and pressure transmitters (Rosemount 3051) have a functionality called Scaled Variable that we have found very useful for a variety of reasons.  In this post I’ll share some of our experiences with this feature. 

The scaled variable allows you to configure the relationship between the transmitter variable and real world engineering units in either a linear or square root function.  You define the engineering units, an upper range limit and a transfer function.

History

The concept of a scaled variable was first introduced in the 3051 Revision 3.   In this revision it was called a Custom Meter Scale.  Configuration consisted of a transfer function (Linear or Square Root), an engineering units string, number of decimal places, and an upper scale value in engineering units that corresponds to the transmitter upper range limit.  The custom meter scale value was not assigned to one of the 4 HART variables.

In Revision 6 of the transmitter, the Scaled Variable as we know it today was introduced.  Configuration was through a “Configure SV” method.  The scaled variable value was assigned by default to the TV HART variable.

In Revision 7 of the transmitter, the functionality remained the same but with the new EDDL, the method was replaced with a pushbutton and the ability to trend variables was added.  By default the scaled variable value was assigned to the TV HART variable.

Why Use the Scaled Variable?

Using the Scaled Variable provides a number of useful benefits.

1.      The scaled variable allows the engineering unit value to be visible within the transmitter from either AMS or the handheld communicator.

2.      The scaled variable configuration documents the associated engineering unit range values within the transmitter.  With the QuickCheck Snap-On (QuickCheck Snap-On) a report can be generated that shows the transmitter configuration for both the DP and engineering units.

3.      The Scaled Variable is not limited at the transmitter Upper Range Value.  The transmitter will continue to calculate the Scaled Variable value all the way to the transmitter Upper Sensor Limit.  For example, let’s look at a 3051S Range 2 (+/- 250 inH2O) DP flow transmitter configured for 10 GPM at 100 inH2O.  At a DP just above 100 inH2O, the transmitter will become saturated and the analog output will be 20 mA.  However, the scaled variable calculation will continue all the way to a differential pressure of 250 inH2O and will indicate a scaled variable value of 158 GPM.

4.      In DeltaV, the Scaled Variable value can be brought into control strategies with a standard AI function block configured to the HART variable associated with the scaled Variable (TV by default).  A word of caution.  The update rate of the HART variables is not fast enough for closed loop control.  Do not use the scaled variable value for control.

5.      If you are using the new DeltaV CHARMS I/O, the HART variables can be viewed within DeltaV Diagnostics.  Having the engineering unit value available along with the primary transmitter variable can be useful for troubleshooting.

Using the Scaled Variable as the PV

It is possible to use the Scaled Variable as the PV.  To do so, you assign the Scaled Variable to the PV in the HART variable mapping.  When you do this, the Upper and Lower range limits of the transmitter must be entered in the scaled variable units.  The transmitter transfer function must be changed to linear because the square root function is now part of the scaled variable calculation.  Additionally, you would not use a square root transfer function in the DCS input block.

We have chosen not to do this for our flow transmitters.  For an instrument technician, using the scaled variable as the PV completely changes the interaction with the transmitter and could potentially be confusing.

Summary

We have found that configuring the scaled variable in the Rosemount 3051 DP pressure transmitters has value.  It takes an extra 30 seconds or so to do the configuration but the value justifies the extra time.  I would really be interested in comments from others that have experience with the scaled variable.