Why doesn’t my LR560 radar’s analog output match the displayed level value?


Siemens SITRANS LR560 radar level transmitterA customer who had had lots of experience with Milltronics and Siemens ultrasonics was installing his first SITRANS LR560 radar level transmitter.  They had worked with it in the shop beforehand, going through most of the settings.  They even tested it by setting it up to shoot against a file cabinet and used a tape measure the check the indicated distance value.

Everything checked out OK in the shop.

When they installed the transmitter on the top of the bin, they changed the transmitter’s sensor mode parameter from the distance mode they used in the shop for testing to level mode.  After aiming, the level value shown in the local display was dead nuts on, but the 4-20mA signal going back to the control room was way off.

The bin was a third full. The 4-20mA showed it about double that.  Not only that, the 4-20 was going in the wrong direction.  The bin was emptying and the HMI reported an increasing level value.  Someone realized that an inverse-acting output was typical of a distance value, so they reconfigured the sensor mode to distanceThat got the 4-20mA much closer to a distance value, but it was still not exactly what it should be, and besides, the goal was to read level, not distance, in the control room.  What was going on?

The LR560 has three modes of operation: level, space and distance.  The sensor mode defines how the process variable is calculated by defining the zero and span of the scale.

Siemens SITRANS LR560 diagram of three sensor modes

The analog output uses the same three modes to define where the 4mA and 20mA points are.

Diagram of Siemens SITRANS LR560 radar analog output scaling

The graphics for sensor mode and analog output look identical.  They are.  The low and high calibration points are common for both, but analog output mode is independent of the sensor mode.

Unlike previously released Milltronics and Siemens level units, the LR560 allows each parameter to be set on its own; so the sensor mode can be different than the analog output mode.

That’s a change.  On the older Milltronics and Siemens ultrasonics, changing the sensor mode (P006 and P007) defined both the operating mode AND the analog output mode.  But on the LR560, they can be set independently.

The customer’s familiarity with older Siemens and Milltronics models worked against him, in this case. When the customer made his change in the field, he’d only changed the sensor mode to level, and not the analog output, which was still set to distance.

The result was that his actual analog output was nowhere near the level value, since its output was a distance value. Once the analog output mode was changed to level, the indicated level in the control room HMI was dead nuts on, where it should be.

When the initial sensor mode was changed from level to distance, the 4-20 mA output got closer, but was still off.  That’s because the distance mode ignores the high calibration point, so its span is typically larger than the level span.  The HMI had been configured to represent a shorter level span but was getting a 4-20 spanned over a longer distance, so the 4-20 values were off by the differences between the level and distance spans.

Anyone setting up a SITRANS LR560 radar should be aware that the LR560’s Quick Start Wizard will set both the sensor mode and the analog output mode to a common setting, one selected for operational mode, as shown here:

Siemens SITRANS LR560 quick start wizard for operating mode

But those who navigate the menu should be aware that the sensor mode and analog output mode settings are independent of one another, and might not match.  (I still haven’t found an example where a user would want a distance value in the local display and a 4-20mA output signal spanned for level, but the LR560 will do it, if needed.)

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Lesman Instrument Company is the authorized Siemens representative in Illinois, Indiana, Missouri, Paducah KY, Wisconsin, and Michigan’s Upper Peninsula. If you are located outside that area, you can find your local sales office or get technical assistance by calling 800-365-8766.

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  1. #1 by Heinrich Baumann on March 22, 2014 - 4:16 pm

    Hi Dan,

    I have been strugling with this some days ago. The problem was not in the LR560 but in the setup of the analog input card. I used a 4-20 mA current loop simulator to isolate the problem. see http://www.divize.com/current-loop/a2007e.html

    Thanks!

    Heinrich

  2. #2 by Tim Little on September 23, 2012 - 6:49 am

    Hi Dan,
    Great post! We have de-coupled the mA output and the operation mode of our newest transmitters in the past few years as recommended by the Namur working group. This group represents over 100 customers in the chemical and pharma industries (BASF, Bayer, etc.) and gives recommendations to developers for process instrumenation devices.
    Tim Little, Product Manager, Radar
    Siemens Milltronics

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