Radar vs Ultrasonic Level Calibration Points (The Devil’s in the Details)

People who have used Milltronics or Siemens ultrasonic continuous level measurement systems are used to the way configuration is done.  It’s not uncommon that when they start up their first Siemens radar transmitter they encounter a stumbling block – the 4-20mA output doesn’t respond correctly. 

The radar will start up and run, and if the tank level is low, they’ll get a valid level reading on the transmitter’s display, but the 4-20mA will be incorrect (too high a percentage).  Or, if the tank level is at a medium or high level, both the indicated level in the display and the output will be maxed out at 20mA.   It’s a misunderstanding about where the span measurement starts from.

The issue is that each technology uses a similar reference point for empty (0% or 4.0mA), but an entirely different reference point for high level (100% or 20.0mA).  Using the wrong reference point gives the wrong output or display value.

Take a look at the diagram:

 The diagram shows a radar sensor (left) reference point for low (4.0mA or 0%) and high (20.0mA or 100%) level calibration points.

Note that the high calibration point (100%) and the low calibration point (0%) are both a measured distance from the sensor reference point at the bottom of the transmitter’s flange.  The subtracted difference (high minus low) is the 4-20mA level span value or the maximum level value.

Compare that to the ultrasonic reference points (right).

The empty point (P006) is referenced from the face of the transducer, (essentially the same as radar).  But the high level point (P007) is is referenced from the empty point, and NOT from the face of the ultrasonic transducer. 

Note also that the P007 arrow starts at the bottom and measures from the bottom up, unlike the radar high cal point where the arrow starts at the top and measures down.

If the high level value is determined from the correct reference point, everything works as expected.  If not, the radar’s 4-20mA output signal is spanned incorrectly and will max out way below the true high level point.

The table below summarizes the reference points.  

Calibration Point



0% or 4mA

Reference from the top down

100% or 20 mA

Reference from bottom up

Reference from top down.

If you don’t remember, it’s in the manual on the CD that comes with every sensor. Manuals are also available free on the Lesman website


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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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