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 Ultrasonic Radar

0% or 4mAReference from the top down

100% or 20 mAReference from bottom up Reference from top down.

How Does an Ultrasonic Flow Meter Work?Posted on February 4, 2013

Ultrasonic flow meters measure the level in a channel by transmitting a pulse of sound from the face of the sensor to the surface of the flow stream and measuring the time for the echo to return.  The transit time corresponds to the distance between the face of the sensor and the surface of the water.   When the meter is initially set up, a pre-determined distance (or level in some meters) is set. This distance is either the zero level (no flow present) or a specific level in the flume or weir.  With this initial calibration it is only a matter of simple arithmetic to determine the liquid level.  This level is then applied to either pre-programmed discharge equations or strapping tables to output a flow rate.Because the sensor is not in contact with the water, ultrasonic sensors have no scheduled maintenance.  Ultrasonic sensors are not affected by chemicals, grease, suspended solids, or silt in the flow stream. Performance of ultrasonic flow meters may be degraded by: Strong wind Solar heating (of the sensor) Foam Turbulence Steam False echoes from obstructions

 To combat these problems a variety of methods are used: Sensor sunshades (for solar heating)

Angling the sensor face (to reduce steam condensate build up) Proprietary control schemes are used to adjust amplifier gain (to increase echo strength) Variable blanking distances (to eliminate early / late echo returns) Dampening factors (applied to reduce peaks and troughs in the output signal).

 For maximum performance, ultrasonic flow meters are not recommended for use on channels less than 6-inches [15.24 cm] wide (due to beam spread) or where steam, foam, turbulence, floating debris / oil / grease are present (although as seen above these conditions can sometimes be mitigated).To obtain maximum accuracy a temperature sensor [either integral to the ultrasonic sensor (preferred) or as a stand alone sensor] is required to compensate for changes in air temperature. Dead span (the distance measured from the face of the sensor in which sensor exhibits degraded performance) is typically 1-foot [0.3048 m], while the range for flow measurement sensors is typically up to 11-feet [3.353 m]. 

Under standard conditions (still air, 40-70% relative humidity, 72° F [22.22° C]), the level measurement accuracy of ultrasonic flow meters can ± 1/4 to 3/8-inch [0.635 to 0.9525 cm] (depending upon the change in level).     When mounting a ultrasonic sensor over a flume (or weir) there is a tradeoff between accessibility (mounting the sensor near the side of the flume) and maximum potential accuracy (mounting the sensor over the centerline of the flume).  If the flow approaching the flume has been properly conditioned and the flume properly set, either condition is acceptable. If, however, the flow has not been properly conditioned, is turbulence, or is poorly distributed, readings at the sidewall may differ from those in the center of the flume.  Don't assume, though, that the centerline reading is automatically more accurate.