Measure the liquid level in the tank with a stick.

Then decide which insturment is more accurate.

Disconnect the unused device to avoid confusion.

A man with two watches is never really sure what time it is.

I noticed that your ranges are different, thats going to give you different readings.I am not sure what Radar level you are using, but confirm you have the correct DK value and you mapped the tank.If both your levels are set up correctly the readings should be close to each other.

Just for my education: what does the M stand for?

Just curious as to what type of water storage tank you have there. I have lots of questions regarding this tank and the sensor installation.

Is it steel or concrete? EMF effects of steel tanks on the radar RF can play havoc with signal returns. Does it have a roof or not? What is the critical tank overflow elevation relative to the radar antenna elevation? Typical daytime and nighttime temperature ranges this tank sees? Is there a lot of water vapor present between the water surface and the antenna?

Was this a retrofit installation? And did a qualified Rosemount radar antenna technician install the antenna, or did your water dept. staff install it? Was it installed in an existing nozzle location in the roof or the tank sidewall? Was the nozzle too large or too small to properly accept

Was the antenna and the software "calibrated"?

High degrees of water vapor or wetness on the antenna can greatly reduce RF returns. Also, very clean clear waters, free of a high degree of suspended solids, can greatly effect RF returns. It is quite possible that you may be experiencing a higher level of return on a lower stratified water level or even the bottom of the tank or it's sidewall.

I would suggest that you contact Rosemount's technical support hotline or have one of their qualified technicians visit the tank site and render an expert opinion and corrective solutions that are guarantee to make this radar unit function properly. Have them warranty the corrective work for a period of no less than 2 years. They know their product line better than anyone else.

Finally, I would in the time being rely on the accuracy of the mechanical level equipment instead of the radar unit.

Here's a good article URL link regarding radar level sensors:

http://instrumentsignpost.wordpress.com/2010/09/10/radar-level-measurement-best-practice-pauto/

How much offset do you have in the two measurements?

As mentioned; Scaling and ranging very much have an affect on difference in readings.

Changes in atmospheric pressure will affect direct reading mechanical level measurement results whereas they will not affect the RF readings.

Any projections, equipment, metal supports, suspended solids, etc into/inside the tank will also adversely affect the accuracy of an RF instrument.

Rent or buy a portable pressure (Piezo) level detector to measure the level then compare the readings to determine which device is correct.

"Rent or buy a portable pressure (Piezo) level detector to measure the level then compare the readings to determine which device is correct."

And what's wrong with using a stick?

Absolutely nothing if degree of accuracy is not tight.

Since there is not an acceptable deviation in measurement range nor the difference in the two derived measurements stated, I'm not sure that taking manual readings is acceptable due to the accuracy required/displayed for the application/process.

Depending on the magnitude of the difference in readings between the two devices, the stick used may not have high enough resolution to accurately detect and/or display the resultant to the user.

Given that it appears the tank is at least 5 meters in height. (16.5 Feet)

If the difference in the readings of the two devices is let's say .0015 Meters and the stick cannot or is not inserted at absolutely 90 degrees to the surface, it would be easy for the reading to vary more than .0015M.

Contrarily; if the level accuracy is not critical and deviation is allowed at .01M or greater, then logically the manual reading being displayed would be subject to interpetation by the "reader" and the accuracy of the scaling on the manual level display device would most likely not be anywhere near as accurate as the resolution of the RF device and therefore use of a stick would clearly be acceptable.

Hopefully the poster is knowledgeable enough about the process to make the appropriate determination using the right equipment.

After re-reading the original post, several times, methinks you are being generous the poster.

GA

Are we saying that "M" should have been "m" i.e. metres? In other words the problem of deciding which device is correct boils down measuring the depth of the tank!!

Radar level sensor mounted in top ie Inverted height of 3.8M from top . Mech. Level range as 0 M starts from its mounted point in Tank . Relative difference in tank's height , if considered - will you have same problem ? , please .