Thread Starter


Dear all,

I have a task to select a suitable instrument for measurement of sphere, tank level. The service fluids are ethylene, Vinyl chloride monomer. Though the radar level insts. are of high accuracy, I am told to go for servo inst.

Can anybody please elaborate on the subject matter? also on problems faced while using such instruments. I have faced a problem of "loss of echo" in case of radar inst. which requires periodic re-re mapping!! please help.

Mihir Ramkrishna

Measurement of servo LT is preferable. Though you do have the problem of a large number of moving parts, the advantages like, accuracy, stable performance under fluid cloud and no interference of other signals, out weigh the disadvantages.

Morover, it can be assumed that the product density whose level is to be measured will not have wide change in density.
The strength of the return radar "echo" signal is dependent upon the dielectric constant of the medium - the liquid you're measuring.

Hydrocarbons typically have low dielectric constants - they are good insulators, not good conductors. The higher the dielectric constant, the better with radar. The lower, the greater probability of difficulty getting sufficient signal strength.

Your radar vendor should be able to tell you what minimum dielectric constant is needed for consistent, reliable radar readings.

There is a dielectric table at


This is the perfect example of "proper selection of the technology to be used".

Either Servo or Radar can be high accuracy instruments but if we use them in the wrong application, then we will lose all.

The type of products you refer are quite light, and the vapor of these products have a big influence in the radar signal, causing loss of accuracy in the measurement. This measurement may "improve" changing some process parameters like vapor saturation, temperature, etc. but it is hard to reach a high accuracy in this kind of applications. The best selection for this type of products is the Servo gauge, which is not influenced at all by the product vapor.

You might also look at guided wave radar...Magnetrol has done primary research on vapor echoes causing errors, and might have beaten the problem by now...so might other vendors.

But Mr. Vila's basic point is exactly correct. I ran a web poll on ControlGlobal.com specifically on this issue, and it amazed me how many people answered that they have level measurement problems because the applications weren't designed correctly.

Consider that we've been measuring level for engineering purposes for over four millennia now (see the photo of the Nilometer in the February CONTROL magazine) you have to wonder when, if ever, that we'll get it right.

Walt Boyes

CONTROL magazine
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Vishwas Deshpande

I am looking for some references where Radar Type level gauges (FMCW Type) have been working on VCM Spheres. Can anybody please help?
Currently SAAB Radar Model #3950 for crude oil tanks and these gauges were working okay. We replaced existing servo type VAREC Model #6500 and L&J 1500 servo type. Accuracy is better in radar.