Hello,

Im using an FMU90 & FDU91 sensor. I have been facing an issue on FDU91 sensor. it is installed in an Elevated potable water tank. This tank has two compartment due to which it has two sensors. This elevated tank is been fed by a booster pump station. The problem which i face sometimes is one of the sensor value is fluctuating from the correct level to higher value and comes back to correct value. This is interrupting my pump operation in auto. The sensor was replaced by another one, but faced the same issue after some trials. The cables were swapped with the other compartment sensor and it works fine with it. Funny part is that the other sensor is working just fine with other cables.

I kindly request somebody share their thoughts to resolve.

Thanks
Mufeedh

 

If the fluctuation is fast, damp the reading. If it takes several minutes to return, here are some steps to take to see what is going on.

1. Go up and look at the inside of the tank. See if there are any internal structures that could be causing a false echo.

2. See if the fluctuation has period. If it repeats exactly the same way, there is something wrong with the sensor, or the cables or the
transmitter. You checked the transmitter against the other sensor, so it isn't likely that it is a problem.

3. See if there is a junction box that the cable goes through between the sensor and transmitter, and see if the cable breaks there and if there is a terminal board. I wonder if this isn¹t a cable capacitance issue, but I cannot be sure.

4. Replace the cable in one continuous length with no splices into the transmitter.

If all else fails, call the factory which is what you should probably have done first. They actually do understand their own equipment nearly all the time.

Good luck,
Walt

Walt Boyes, Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC

**Spitzer and Boyes LLC publishes the Industrial Automation and Process Control INSIDER (www.iainsider.com)**

 

Thanks Mr.Walt for your reply.

Well there isn't any structure which is in sensor's way.

The fluctuation is random. i did observe for about 4 hours but didn't see once. But it happened after some time.

I did forget to mention something. The sensor cables has been extended via junction box. The cable was swapped from the other sensor's extension cable. But still it was fluctuating.

Ill try connecting the sensor directly to the transmitter and see how it responds.

Could this be due to the humidity/temperature? Transmitter was indicating 42 deg C via HART during the problem was occurred.

 

Temperature could be a factor. Check inside the j-box for condensation that may be wicking down the cable.

Let us know how running a direct cable works.

One other suggestion. After you run the direct cable, take the sensor out of the tank and aim it at a flat piece of wood or steel (a target) where you absolutely know the distance and observe what happens over a day or so. If the fluctuation happens, it is independent of the application.

Walt

Walt Boyes, Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC

**Spitzer and Boyes LLC publishes the Industrial Automation and Process Control INSIDER (www.iainsider.com)**

 

Are the two compartments are open across the top where the ultrasonic energy from one transducer could affect the other? If they are totally separate compartments walled off from each other, then that can't be a problem. Milltronics/Siemens ultrasonics uses a 'synch' connection between multiple transducers to avoid crosstalk problems between two transducers within 'hearing' distance of each other.

 

I agree with David, the transmitters should be synced together.

> Are the two compartments are open across the top where the
> ultrasonic energy from one transducer could affect the
> other? If they are totally separate compartments walled off
> from each other, then that can't be a problem.
> Milltronics/Siemens ultrasonics uses a 'synch' connection
> between multiple transducers to avoid crosstalk problems
> between two transducers within 'hearing' distance of each
> other.

 

Thanks guys for your response. I have replaced the extension cable with a spare one which i doubt there could be slightest of the chance of loosing its insulation while pulling. It's under observation since last evening. Will let you guys know about the outcome.

 

If you are still having issues, here is a suggestion. I have ultrasonic sensors in my shop for loop detection. I have problems with them with air blasts and blow off. The frequency of the air has an effect on them. If you have any air pressure letting off in the area, you could be experiencing the same problem.

 

Guys. It was the cable that was causing the issue. Its been over couple of weeks after replacing the extension, and i haven't had any issues till now.

Thanks for the response everyone.