4-20ma tank level sensor

I am using an XP88 ultrasonic level sensor. I have placed all of the formulas within my PLC code. It all seems to work properly other then the fluctuation of the 4-20ma signal. Should I add some type of filter with math to allow the values to keep from bouncing on the HMI? I read thru some of the other posts and it seemed that maybe some of the other ways to program the plc for tank level monitoring instead of utilizing a math equation is to crate a table. any opinions on what approach to take?

What is causing the fluctuation, noise?. Are you using shielded wiring?

Some analog cards have a filter build-in. What PLC and analog card do you have? If not you can use a simple filter see txt below.

I am using 16AWG wire to the sensor. It is powered loop device. I have not wired in a tank level sensor before and talked to the tech guys that support the unit and they felt everything should be good. I just noticed as it sits there the signal bounces...not a lot but a little. which in my case the scaling is set so that every inch is 72 gallons. these tanks are plastic so I am assuming I am getting some fluctuation from that variable.

lefeverj said:

I am using 16AWG wire to the sensor. It is powered loop device. I have not wired in a tank level sensor before and talked to the tech guys that support the unit and they felt everything should be good. I just noticed as it sits there the signal bounces...not a lot but a little. which in my case the scaling is set so that every inch is 72 gallons. these tanks are plastic so I am assuming I am getting some fluctuation from that variable.

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Well if you don't want to replace the wiring or attempt to get rid of the noise if that is causing the problem then that simple filter should do the trick for you.

Mickey said:

What is causing the fluctuation, noise?. Are you using shielded wiring?

Some analog cards have a filter build-in. What PLC and analog card do you have? If not you can use a simple filter see txt below.

Click to expand...

I have an IDEC PLC with analog card. Is it a common practice to use a shielded wire for an ultrasonic level sensor? The onsite electrician wired in the sensor so I am wondering if I need to push back on the customer to a run shielded wire? I will try the filter..through a math function on the PLC i am assuming..and see how it works

Is it a common practice to use a shielded wire for an ultrasonic level sensor?

Click to expand...

It is good practice to run shielded wiring on any loop powered 4-20ma device. Or analog signal.

I'm not familiar with your PLC, so no help with that from me.

Please be a little more specific, as you have only spoke in generalities.
What is the linearity and repeatability spec of your device?
(I'll answer my own question :
Accuracy +/- 0.2 % of max range.
Resolution: 0.079" (2mm)
Dead Band: 8" (20cm)




What is the resolution of your analog input when using a 4-20mA range?
Give real world numbers for the amount of fluctuation you are seeing. "Not a lot" also tells us little.
What is the frequency of the fluctuation? Can you put a scope on it or does the IDEC software allow you to trend it a high enough frequency to see what is going on?


We need to determine, based on your resolution, what level of fluctuation you should expect.

We need to determine if this is high frequency noise, or somthing low frequency such as waves in the tank.

Ultrasonic sensors by nature are not super precise. I would expect fluctuation.

Regarding your question about calibrating to a table, that depends both on how accurate you need to be as well as if the tank is irregular. If the tank gives you a very linear relationship between displacement and volume and repeatability is more important than accuracy, by all means stick with the equation. If your tank is irregulary shaped or you need precision beyond the linearity specification, then you should calibrate to a known standard and extrapolate between data points.

In either case, the calibration and scaling will not affect noise or fluctuation, so if that is why you are thinking about doing it there is no point.

I personally am not a big fan of filtering a signal just to make it look more stable that it actually is. There are times when you may need to, but all too often it is used to cover up a problem.

If every inch is 72 gallons, then every tick of the sensor ouput is worth 0.079* 72 = 5.6 gallons. Thats just based on the field devices resolution.

I am not currently in front of the PLC/HMI but I will note the fluctuation hopefully by the end of the week. I just noticed that it was constantly flickering or changing etc. This is my first application with a tank level sensor and I was hoping to get a feel from other users if they see this and if so how do they handle it or do you disregard it as an application. I noticde on the analog setup block that there is a filter option I will have to play with those setting to see how it effects the over range. The tanks are plastice so I am sure when mixing we w ill see turbulance and we will see changes with the cold air nights and hot days. Just trying to get a feel from the users on how they have handled their individual cases with a 4-20ma sensor.

This is a snap shot of the options I have on the analog setup block: