False readings on ultrasonic level transmitter 3

[formsand]

Hello all,

This is my first post here, if there is anything I did not understand from the new member and new posters forums, please correct me.

I am working on a project that is using aqueous waste tanks. These tanks fill with sudsy waste water, and when they are filled enough, the pumps kick in and pump the fluid out. To gauge the tank level, a Ultrasonic Level Transmitter is being used. The problem is that the soapy aqueous solution is fairly sudsy. Sometimes, the level of solution in the tank is fairly low, and there is a lot of suds. The level transmitter reads these suds as actual fluid, and triggers the pumps on. This can be a problem if the tank is actually dry except for the suds.

Does anyone have suggestions for a better device to determine the liquid level in the tank? Or a source of desudsing agents?

Help is appreciated! Thank you,
Aaron

 

[btrueblood]

Use a float switch. Or measure the hydrostatic head with a pressure sensor mounted near the bottom of the tank. If you are worried about debris clogging the pressure sensor, use a bubbler tube (of course, this will also help generate/maintain suds in the tank) Or use a radar $en$or.

Whatever you use, mount it in such a way that it's easily replaced from the outside of the tank.

 

[formsand]

Thanks trueblood, I have looked into the float switches. I should add that there is a signal output from the ultrasonic transmitter, so I would hopefully want to find a solution that we will not have to change the control system for.

 

[btrueblood]

Right now your pump starts at a high level, and stops at a low level, right? Upper and lower setpoints can be picked up by two float switches, and have each switch feed the proper analog voltage to mimic the existing sensor's output at those positions - a few resistors and maybe a diode is all you'd need.

Alternatively, junk whatever is reading the analog sensor, and replace it with two relays, the upper one latches the second relay, which is unlatched by the bottom float switch.

 

[danw2]

Use a submersible head pressure transmitter. It'll have a 4-20 mA output similar to the ultrasonic, so that it will connect to the control system. You might have to scale the analog input to match the submersible, but that's a keyboard change, not a hardware change.

If you can mount an ultrasonic, you can easily mount a submersible. It hangs down into the wet area from the top and needs to reach down to the level from which you want to measure 'zero'. Sometimes people lift it up a couple inches, being willing to sacrifice a true zero reading to keep the crap that collects on the bottom from fouling the sensor.

Foam is typically so light that it doesn't unduly influence the hydrostatic level reading much.

All submersibles that I'm familiar with are loop powered, with a 24Vdc power supply in the loop. Ultrasonics can be either loop powered or active outputs from 4 wire or 3 wire instruments. So you might have to have 24Vdc at 25mA to power a loop powered level transmitter.

Submersibles are used extensively in sewage lift stations and treatment because they work in a dirty, aqueous environment.

pdf file shows a couple different brands.

 

[formsand]

Thank you both for the answers. It looks like the submersible is a pretty good replacement for the ultrasonic, seems like it will be just a plug in replacement for the current ultrasonic.

 

[formsand]

I have another question for you dan. If the submersible only measures hydrostatic pressure, then does this mean that when the pump turns on to evacuate the liquid, it will not affect the submersibles readings?

 

[danw2]

Good question. The answer is yes, but . . .

Yes, pressure is force/area. So any force other than the head pressure (gravity's pull on the water column) will affect the reading because the area of the sensing diaphragm is fixed.

The movment and swirl of liquid is an additional force applied to the sensor diaphragm. Sometimes the force adds, sometimes the force subtracts.

The effect can be extreme when pressure sensors are installed in the flow pipe that pumps out of the bottom of a tank.

The effect can be minimal if the transmitter is located away from direct flows.

But, you described the vessel as a 'tank', so I assume the tank has some fair amount of horizontal area. Is there an accessible location that isn't right next the pump's suction? Remember, it's on a flexible cable that can be run horizontal some distance if needed.

Frequently, in level measurement, a vertical pipe called a still well or a stilling well, is used to quiet the flow around a level sensor (it's used with many technologies, not just head pressure). I've seen still pipes with holes, slits or notches at the bottom. But if it were me, I'd re-locate a submersible.

 

[formsand]

Ok that was one of my concerns. I'm guessing it is not worth the trouble trying to quantify the effects on the submersible and to just relocate it. Great, I think the submersible will work well.

Thank you both dan and trueblood!

 

[btrueblood]

Do think it through - the signal from the submersible is going to increase with increasing tank depth - does that match the direction of the output of the ultrasonic device? If not, you will have to use an inverting amplifier, or play with the pump control electronics.

 

[danw2]

True, the scaling for 'level' is the inverse of 'distance'.

If the ultrasonic was setup for distance (20mA = low elevation level but farthest distance from transducer, 4 mA high elevation level, but closest distance to transducer), then invert the scaling so that it reflects level.

If the ultrasonic was set up for 'level, then make sure the new range matches the old range, if not adjust the range values.

 

[formsand]

That's true, though the controls company will be handling that. I wanted to find a solution that would be as close to possible as the current system, though I knew an easy replacement wasn't available. Thank you all for your help again!

 

[jpolson]

Radar level transmitters. They will give you the same analog feedback you are getting now. We use the Siemens LR560 on many silos (lime - CaO). On 1 particular silo it is programmed to ignore airborne dust and pick up the bed of material below it. While playing around in my office I could get the sensor to ignore the floor in my office and pick up the floor in the office below me.

We used to use the ulrasonic sensors on some applications, but the dust would really mess with them The radar has been far more robust.

Krohne also makes some units that we like. I think the krohne is a little easier to program. The Siemens is a little less expensive. Krohne may also give away free programming software. Both units can be programmed from a lcd on the unit itself. There are other manufacturers, we just don't have any of them.

Good luck

Joel Olson

 

[hendersdc]

The ultrasonic sensor in the link below from Introtek will measure the liquid level through the bottom of the tank, avoiding all the froth on top. I haven't used one but I remember it from when the rep stopped by our office. Might be worth a look.

Introtek Continuous Level Detection

Doug

 

[formsand]

Did he give a hint on how reliable it is? It doesn't seem like the technology can change that significantly to allow accurate detection through the foam, though I could be mistaken.

 

[hendersdc]

I can't really speak to the accuracy or reliability of the sensor. I would suggest giving Introtek a call and telling them about your application. They are very willing to customise a sensor to suit your needs.

DOug