TDS (Total Dissolved Solids) Sensor?

Does anyone have experience using a TDS (Total Dissolved Sensor) w/ a PLC analog input. I have a customer trying to move from a control box and take the sensor directly into an analog input.

He had given me a table of resistances from the sensor which relate to a TDS reading in parts per million (ppm). I had given him a circuit based on DC current but the output from the sensor varied all the time.

Today I finaly got a chance to visit his facility and look at the control boxes he was using with the sensor previously. They all used some form of AC current with the sensor. One was 30V AC 60Hz and another micro-p board seemed to be based on a 5v 30khz signal.

Has anyone had any success with a TDS sensor and straight DC or if not, how would you convert an AC signal to be used with a DC analog input?

Thanks,

Well, Norm, I've never heard of a TDS sensor, but I did a little investigating anyway. I found THIS description, which states:

"Conductivity electrodes are not stand-alone units and thus require external signal analysis or data acquisition equipment"

I'd look into replacing the 'control box' with one that provides an analog output. Something like THIS maybe?...

beerchug

-Eric

it's been a while, so excuse my fuzzy memory. I thought TDS corrspond to conductivity for most solution type and i know Foxboro makes a conductivity meter with the standard 4-20 output. Here's some of the stuff I googled.

http://www.hydroponicequipment.com/infopages/tds.shtml

...

The last time i did something like this was for CIP in the food industry, I think.

I guess I should have said I'm looking for something "down and dirty" one might say. This is not related to human consumption water. It is for use in a system that filters car wash water (and no, this is not a student car wash project but listen up kids... maybe you could get extra credit by adding water recycle/filter to your car wash projects...).

The simplist board they currently use is basically a comparator to turn on a red light when the TDS in the water goes over a preset level. They calibrate it with a 10kohm resistor and turn a pot until the light goes on. Very simple, but again, the voltage across the sensor is AC. When I put my scope across the sensor wires, I see the RMS voltage change when the sensor is placed in different water samples. I can duplicate the circuit easy enough.

I guess the real question might be what the easiest/cheapest way to convert an analog AC signal to a analog DC signal is?

Of course, someone before me sold them the idea that we could incorporate the TDS sensor into our control and give them a number output on the Moeller MFD display. Then someone designed and built a control box with about 5 square microns of extra panel space. Then the project gets dumped to me to try to make it work.

Oh well, it guess it still beats flipping burgers.

For a steam boiler application a TDS sensor (Total Dissolved Solids) is usually a sensor which measure the conductivity of the water. There is a relation between the conductivity and the TDS. The question is will a conductivity meter work for this application?

ndzied1 said:

I guess the real question might be what the easiest/cheapest way to convert an analog AC signal to a analog DC signal is?

Click to expand...

Um, maybe a diode or two (or 4), with a capacitor thrown in for good measure?...

beerchug

-Eric

Little One:
Yes, it is really a conductivity sensor.

Eric:
I'll have to try that and see how it works. Hopefully the analog input has a filter on it to smooth things out. I guess the cap will help with that too...

First off, Hach, ATI, ROyce, and a lot of other suppliers make conductivity meters that could be used. You will have to put up with frequent calibration and electrode replacement because the abrasives in your water will erode the electrode.

An AC signal can be measured as DC. You will have to play with the calibration. M-Systems and A-Pi and others also make AC voltage input DC output signal conditioniers.

Would a turbidity sensor like these work?

http://catalog.sensing.honeywell.co...idity&ITEMLIST=143465,143466&P=9929&CURRITM=0

I use Endress and Hauser products for conductivity measurements. Mains supply voltage but 4-20ma out, so easy to integrate in analog in.

http://www.endress.com/

and visit their measurement analysis section in products.

the total suspended solid meters we have are usually 4-20 ma signals
would not the total dissolved solids usually output the same voltages? If so wouldn't an analog input card be better suited for your PLC hookup?