FlytrapCare Carnivorous Plant Forums

I have some potentially exciting news, but I need a little help. I believe I've found a way to easily find the TDS of water using a tool that, although it is common place for someone that dinks around with electronics, is still not super common in most households, but it is certainly more common than a TDS meter. So, here are my thoughts:

TDS meters use the conductivity of water in order to measure how much is dissolved in them. Conductance is simply the inverse of resistance (1/R). Most households that have someone interested in electronics or almost anything that has to do with working with electronics probably has one of these:


This will allow you to measure the resistance of the water. The trick is, for the number crunching we need to have the probes at a set distance apart so the numbers are accurate. Using numbers from this website http://www.engineeringtoolbox.com/total ... _1062.html , we can see we need to keep the probes 1cm apart.

So here's what we need to do. With the probes 1cm apart, measure the resistance of the water ( for our case we want LOW conductivity, which means we want a HIGH resistance, so set the multimeter to as high of resistance measurement it can get). From there, we need to calculate the inverse of the resistance measurement. This will get us conductance. According the the website above:
TDS = 0.7 * conductance
Where conductance is measured in micro-siemens per centimeter.

In more mathy way of writing it out (and it makes more sense to me...)




Here's where I need some help. I don't have a real TDS meter to verify this works correctly. The portion where we invert the resistance per centimeter to conductance leaves some units not quite where I'd like them, so I'm not 100% sure how valid of number we'll get out of it. I tested some of my distilled water, and some water I allowed to run through my soil and I got about 0 for distilled and about 1 for the run through, but I'm not confident in those. Is there anyone that has a real TDS meter and a digital multimeter that may be able to see if this actually works?

TDS meters are super cheap. I think I paid $10 for mine. I would check it for you but idk where my dmm is. If I can find it i'll post my results.

Very cool Blake! If I get a few minutes sometime in the next few days, I'll try it out and see how accurate it is.

Dionae wrote:TDS meters are super cheap. I think I paid $10 for mine.

Agreed, but they're quite a bit harder to find than a multimeter. A lot of this is just based on curiosity as well...I'm an electrical engineer, what do you expect!

Thanks for the help on verification everyone! If there is anyone that doesn't feel comfortable with the math, feel free to get the resistance of your water and the reading from your TDS meter and I'll be happy to run the numbers.

It is quite easy to find a TDS meter any saltwater fish site that sells RO systems,, and One can go to amazon.com,,, do a search and one can find a TDS meter and calibration solution. Along with ebay,,,, the list goes on. no need to find a replacement and or use a multimeter when the real thing is a click away.

I too have only a multimeter, and no tds meter. I would love to see this tested, so then i can check to see if my tap water is clean enough to use. I remember reading somewhere that tds meters only work correctly at a certain temperature. What setting did you use on the multimeter for your calculations?

A TDS meter is going to be calibrated for a certain temperature. Since water conductivity changes as temperature changes, both the TDS meter and the multimeter will report different values based on temperature. Its best to just test everything at room temperature.

Since the TDS of the water should be low I used the highest resistance measurement (2000 ohm or the Mohm setting) with the leads ~1cm apart.

i found another website with a similar idea, http://www.nano-reef.com/forums/index.p ... pic=188790 nearly every website i found during my search was either about fish tanks, or growing marijuana hydroponicly. According to their calculations, water would need to measure 10000K or above to be under 50 TDS. Unfortunately most multimeters only go up to 2000K. also,all the waters i tested were in the 300-800K range, with 300 being tap water with salt added, and 800 being bottled RO water. this puts them at 1660 to 625 TDS, respectively. Hopefully someone a little better at using multimeters can correct me as i really hope that isn't the TDS of the water of my plants!
800k ohms : (1/800000 ohms) = 0.00000125 siemens, (0.00000125/0.000001) = 1.25 *500 = 625

I think the person on that site got a bit confused with their numbers. The number for pure NaCl that I've been finding when measuring PPM/TDS with respect to conductivity is 0.5, not 500. They suggest that if you're measuring something that has other components in it (not just pure water with NaCl) you should use 0.67 or 0.7.

That would make your measurement 0.875 PPM for your RO water.

Check these out:
http://www.globalw.com/support/hardness.html
http://www.appslabs.com.au/salinity.htm

I think we are getting closer, but something must still be off, because then the ppm of the super salty water would be way to low. also i think .857 ppm is just too pure to be possible. What is the conversion between (uS/cm) and Resistivity? Knowing that could make the chart on http://www.globalw.com/support/hardness.html very helpful

Siemens (conductance) are inversely proportional to resistance with no conversion factors in the middle or anything of that sort. As such, 1M ohm -> 1 uS (as seen in the chart posted above). The /cm is the part that I'm a bit shaky on and that's why I would like someone to test with a real TDS meter to see if we're close. Running the calculation and the units that go along with it theoretically puts out uS * cm rather than uS/cm. To get the units to work in our favor we would need to somehow hold the multimeter probes 1/cm apart...and since I can't hold something anti-apart I want some verification. Maybe I'll pick up a cheap TDS meter here in the next few days to test this, although I really don't feel I have the need for one other than a one or two time use.

just to verify, 50 ppm is equal to 50 TDS, right? Because that is the magic number i am looking for. I think (by putting a couple of those charts together) that a reading of 10K would produce ~50ppm water. Which would mean there is definitely something wrong with my measurements. it there any way to test the accuracy of my multimeter? I am going to bottle up some water and take it into school. I think my chem teacher will probably let me borrow some more technological salinity/PPM/Resistance/conductivity meters.

If you care for your plants or for that matter care for any life you put in your charge, dont go cheap. Go on line get a REAL TDS meter and go from there. They do not cost that much. A few dollars to ensure your plants have good water is well worth the price. It is good that we are all trying new things and methods to save $$ and all. But the basics never fail. when in doubt always fall back to the basics,, Get a TDS meter or just spend a DOLLAR on a gallon of distilled water.

If you care for your plants or for that matter care for any life you put in your charge, dont go cheap.

I'm not trying to go cheap here, i would say i am more interested in the science of it then then the money saving. I buy distilled water for my plants. I just think this could be an interesting use of my otherwise useless multimeter.

Jimmy wrote:

If you care for your plants or for that matter care for any life you put in your charge, dont go cheap.

I'm not trying to go cheap here, i would say i am more interested in the science of it then then the money saving. I buy distilled water for my plants. I just think this could be an interesting use of my otherwise useless multimeter.

I purchase distilled water for my plants as well. I think I'm in it for curiosity sake. I agree that a multimeter is never going to be as accurate as a real TDS meter, but it has an easy potential to give ballpark figures.

just to verify, 50 ppm is equal to 50 TDS, right? Because that is the magic number i am looking for.

Yes, that is consistent with everything I've read.

it there any way to test the accuracy of my multimeter?

You can always go to your local electronics store (or online) and purchase some 1% tolerance resistors. Typically they're blue/green in color, use a 5-band resistor color code in which the last band in the sequence should be brown to signify 1% tolerance. Unfortunately they're going to be a bit harder to get a hold of since common places such as Radio Shack only stock 10% tolerance resistors (these are the common brown/tan color with a 4-band code, the last band being gold to signify 10% tolerance). Once you have a good resistor of known value, you can measure it and see what the multimeter says.

Let me know what the equipment at you school says it is, I'm interested to see!