Cheap fleabay TDS meters

[Brad_C]

I spent (not very much) and picked up a cheap clone TDS meter from E-bay :


Knowing full well it was a clone I totally missed the fact it doesn't have a calibration pot on the back (like some of the better clones) and is actually one of the "cheapest of the cheap" clones.

Anyway it arrived, so I pulled it to bits to find a serious "cost down engineered" version (which is what I expected). Temperature is measured by a thermistor on the PCB just above the probe terminals. The lump on the bottom of the unit that is supposed to be a thermistor is just a blob of plastic. The probes are friction fitted up against blobs of solder on the board, and there are absolutely no adjustments or even trim resistors outside of the IC. So I put it back together and shoved it in the pool and it read ~3300ppm. My chloride level is ~4600ppm at the moment (tested last night with a drop test) and I have somewhere around another 200ppm of other grunge in there.

So I started to fiddle with button combinations until I found if you hold the temperature button down for long enough the display starts to flash. Put it in a liquid and press the hold button to increase the reading and the power button to decrease the reading. Hold the temp button down again until the display stops flashing to save the calibration.

Righto. Let's put 250ml +/-1ml in a beaker on the mag stirrer and add 1.504g +/- .005g of table salt. Now I don't actually know the moisture content of the salt, nor any additives it may have, so this is all really loose. The water went cloudy when it went in, so there's something else in there with the NaCl. My error budget says it could be anywhere between 5600ppm & 6400ppm.

I mixed that up and drop the probe in, and sure enough it's reading ~5000ppm. Calibrate that up to 6000 and call it good. Pop it back in the pool and we have ~4700ppm. Near enough for Perth.

A few dilution tests indicates its linear enough (+/- 100ppm) and for what I want it for (+/- 500ppm) it'll do. Nice. I now have a quick and easy salinity meter which will give me readings +/- half a bag of salt without busting out the silver nitrate. *and* I know I can re-calibrate it when I need to. I don't think I'm serious enough to go and buy some calibration solution. +/- 500ppm is fine.

 

[Brad_C]

This quote is from another thread back in March after I wasted 4 months on that cheap meter (and another that was worse).

Thus far I can firmly place the TDS meter in the "don't bother" category. I'll continue to try and chase a good & affordable one down, but that is because I have other non-pool uses I can justify it for. Unfortunately the relationship between salt and other TDS in the pool is handwavingly nebulous if you are chasing accuracy rather than precision. Even if you get an accurate meter and keep it calibrated, TDS in your fill water and chemical addition will continuously move the conversion factor you use to estimate chlorides from the TDS values.

Ok, so the meter I was waiting on back in March was a real HM Digital TDS-3. Since it arrived I've been through winter with various changes in water temperature and salt concentrations (leaks, backwashes, overflows ... ) and I've done a *lot* of testing. I whipped up a 4600ppm calibration solution and I've compared every reliable data point against the salt drop test. I revise my recommendation to "if you have a salt pool you should at least glance at one of these sideways".

After an initial tweak of the calibration to accurately read 4600ppm at that data point, it has remained +/- 100ppm (the accuracy of my high-res drop test is 80ppm). Calculations doing before and after measures on specific weights of salt verify out within the margin of error. I'm super impressed and there is absolutely no comparison to the accuracy and stability of the cheap clones I wasted my initial purchases on.

The thing I'm struggling to understand is my fill water has a TDS of about 400ppm, plus I have about another 250ppm of other ions, yet the TDS meter tracks the chloride test +/- 100ppm (for example I've just tested and the meter says ~4230ppm and the drop test 4160). Regardless, it tracks the drop test really well.