I bought a really cheap TDS/conductivity meter.

It looks like this:


It's completely useless as it does not adjust for temperature. Water from my mains cold water tap (maybe 10C?)reads 330ppm. Same water 30s later from the microwave (maybe 50C?) reads 700ppm.

Have I just got a broken unit? It seems the temperature reading in the meter itself doesn't really reflect the temperature of the water at all. If it did would one expect the meter to adjust for temperature properly? Or is it that really cheap "TDS" meters don't adjust for temperature and are therefore all useless?

I have used a similar cheap unit just fine. Test with water around 25C or 77F. Let a glass of water come up to room temperature. I'm not saying this particular unit might not be defective but try it with a couple different sources at room temp.

There are dozens of these cheap TDS meters that look and work the same. None of them, as far as I know, are temperature-compensated. The destruction manual/sheet of paper that came with it should mention the standard measurement temperature for the device. I've lost the instructions for mine, but if memory serves, it is 20 °C. However, if you don't have the time to let the sample come to the standard temperature, you can compensate by simple math. For each 1 °C above standard temperature, the reading goes up by 2-4%. This has nothing to do with the meter, it's simply physics; as water temperature rises, so does conductivity. Those that have temp compensation inbuilt cost way more.

I use this one. Not expansive and has temp compensation.

https://www.amazon.com/gp/aw/d/B007KDYO ... asin_title

cpreston wrote:I use this one. Not expansive and has temp compensation. https://www.amazon.com/gp/aw/d/B007KDYO ... asin_title

Wow, that's an unbelievably low price for a TC-TDS meter. Have you checked the accuracy? There are two checks to do. One is to test it against a known solution (cal solutions of various strengths can be bought inexpensively if one shops around). The other is to test the same water sample at several different temperatures. Incidentally, one can easily make a calibration solution. Simply add 1g of salt to a litre of distilled water. This will yield a calibration solution of 1000 PPM. To get a 500 PPM solution, simply take half of your 1000 PPM solution and add a like amount of distilled water. Do this again to get a 250 PPM solution, and again for a 125 PPM solution, and so on.

I hope this is related enough to not be deemed a "hijack"...

If the machine is plumbed-in (making sampling water difficult), any reason to think that sampling water from the boiler after letting it cool to room temp wouldn't give accurate TDS and hardness reading?

Nunas wrote:One is to test it against a known solution (cal solutions of various strengths can be bought inexpensively if one shops around

Yes, I calibrated it with a test solution.

In the UK this looks to have the same case shape and also claims to be temperature compensating:

https://www.amazon.co.uk/Yakamoz-Qualit ... B06Y3133HF

We'll soon find out if that's the case.

My understanding is that most of these TDS meters have temp correction, including the HM Digital TDS-3 and TDS E-Z. They aren't necessarily good at it, and the recommendation is to calibrate and use the meter at 25 ℃ (77 ℉) if you want accuracy.

The OP's meter might well be defective. If it doesn't read temp properly and auto-corrects for temperature then it could be terribly off. I believe these can be calibrated per HM Digital: https://assets.freshwatersystems.com/im ... struct.pdf


tl;dr Warning -- What follows is my somewhat long argument in favor of inexpensive NaCl calibrated TDS meters for us coffee people:

The inexpensive ones almost all use a linear NaCl calibration factor of 0.5 ppm per µS/cm. To my mind that's convenient and good enough for us coffee people. A professional water person might use a meter that can be calibrated to a 442 Natural Water™ solution, which would better approximate natural waters across a wide range of ppm values where the conversion factor is non-linear, and closer to 0.60 to 0.65 for the lowish TDS water we are measuring. 4-4-2 is also reportedly the calibration that the SCAA used when they chose 150 ppm as their ideal TDS back in 2009. So if for some reason you wanted to hit that same conductivity measure using your inexpensive meter you would want to shoot for a reading of 115 - 125 ppm. Interestingly, the TWW water formula is geared to read 150 ppm on an inexpensive meter that uses the NaCl calibration factor of 0.5. Makes sense because most of their customers have an inexpensive meter and have read somewhere that they want 150 ppm TDS for their coffee water.

One advantage of this 0.5 calibration factor is that you can just multiply your TDS ppm reading by two to see what the conductivity is in µS/cm. That can be handy when doing dilutions of tap water where the conductivity is available in your water report. For example, if your tap water report indicates an alkalinity of 120 ppm and a conductivity 300 µS/cm, then you can dilute it down to 40 ppm alkalinity and a conductivity of 100 µS/cm , which would read 50 ppm on your inexpensive meter.

Also, that factor is best for evaluating calcite remineralization. The conductivity of dissolved calcium carbonate is about 2 µS/cm per mg/L as CaCO3*. So If you are using a TDS meter to gauge how much mineral you are getting out of a calcite remin cartridge, if you get a 50 mg/L increase in calcium hardness as CaCO3 it should show up as around a 100 uS/cm increase in conductivity at 25 ℃, which would read 50 ppm increase on your cheap TDS meter.

P.S.
Some have argued that .6 or .7 is a better factor for use with tap water. That's not necessarily true. If you live in Seattle, for example, 0.5 would be the better factor. It's best to just be aware that conductivity can be used as a crude estimate of TDS, and an actual calculated or dry residue TDS can be off from that depending on what's in the water.



* Aqion calculation for 0.2 mmol CaCO3 (20 mg/L as CaCO3)