Calibration of different TDS testers - help!

Wondering if you could help me answer a question. My coworker got the kkmoon 3 in 1 tester. I’m trying to calibrate it using the GH calibration solution, but we don’t know which scale it is supposed to be calibrated to.
We tried to drop it down to 1500 for the NaCl scale, but it will not go that low. It recommends using the 6440 Solution to calibrate it, and from what I have found it says that is best used for the Siemens scale, but his device measures in ppm not in S as the calibration solution says for that scale.
Should we be calibrating it to the Siemens scale even though it measures in ppm?
My TDS tester is reading close to correct on the solution at 1500 ppm. And my tap water reads at 62 - we cannot get his meter to read below 70 to 80 on my tap water at its lowest setting, and on the solution it is showing about 2890.
Any help would be greatly appreciated! We’re at work trying to figure this out LOL

Microsiemens is ppm.

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Ohhh!! we were just confused because the bottle says that it should read 2930 S - and the others say 1500 ppm and whatever - so if his device recommends using the 6440 solution to calibrate ( most of what I found says that solution is best used on siemens devices ) - should we assume it to be on the Siemens scale? And calibrate accordingly?

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I think you are over worrying the accuracy ph is far more sensitive than ppm/tds a matter of 50 ppm is not like ph scale where being out by .3 factors hugely ppm is rough measure of how much dissolved solids are in a liquid in parts per million when you here hydro growers talk ppm we always talk in ranges usually with 100 ppm ± variable :wink: so for instance when asked if soil is properly flushed I answer with 500-600 rarely an exact number since you would be fighting forever to set something at exactly 450ppm for example. Instead you will hear around 4-500 or around 2-300 etc. what it does help you track is drops and minor changes in ppm between res changes with a percentage of accuracy and if something is out by 200-300 etc.
Best way to some it up is it is used to set decent start point then track and monitor changes from that known point

when you look at a schedule it refers to ranges not an exact number I think you would go nuts trying to set a nutrient solution to an exact number let alone have it hit that number more than once on purpose
It’s more like guidelines than rules :stuck_out_tongue:


I’m just trying to find out if my TDS meter is correct by comparing it to his - wanted to make sure his is calibrated correctly before comparing results on tap water.

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So if his device is set on the Siemens scale, then his reading in the solution would be 2930 - so does that mean that he would need to do a conversion or something to match his readings to mine which are on the NaCl scale?
If his reading is so much higher than mine in the same solution, then that would mean his reading would be much higher than mine in regular tapwater as well, correct?

Correct ppm meters convert MS to ppm to make sense many meters have the option of changing reading scales mine reads EC MS and PPM just a matter of common preference


Uh, no.

NaCl solution is, I believe 1,413 ppm or microsiemens. If your meter is in ppm that should be one half of that. IIRC. Unless your meter is displaying in either of several different scales like EC, which can be calibrated for salt on one scale or potassium chloride on another, which is slightly different. But ppm should be 1/1 NaCl scale and equivalent to 1/2 a reading in microsiemens.

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but the readout says ppm on the actual device

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Let me find another treatise on it. I might be off too.

Maybe this will help.

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I had a good idea, for my own readings. I called my local fish store, and ask them if they could test my water for me… I was going to use their reading to check my reading. He told me their water comes out of the tap at 45 ppm - this place is relatively new and I’m sure they had new piping put in because they deal with water so much. I live in a older house and mine reads 60-65 ppm - so that would mean my cheap TDS tester is actually correct! :sunglasses::grinning:


NaCl is a conversion factor based on Sodium Chloride (regular table salt.) The conversion factor range is 0.47 to 0.5. Non-linear meters based on NaCl typically use: 0.5 x the EC level (if converting from µS to ppm or mS to ppt) or 500 x the EC level, if converting from mS to ppm. TDS 442™ 442™ or Natural Water™ is a proprietary scale based on properties of naturally occurring fresh water. The 442™ part is an abbreviation of 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride. The conversion factor range is 0.65 to 0.85. Non-linear meters based on 442™ typically use: 0.7 x the EC level (if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.


KCl is a conversion factor based on Potassium Chloride. The conversion factor range is 0.5 to 0.57. Non-linear meters based on KCl typically use: 0.55 x the EC level if converting from µS to ppm or mS to ppt) or 700 x the EC level, if converting from mS to ppm.

TDS 640

A less popular conversion factor. The conversion factor range is 0.64 to 0.67. Non-linear meters based on 640 typically use: 0.64 x the EC level if converting from µS to ppm or mS to ppt) or 640 x the EC level, if converting from mS to ppm. Yes, four different possible conversion factors means that four different meters that give measurements in ppm may all give different readings from the same solution! However, all EC meters should give the same reading in the same solution as there’s no conversion factor necessary. I know, I know … TDS sounds like a confusing thing - but it’s really just a measure of the total ions in solution. For every gallon of water you have X mg’s of stuff in it. If one of your friends starts talking about their nutrient solution in terms of TDS, be sure to find out what scale they are using. Many growers, especially in Europe, in an effort to avoid confusion, use EC. If you are still confused, contact the manufacturer of your nutrients and find out what they recommend. Remember to ask them what TDS scale they use if they give you dosages in terms of ppm. Likewise, if you are working with a TDS meter that only has a ppm display, remember you need to be sure of the conversion factor being used. TDS comes into its own when you need to measure individual elements in applications such as nutrient and water quality, tissue analysis results and soil analysis. Results from these laboratory tests will give individual elemental readings in ppm or mg/l. Remember, a TDS meter will only give you an approximation of the overall nutrient concentration, based on the conversation factor used. Below is a table to show the relationship between the various methods of displaying the strength of a nutrient solution.

So an EC of 1.0 or a us of 1,000 would be 500 ppm on the American NaCl (500) scale. There are two others: Australian and British/Canadian.

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Thank you @Myfriendis410 that actually makes a lot of sense. Explains why two different meters can get 2 diff readings from same water!
Seems like I shoulda gotten an EC meter - or figure out what conversion were using…
Thanks again, extremely helpful article!

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What is interesting upon further reading is that this is a compromise and only marginally accurate anyway. As it relies on electrical conductivity in water it cannot take into account non conductive elements as well as any chemical compounds that may form. So it’s really only a reference and not something to wig out about lol.

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