glivo
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Joined: Fri Sep 11, 2020 7:43 pm

Reading TDS meters

Hi. I'm a newb to the forum. I have a question that hopefully someone will know the answer to (if there is AN answer).

I've recently bought 2 different brand TDS / EC meters and I'm getting very different readings from them. Not to mention the fact that they are also reading my "known" nutrient concentrations way off the mark. (Both meters are lass than 3 weeks old.)

The first one is "manufacturer calibrated" to a 1413 solution and measures temperature (C & F), TDS in ppm and EC. I'm assuming it is using the 500 scale since the readings are multiples of 2, ie; 500 ppm = 1000uS/cm = CF 1.0

The second meter measures only TDS and also comes pre-calibrated, but to C342 NaCl. However, it can also be calibrated to a C1000 NaCl solution to give better accuracy for testing different in applications.

I had doubts about the accuracy of the first one, which is why I bought the second (better brand) unit. Yesterday I made up a C1000 NaCl solution and tested at 25 'C using both meters, The second unit measured spot on at 997 ppm - 1000 ppm exactly but the first unit was only measuring at 795 ppm - 800 ppm. So this would suggest that the better one is good and the first one faulty. The first meter is reading lower than it should. Maybe not???? Read on.

BUT, I have a made up solution of Dr Huett's Lettuce nutrient. Mixing this nutrient from base salts involves using some that are hydrates in their individual crystalline form so the calculated TDS concentration of the diluted nutrient solution should be around 445 - 450 ppm. The first meter is reading it higher at around 580 ppm and the better quality one is reading even higher still at nearly 800 ppm. The first meter is still reading lower than the second but they are bother reading higher than I'd expect knowing what I've put into solution.

C342 calibration solution says this on the bottle. 342 ppm NaCl @ 25 'C. (362 KCl, 478 as "442", 700uS). So here is the question.

What does this mean? My meters are supposedly calibrated to C1413 standard (* see below) and C342 NaCl. How do I interpret the readings given when measuring a nutrient solution made up from 10 basic salts for macro and micronutrients? Do I need to be applying a conversion factor to the readings I'm getting?

I understand the difference between 500, 640 and 700 scales in EC / CF / TDS in principle, but do I need to be applying these conversion factors to my readings from the different meters?

* Note: CONDUCTIVITY 1413 us/cm referred to 25ºC. A solution preparation is based on the procedure in the HANDBOOK OF CHEMISTRY AND PHYSICS 1963 Chemical Rubber Publishing Company. Conductivity of Standard Solutions using KCl.
Preparation: Dissolve 0.7459 grams of potassium chloride (KCl) in distilled water made up to one litre of solution.
At 25ºC this has a conductivity of 1413 us/cm.

imafan26
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Location: Hawaii, zone 12a 587 ft elev.

What kind of water are you using? Depending on your water quality it can read differently. Although, I admit it should not range that far off.

glivo
Newly Registered
Posts: 2
Joined: Fri Sep 11, 2020 7:43 pm

The test water is made from distilled water reading 1 - 3 ppm and the nutrient solution is made from tanked rainwater reading about 30 ppm. Nothing to do with my problem interpreting the meter results.

Edit: Well it would appear likely that the cheaper first meter is not callbrated or is defective. Both devices appear to be using the 500 scale (NaCl) and so they should be giving the same reading. As to why the meter that is reading correctly reports nearly double the actual ions in solution for Hydroponic nutrients, I still don't know.



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