How to read tds meter

Hi guys just got a new tds meter and this one has two settings one is (0.5-500ppm)and (0.7-700ppm)which setting do I use since they both give me a different readings. Cheers if anyone can help


Need the make and model, there’s so many differences between different meters there’s nothing that can be done without knowing WHAT you have and maybe looking for the manual, but I’ll hazard a guess as you have two ranges, TDS and EC, or “Total Dissolved Salts” and “Electrical Conductivity”, which are two different testsaltogether, although the test is done the same way, how that conductivity is registered and displayed is different, which is why you get two different readings. It’s not two different “ranges”, it’s two different interpretations of what comes off the sensor.

So, make and model please, just so people can be sure they give you the right advice.

It depends on what scale your feeding charts & such are in. That just means the tds is found by one of the following equations: ec x .5 or ec x .7.

TDS is actually found using the EC. EC is basically the number that truly matters, but then there are equations that turn the EC into the TDS.

The .5 & .7 are not dependent on make/model, but rather which country/area/etc a person is from, which determines the conversion typically used in that area. It’s also dependent on what you’re measuring, though for growing cannabis, the .5 or .7 factor are what’s used.

Here’s a chart from an ec/tds meter’s website:Screenshot_20180202-000922


Yes, but we still need to know WHAT he has so we know what we’re dealing with, as that will have been calibrated a specific way.

And it is specific to make and model as some pens are calibrated for TCS KCI, some for NaCL, some TDS 640, some TDS 442 or natural water, some allow you to choose, some allow you to adjust between a range as a conversion factor. So, your pen is what decides which conversion, and therefore which TDS range, you use.

For example, the Tri-Meter 202 model uses the NaCL conversion factor of 0.5, the 203 model uses the 442 conversion of 0.7. Same manufacturer, two different displays on two different models. I can go onto the Hanna Instruments site and find the same, of the first three TDS meters I looked at, 2 were adjustable between 0.45 and 1 and the other was preset at 0.5. So it very much depends on the meter itself to ensure he gets the best advice possible for HIS meter.

No, we know he has a tester that gives him one of 2 options. He’s trying to figure out which of those 2 specific conversion factors to use. Deciding on which factor to use has nothing to do with the brand of meter. We need to find out what conversion factor the feed charts & such are that he’s using. Also, he can change his conversion factor, as needed.

How do you know he can change his conversion factor? How many examples do you want that show either a fixed or adjustable “range” from a single manufacturer?

We know he can likely change from either 0.5 to 0.7, or it could be an EC/TDS pen giving two completely different readings in which case said TDS range is fixed.

You don’t know, neither do I, not without the make and model, we’re only guessing based upon what he posted and we don’t know whether he is reading things properly or not. Best to know what we’re dealing with with absolute certainty, isn’t it, because I don’t know what he has in his hands.

Well, since he can choose between 2, I assume he can switch them whenever. It’s not rocket science.

The thing the o/p needs to figure out is which of their 2 options is best for their nute line/schedule/etc. Once that info is secured, the choice will be made. @Pati, you should be able to find that info fairly easily. :+1:

I use an ec tester, myself, to greatly reduce dealing with which conversion factor people are talking about.

As I say, you assume. I assume. Reality is we need to know to be certain.

I keep mine on EC, simpler to go by when I use it because there’s no faffing with different factors, and the plants will start to tell me if something is wrong anyway.

Everyone has their own preference

Thanks a lot guys I think all info is great. My old tds meter did not have 2 settings so I knew wat is was reading this one is a ec and tds.

So just to make it more clear i did a little example for u guys tasting it just in water so it shows different results in the same water but one on 0.5 and 0.7 i just don’t get it why same water different readings.

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They are different b/c the conversion factor is different. EC is the base reading; tds meters are just ec pens which include the algorithms to convert EC into TDS. The 2 conversion factors will give you 2 different TDS’s, but your EC will always be your EC, if that makes sense?

Check out that chart I posted above. You’ll notice that for a tds of 280 (.7 conversion) and a tds of 200 (.5 conversion) both equal the same EC of .4.

So, what the tester measures is the EC, then the conversion factors change it to TDS. The different conversion factors absolutely end up giving you different TDS numbers, but the EC will not change.


Thanks mate that makes sense.ive only started reading my ppm about a year ago so for the past 5 years just been making sure that my ph and ec are on point and I have never had a problem but I’m always wanting to learn some new tricks so thanks for the chart schedule very helpful :grinning:

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That chart is the best cheat sheet.

Looks like @blackthumbbetty got this covered … my job is done here… back into the shadows I go…


U were on point as well mate your info was correct about different meters given different reading I’m just learning about the ppm stuff so I appreciate all help thumbs up :+1: to all

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I’m also shopping for TDS meter. Looking at ISPRING 3-button digital TDS/Thermometer with Auto Calibration. Will this work?

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No idea. But look here and it might help you.