EC meter readings

B

Bertie

New Member
Hi all,

I bought an EC meter a while ago (2 years ago or so), and have looked after it. Its from when I put together an NFT outdoor system for fruit and veg. Total waste of time as it turned out, though was still fun (ish).

Anyway, I spent ages last time finding out how to convert the reading on this meter. It is expressed in uS. A few hours ago I took a reading from my tap water (just to see), and it was 67uS, what does that mean? I have read that I should multiply by 1000 and divide by 2. This is supposed to give me the PPM.

This would mean I have a PPM in my tap water of 33,500. Which know for a fact is wrong (though it does have a whiff of chlorine, which I know will dissipate over a day or so).

When I eventually found the info on-line last time, the readings were similar, but that certainly wasn't the equation used. I remember it had something to do with 0.7 or 0.6 and some forumlae to calculate along those lines.

I've just spent two hours trying to find out, and I'm stuck, and in desperate need of a doobie, before I start throwing things around.

Anyone know of a conversion chart I can look at, or the formula to use? I see people talk in PPM and EC readings of 1.5 etc, but not this.
 
No it isn't, but thanks anyway.

I looked at that earlier, and it doesn't show my readings. From what I can now gather, my uS reading is 1000th of a standard EC reading.

So my 67uS = 0.067 EC

Though for some reason that doesn't look right either. I have read people saying that they have a starting EC reading of higher than that from the tap.

My tap water PH is around 8.2 - 8.5, and does have a whiff of chlorine, but I don't know what difference that would make to the readings. There is also fluoride added.
 
Says in the description it's factory-calibrated with a 700 µS NaCl solution.
 
On the rear of the device itself, it says "calibrate with KCl".

Either way, I still don't know the formula to convert from µS to more common EC levels which are usually referred to in forums, or even PPM which is what I'm more familiar with.

Having done some more googling etc, and taking another look at the product in the link I posted...

I think their site is a bit messed up, as if you click on the link to view picture of product in it's case, it is a different product altogether. Saying as its stamped on the reverse of the one I have with "calibrate with KCl" I'm going with that. Actually, it says 'calibrate wiht KCl' so obviously a bit shonky anyway.

The conversion ratios are...

TDS - NaCl: 0.47 to 0.50
TDS - KCl: 0.50 to 0.57

So I figure I'll go for a conversion ratio of 0.55

So my reading of 67µS yesterday, would convert to 37ppm (rounded up).

I'm still unsure of the conversion between EC readings as is often expressed on forums though. While googling I came across a link to this place, and a post saying the ratio was 1:10, yet elsewhere it says it is 1:1000. So my 67µS would either be EC 6.7 or EC 0.067. One looks way too high, the other looks possibly low. Maybe its 1:100:laughtwo: I think 1:1000 might be correct.

If I multiply 0.067 by 30 I get an EC level of 2.01
If I multiply 37ppm by 30, I get 1110ppm

Does an EC of 2.01 = 1110ppm? I read that EC for weed should be between 1.5 and 1.8, yet had assumed a good ppm range would be between 1200 - 1400 when on full feed. If that is the case, it doesn't add up.


Am I on the right track here guys? 37ppm seems mild considering it has a whiff of chlorine and a PH over 8. I thought that would suggest plenty of Ca present, so 37ppm would seem pretty low, considering there'll be a load of crap in there also (fluoride/chlorine and God knows what else).

======

Years ago, I was going to make my own, using a couple of stainless steel knives fastened onto a piece of timber and connected to an electricians meter. I couldn't have jumped straight in and followed hydro guidelines. There wasn't any around back then anyway, but figured I'd have been able to see what the plants were using, or not, and develop my own scale over time. I was running low on cutlery though (hot knife sessions), so it never really got off the ground and ended up playing in the dirt instead.;)

Anyway, am I getting close with those conversion ratio's?

Cheers
Bertie
 
"Does an EC of 2.01 = 1110ppm? I read that EC for weed should be between 1.5 and 1.8, yet had assumed a good ppm range would be between 1200 - 1400 when on full feed. If that is the case, it doesn't add up."

I think I may be looking at the conversion as if it were 0.7 ratio. Hence me thinking 1200 - 1400ppm would be good. At the lower rate (0.5), 1100ppm would seem to be the high end, or 2.0 on the general EC scale. So it looks about right.

Would still appreciate confirmation though.
 
An EC meter does not measure the PPMs of TDS in a solution. It measures electrical conductivity. A "PPM meter" is just an EC meter which multiplies the measured EC by a particular conversion factor (determined by the manufacturer).

Neither device ever actually measures the TDS of a solution. For that, you can do what my buddy does as part of his job: Take a sample, weigh it on a scale that reads to 0.0001 gram, evaporate the water from the sample, and weigh the solids that remain. Then do the math. Then identify exactly what the makeup of those solids are (both in types of substances and the ratios at which they are present). Then do more math. But you're not going to get it from a meter.

Different substances, when dissolved into a solution, affect the EC of that solution at different rates. A liter of distilled water that has 1000 PPM of NaCl dissolved into it would have a different EC than a liter of distilled water that has 1000 PPM of KCl would.

You can come up with an approximation. How close you come depends on a few things. If the solution that you are checking has only ONE type of solid dissolved into it, you just figure out what CF to use for that particular substance. But that won't work with hydroponics nutrient solutions because they are made up of a number of substances - which, as I mentioned above, would all affect the EC of that solution differently. See the problem?

So... you are back to approximating. The manufacturers of "TDS meters" will include a CF factor. They pick one which is expected to give you a best guess based upon the type(s) of substances in the solutions that you will be working with; a gardener would probably use a different one than someone who is concerned with the quality of public drinking water.

I strongly suggest that you read some of the information that is available on the Milwaukee Instruments website. Specifically, the articles that are listed on their Tech Tips and FAQ's(sic) pages. The information there will give you an understanding of what your EC meter is, what it does, how it does it, what it can - and cannot - do, et cetera. They have some decent meters as well, lol.
 
Thanks for the reply, but I'd already checked that out. Like I say, I was considering making my own many years ago. I know what they do, and how they do it. It's just the electrical current between two points, which increases as more minerals and salts are in the solution, and decreases as there are less.
Yes, you could wait for the H2o to evaporate and weigh it, you could analyze the ratio's within those left behind solids also. I'm not prepared to go to that trouble though, nor the expense of purchasing a set of scales that will go down to an incremental level that small. I could put my feet up and send a sample off to a lab also, but I wanted to use the meter I have, and to understand the measurements given from it and be able to translate those readings to other forms (the ones usually used on forums).

Personally, I would think KCl used as a calibration solution would be best for horticulture. Certainly for most fruiting or flowering plants imo.

What you seem to be saying is they can only be a guess. That might be so, but it's all any of us have. You cant be suggesting we don't use them, yet it does read a little like that.

Anyway....

What I wanted to know was if there was a simple conversion chart or formulae that would get me as close to the middle of the ball park as possible using the said device.

Your earlier reply of "So... 335pm, 428.5ppm, or 469ppm... take your pick." left me a little confused.

There is a post in here (only post in thread, and posted as a guide by a Mod I think) that is incorrect as far as I can see. It says, the ratio is 1:10, I now believe (from what I've since read) that it is 1:1000. Maybe you'd followed that to come up with your earlier three possibilities, I don't know.

========

I'll ask again, as my question seems to be getting a little lost.

Is the uS measurement a ratio of 1:1000 of the general EC measurement used on weed forums. Would my uS conversion of 0.55 with a KCl calibrated meter be correct to get a workable ppm level?

and/or

Is EC 2.01 considered equal to 1105 ppm if device is calibrated using KCl (give or take a few ppm)?

Anyone?

Cheers
Bertie
 
Bertie said:
Thanks for the reply, but I'd already checked that out.
Click to expand...

My apologies for thinking that you hadn't read them. But since they cover the basic info, I assumed from your questions that you had not.

Bertie said:
You cant be suggesting we don't use them, yet it does read a little like that.
Click to expand...

No, only that people understand what their meters do and how they do it. It is good to know that the TDS meters (and EC meters, after converting the result to PPM) give only a best-guess approximation and that what is dissolved in their solution will change the reading.

Bertie said:
Anyway....

What I wanted to know was if there was a simple conversion chart or formulae that would get me as close to the middle of the ball park as possible using the said device.
Click to expand...

I started to reply that someone posted a chart earlier in the thread, lol (whoops). Got confused as to which "EC conversion" thread I was in. Must be one (or two) others active. Let me hit the site's search function for a second...
How do I convert between EC, CF, & PPMs?
Hopefully that will help. I have seen the different conversion factors listed as NaCl, KCl, and 442TM. Also as Hanna, Eutech, and Truncheon. I don't remember offhand which is which.

Bertie said:
Your earlier reply of "So... 335pm, 428.5ppm, or 469ppm... take your pick." left me a little confused.
Click to expand...

I just took the EC reading you listed and gave you the calculated PPM reading using the three common conversion factors. Sounds about right for high-pH water with a fair amount of Cl and Ca in it. Possibly a little higher than average - but so is mine.

Bertie said:
There is a post in here (only post in thread, and posted as a guide by a Mod I think) that is incorrect as far as I can see. It says, the ratio is 1:10, I now believe (from what I've since read) that it is 1:1000.
Click to expand...

It's probably not incorrect. However, it may well make incorrect assumptions about the reader's particular EC meter and which unit of measurement it uses. Then again, I am just guessing as you did not include a link to the post.

Bertie said:
Is the uS measurement a ratio of 1:1000 of the general EC measurement used on weed forums.
Click to expand...

EC meters display in microSiemens/cm (mS/cm) or milliSiemens/cm (µS/cm). As the prefixes indicate, moving the decimal place over three spots (multiplying or dividing by 1000, depending on which way you are going) would change one number to another. Converting between one "metric number" and another is always done like that (with the prefixes determining the amount of decimal places to move) and this is no different.

But I'd think that just listing the number in µS/cm or mS/cm would mean that no "converting" is necessary. Like when asking someone how far it is to the store - ok, assume we are in a country that uses the metric system, lol. They won't say, "21.5," which might have you wondering if you are supposed to travel 21.5 kilometers, meters, centimeters, et cetera. They'll say, "21.5 km."

<SHRUGS> Of course, I would tend to assume that since the difference is a factor of a thousand, it's not exactly necessary to mention whether your EC meter's reading is in one or the other.

Bertie said:
Is EC 2.01 considered equal to 1105 ppm if device is calibrated using KCl (give or take a few ppm)?
Click to expand...

According to the chart I pulled from another thread here via the search function, 2 might be roughly equal to 1000 PPM, 1280 PPM, or 1400 PPM. Depending on the conversion factor you choose to use;). Milwaukee states that since nutrient ions are on average much heavier than NaCl ions, TDS meters that use the NaCl conversion factor (.5) typically underestimate the actual PPMs by ~30% - so perhaps the .64 or .7 conversion factor would be better to use. Or... Check the website of the manufacturer of your nutrients-of-choice. It probably has a "nutrient calculator" which will tell you how many ACTUAL PPMs are in a solution with the strengths that you input. Mix it up and measure the EC with your meter and you'll know. Don't forget to use distilled/RO water so that you won't be inadvertently adding an unknown amount of dissolved solids.

Here is the chart I found in one of the results of the search I did. I'm not sure what CF means in it, but it's clear that I shouldn't have abbreviated "conversion factor" to "CF" in an earlier post. Hmm...

PPM-EC-C.jpg
 
I don't think thats relevant to me. If I had a TDS meter, then yes, I would have to convert to get the recommended PPM levels from the nutrient bottle/manufacturers website, etc .

However, I have an EC meter, not a TDS meter. My EC meter displays it's measurements in uS (Micro Siemens). We have established that a reading of 1 Micro Siemen is equal to 1000th of a Milli Siemen, yes?

======================

"Your earlier reply of "So... 335pm, 428.5ppm, or 469ppm... take your pick." left me a little confused."

"I just took the EC reading you listed and gave you the calculated PPM reading using the three common conversion factors. Sounds about right for high-pH water with a fair amount of Cl and Ca in it. Possibly a little higher than average - but so is mine."

======================

I think you may be quite a bit off with your readings if you think a PPM reading from the tap of 300 - 470 is merely a little higher than average.

Here is a calculator I've found.

Conductivity convertor, Siemens into TDS PPM and MHO

If you stick 67 MicroS/cm in the input, which is what my reading is, you'll get a converted PPM of 42.9 (42.88 rounded up). They're obviously using the European conversion ratio of 0.64.

However, on the back of my meter it says calibrated with KCl, which has a conversion ratio range of 0.5 - 0.57. I decided to go with 0.55 as it's the higher end of middle. This gives me a reading at the tap of 37ppm once converted. Even using the 0.7 conversion it doesn't get close to 300 - 470ppm as you quoted from the chart. Indeed, at a 0.7 conversion it's still only 46.9ppm.

Fluoride is added to tap water at a low rate (1ppm or less usually), and chlorine doesn't conduct electricity (I've since found out). So it will be mainly CA, a bit of Mg and a few other bits of trace elements.

Tap water with Ca @ 400+ppm would probably give people the wild shites imo.
 
As an after thought. I wonder if people are reading high PPM's at the tap, and are opting for a RO system. When really it would seem it's quite low. There is the hassle of leaving the water exposed or bubbled to get rid of the chlorine, but I cant help thinking a few people may have gone for the RO system when it wasn't really needed.

Don't get me wrong, I want one myself. Thats so I can drink from the tap again though, not for growing reasons. Damn governments and fluoride, who do they think they are?

Another little thing I picked up, and this isn't really the correct forum, but saying as I'm here and it's kind of relevant. Lemon juice is the only thing that actually turns from acidic to alkaline once entered into the bloodstream. This is good for our high acidic diet we have these days.

So for those of you with a RO machine, a little dash of lemon juice in your drinking water is a good thing to do. It tastes nice too.;)
 
Bertie said:
"Does an EC of 2.01 = 1110ppm? I read that EC for weed should be between 1.5 and 1.8, yet had assumed a good ppm range would be between 1200 - 1400 when on full feed. If that is the case, it doesn't add up."

I think I may be looking at the conversion as if it were 0.7 ratio. Hence me thinking 1200 - 1400ppm would be good. At the lower rate (0.5), 1100ppm would seem to be the high end, or 2.0 on the general EC scale. So it looks about right.

Would still appreciate confirmation though.
Click to expand...




Bertie, isnt there a mode button on the EC Meter..u can change it to PPM.
700 uS = 342 ppm
 
To me the problem is people talk about ppm and what it should be, but they dont take into account the conversion factor. As far as i can tell, my meter has a .7 conversion factor, there fore when I add nutes and some one says it should be at 500ppm for veg, that number is at a .5 conversion factor, for me, that number need to be around 700ppm. If you can decide which solution your machine was calibrated too that will make it easier for you to determine your ppm... if it is KCL
TDS KCl
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.


Going by that and looking at the chart, I would say you have a ppm somewhere in the 300-400 range

if your getting 38ppm at your tap your doing something wrong in your conversion, my RO water comes back that clean...
 

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