Thread: Thermo Scientific Eutech Elite CTS Salinity meter. TDS conversion factor.

Hello,
I hate TDS meters, they are all crap for salinity, even when calibrated, when you actually test them with 1gram salt in 1000ml de-ionised water, none of them are accurate.
So, I copied Mike Snaden after seeing him use one on YouTube, because there was one on eBay cheap, and bought a-

Thermo Scientific Eutech Elite CTS.

Having acquired it off eBay sometime ago I have finally got round to sorting it out. I need some 1413 us/cm calibration fluid.
However, it wants to know what TDS conversion factor to use? It's set to 0.71, I googled it and some say for NaCi (salt) I need 0.55, some say 0.5.
Anyone got a clue?
I've never heard of that before.and the instructions just tell you how to change it, but no help as to what or why?

This is the first thing I've actually found on the subject. Looks like 0.5 ish is for pure water and more like 0.71 for 'mixed' water.
Which actually explains why adding salt to de-ionised water never quite does what you're expecting it to. Because the TDS factors are different.
-so perhaps the cheap ones aren't so bad?

"TDS Units
Total dissolved solids are reported in mg/L. TDS can be measured by gravimetry (with an evaporation dish) or calculated by multiplying a conductivity value by an empirical factor 13. While TDS determination by evaporation is more time-consuming, it is useful when the composition of a water source is not known. Deriving TDS from conductivity is quicker and suited for both field measurements and continuous monitoring 42.

When calculating total dissolved solids from a conductivity measurement, a TDS factor is used. This TDS constant is dependent on the type of solids dissolved in water, and can be changed depending on the water source. Most conductivity meters and other measurement options will use a common, approximated constant around 0.65 32. However, when measuring mixed water or saline water (with a conductivity value greater than 5000 uS/cm), the TDS constant should be higher: around 0.735 and 0.8 respectively 20. Likewise, fresh or nearly pure water should have a lower TDS constant closer to 0.47-0.50 36.

Standard Methods for the Examination of Water and Wastewater accepts a TDS constant of 0.55-0.7, though if the water source is known to be high in calcium or sulfate ions, a constant of 0.8 may be used 13. Several conductivity meters will accept a constant outside of this range, but it is recommended to reanalyze the sample by evaporation to confirm this ratio 13."

Taken from here:
https://www.fondriest.com/environmen...47%2D0.50%2036.

...but then I cannot pretend to fathom this table.
So if I expect a TDS of 300 I set it a TDS the factor of 0.67?

Explanation The equation for estimating TDS from \(EC\) is \(TDS(mg/L)=k\cdot EC(S/cm)\).* The conversion factor (\(k\)) is related to the water's chemical composition.* For natural waters like lakes and streams, \(k\) is usually between 0.6 and 0.7.* For solutions with mostly sodium and chloride ions, \(k\) is usually between 0.49 and 0.56.* The ionic composition of the water can affect the recommended \(k\) value.*

...I'm gonna leave it at 0.71

Originally Posted by Alburglar

Explanation The equation for estimating TDS from \(EC\) is \(TDS(mg/L)=k\cdot EC(S/cm)\).* The conversion factor (\(k\)) is related to the water's chemical composition.* For natural waters like lakes and streams, \(k\) is usually between 0.6 and 0.7.* For solutions with mostly sodium and chloride ions, \(k\) is usually between 0.49 and 0.56.* The ionic composition of the water can affect the recommended \(k\) value.*

...I'm gonna leave it at 0.71

Water Molecular Chemistry 101
The simplest way to explain the table is that this, and all similar meters, measure the current that flows between its two electrodes which is termed Electrical Conductivity (EC).

Very pure water won't conduct electricity and the amount of current that will flow through water depends on stuff dissolved in it. People who should have got out more and played football instead of studying books when they were kids call this stuff "ions" which are bits of molecules that carry a weak electrical charge similar to the static that sticks balloons to walls if you rub them on a jumper.

The electrical flow depends on which type these ion thingies are because some carry a greater charge than others.

Since some ions cause more electrical flow than others it's possible to say:
"That much electrical flow between the electrodes must have been caused by this type of ion thingies".

Sad gits can take that statement and then develop equations which give conversion factors for what concentration of different stuff dissolved in the water would cause that amount of flow so there are conversion factors for all sorts of ion thingies but the most common factors we use are for a salt solution or for a general approximation of a typical mixture of stuff dissolved in freshwater in natural bodies of water.

Since all these factors are calculated assuming typical amounts of ions, they are approximate and can be in a range depending on what else might be in the sample. E.g. the TDS factor for natural fresh water can be in the range 0.6 to 0.7 and the factor for salt concentration can be in the range 0.49 to 0.56 .

Below is diagram I drew donkey's years ago to illustrate how a TDS meter works that may be helpful to understand the general principle of how these meters work.

CLASS DISMISSED


Beginner TDS.jpg

All read, thanks Sid.
And theory understood.
But what's the answer?
So I set a salinity meter to the fresh water factor or the salt factor.
...or half way between the two?

Isn't it a bit chicken vs egg:

Me: I want to test my water.
Science: Ok to do that, tell me what's in it?
Me: I don't know, that's why I'm testing it

I've emailed Thermo Scientific to see what they suggest and I've messaged Mike Snaden of Yume Koi, to see if what he does too.
... probably more likely to get a response from Mike, but we'll see

You can either compromise and pick a half way setting or adjust it according to what you want to measure each time you use it or have separate meters like I have.

Alternatively, to save messing around or the expense, you can be realistic about the value of what you intend to measure. Salinity is an important parameter to get right but, since there is no particular value for TDS as it's only an indication of how dirty your water is, you could set the correction factor for salinity in order to salt baths right, then just use the slightly incorrect value of TDS to check that it hasn't suddenly increased for no accountable reason.

TDS can be a good indicator of the cleanliness of a pond but we should look for variations rather than an absolute value. The best way to assess TDS is to get your pond in as perfect a condition as you can make it, add any normal treatment such as clay or salt (for those who refuse to believe that a constant saline environment is detrimental to koi!!) then measure its TDS. That will be its ideal value which can be used as a base to judge any future value. Any increase in the value may well be due to an increase in pollution such as DOC etc., but a sudden increase is likely to be due to something that has been deliberately added.

Do I leave it high then because our water is from chalk aquafer so it's really hard with high TDS, I think if I recall correctly it's 340ppm out the tap.

If you mean leave the TDS high because of its calcium carbonate content, apart from going down the RO route, there isn't much you can do other than learn to live with a high TDS rather than chasing a particular value as recommended by the "Facebook experts".

I've also learnt that if I Google 1413 us/cm calibration fluid it's £35-£150, for 500ml but if you Google 1.413 mS/cm EC conductivity solution it's £6.50 for 300ml, as it's what the 'home grown' hydroponics guys use.

Well no that's the point, you can't do that using a salinity read out like I was trying to find 0.6%. That's what I was doing, but the TDS factor changes from distilled (de-ionised)water, to pond water. So you can't compare distilled water to pond water. Especially doing what I was, by using the Salinity read out of what is basically a TDS meter, without changing the TDS conversion factor , which I now think I understand enough to know is a best guess depending on likely water makeup. The conductivity of the distilled water is different to the pond water, which has all the extra ions.
This means that the maths you use to convert TDS to Salinity will be slightly different when testing de-ionised water ( with little or no conductivity initially) than when you're doing the maths, converting TDS to likely salinity in the pond.

So I did lots of testing with the cheap TDS Meters and I added 1 gram of salt to 100ml it wouldn't read 1%
I would repeat that at 200ml and 500ml and 1000ml and the results from the cheap TDS meters seemed like gibberish.

If I had recorded those results I could have posted them up. But I could not predict any results with known quantities of salt in known quantities of distilled water. Even in small volumes. Which is why I thought all those cheap TDS meters are crap.
...which is why I bought the posh meter

Once I crack it, I'll test the posh one against the the cheap TDS and the DD solutions one I have.

But I think I understand it enough now, to know that the 0.4% salinity I currently have in my pond is residual from when I salted to (by my maths) to 0.75 in the summer (only thing I could do the day before 10 night Holiday when parasite s found).

But in reality you have no way of knowing what TDS is salt or clay or other dissolved organics so it's all about educated guesswork.

I bought a proper Salinity meter, to get around that and just show salt, basically because I didn't understand it was using TDS at a variable conversion rate to figure that out.

...so back to square one. I'll use a TDS converted salinity reading ( because why not, I've bought the posh meter already) and use that reading to give me a ball park figure, but really it's down to sensible maths when salting the pond.

Which is pretty much what you and I have been doing already it seems.