TDS meter calibration

[Brad_C]

Wife has been commenting "the pool tastes a bit salty". I run the salt about 4400ppm as it's a small pool and it doesn't take much washout/top up to drop the levels. Life has been "a bit hectic" since 2019 so I thought I'll check my logs. Found I hadn't adjusted the calibration on the TDS meter since about 2000 and it was 700ppm low. So the pool is actually over 5000ppm. That explains the salty taste.

I calibrate the pH meter once a year (or when the storage solution deviates from about 3.77), but had forgotten to calibrate the TDS meter.

I know once a year sounds a bit lax with the pH meter, but I've had it for > 12 years now and it's generally less than +/- 0.05pH out over the year, and that's frankly within the error for the meter+calibration solutions.

 

[AUSpool]

Same with mine, I think from memory my salt meter was reading about 500ppm low and I believe I had a bad batch of calibration fluid. Replaced the batteries and the probe. New calibration fluids. I made a 4000ppm reference solution with distilled water and salt to test its variation every time and now rinse and dry every time in distilled water.

 

[Brad_C]

Yep. I knocked up some 4600ppm test fluid back in 2019 which is still good. Distilled water and hot-plate dried non-iodised table salt.
Whether it's my TDS meter or pH meter, I *always* rinse of with deionised water before putting it away. The TDS meter dry and the pH meter in storage solution.

Must remember to check the calibration of the TDS meter a bit more often.

I made some pH balanced 70ppm CYA test solution back then as well. Next time I get a fresh set of test reagent I must test it to see if it has degraded.

 

[JamesW]

No salt reading from the SWG?

 

[AUSpool]

No salt reading from the SWG?

Some of ours do but not many.

 

[JamesW]

Also, why the high salinity requirements for your units?

 

[AUSpool]

I don’t know, our older ones used to operate at 6000ppm and seamed to last forever. Most of the newer ones have an ideal target of 4000ppm with an operating range of 3000-7000ppm. I keep mine at ~4000ppm. There are a couple of units that run on low salt, 900 and 1500ppm.

 

[Brad_C]

Mine specifies a minimum of 3000. I ran that for years, but found 4000 gave me a buffer for washout & rain. Targetting 3000 meant I was often under. In my pool a bag of salt is 1000ppm, so an extra couple of hundred over means I never drop below 3000.

No salt reading from the SWG?

Nah. I have a "production meter" & 2 LEDS. Both green, ok. One red, add some salt. Both red, have a closer look at what's going on (water temp, salt, cell).

I'm not sure I'd trust a readout on the SWG anyway, even if it was temperature compensated. The TDS meter is +/- 100ppm (when it's actually in calibration anyway).

 

[AUSpool]

Mine specifies a minimum of 3000. I ran that for years, but found 4000 gave me a buffer for washout & rain. Targetting 3000 meant I was often under. In my pool a bag of salt is 1000ppm, so an extra couple of hundred over means I never drop below 3000.

Mine is kind of the same. It says never go below 3000ppm. Ideal 4000ppm. And I’m sure there is something about 7000ppm. My salt meter reads low as it drifts out of calibration, not sure if that’s a general trend with all meters. I was targeting 3500-4000ppm so found myself running high but still within range.

For me 1/2 a bag (10kg) = 500ppm. 200g CyA = 10ppm. Apart from acid, everything else I need, or don’t, comes in the tap water. If I ever build again I’m ordering a pool by volume in 10,000L increments. It makes the calculations so much easier.

I just realised that auto correct does conversions, ie. 10kg =22.0462 lb. It just took me ages to right ‘= 500ppm’ because auto correct was adamant that I wanted to say 22.0462 lb.

 

[Brad_C]

Yeah, your pool is only 1000L bigger than mine. Makes it dead easy.

My fill water is CH 90 but TA 140, so I generally add calcium, stabiliser, acid, boric acid & salt. I found running low calcium levels seemed to impact the gel coat more (not that there's much of it left these days), so if I keep the calcium > 250 everything seems happier. Still don't need to ever clean my salt cell.

I used to let the pH float up to about 8 and then knock it back down to 7.2. That generally used to take 2-3 months. I'm going to try and keep it at 7.2 with more frequent testing/dosing to see if that helps with my ongoing algae issue, but as I barely get through 5L of acid a year I don't mind using a bit more if it turns out to help.

The only real thing that annoys me is my pool tends to eat 10-20ppm of stabiliser a month. I hate that test. That reminds me, I'd better order some more reagent.

 

[Bperry]

Yeah, your pool is only 1000L bigger than mine. Makes it dead easy.

My fill water is CH 90 but TA 140, so I generally add calcium, stabiliser, acid, boric acid & salt. I found running low calcium levels seemed to impact the gel coat more (not that there's much of it left these days), so if I keep the calcium > 250 everything seems happier. Still don't need to ever clean my salt cell.

I used to let the pH float up to about 8 and then knock it back down to 7.2. That generally used to take 2-3 months. I'm going to try and keep it at 7.2 with more frequent testing/dosing to see if that helps with my ongoing algae issue, but as I barely get through 5L of acid a year I don't mind using a bit more if it turns out to help.

The only real thing that annoys me is my pool tends to eat 10-20ppm of stabiliser a month. I hate that test. That reminds me, I'd better order some more reagent.

The pH doesn’t affect algae. Trying to keep low ph will just make you have to add baking soda to avoid crashing the TA.

 

[Griswald]

TDS is not the same as salt.

The "T" means TOTAL, so everything dissolved in your pool affects that number, not just salt.

 

[JamesW]

So the pool is actually over 5000ppm.

Note that a TDS reading of 5,000 ppm is not accurate and it is really a reading of 3,300 ppm for salt.

The meter has to be set to salinity (NaCl)(Salt).

A setting of TDS is not a valid reading.

 

[AUSpool]

I use salt meter set to salt and calibrated with lab grade 3000, 4000, or 5000ppm NaCl calibration solutions, with +/-0.05% variance.

But surely any TDS meter calibrated close to the read value with a half decent calibration solution would be just as close to the +/-200ppm accuracy of the drop test?

I can get my meter to be within a bees whisker of the drop test.

 

[Brad_C]

Yeah, me too. My meter is +/- 1 drop of the drop test when its calibrated correctly.

 

[AUSpool]

The trick is knowing how do use one correctly. TDS meters work pretty good strait out of the box in solutions up to about 500ppm. Anything over that and they need calibration as close as practical to the measure point and ideally set to salt. They measure conductivity and then apply a conversion factor that best represents the solutions. TDS and salt conversion factors are a little different. In a salt pool at normal levels the TDS is mostly salt as NaCl. A meter needs to be rinsed, calibrated, cleaned, and stored properly to get a half decent result. And I cross reference mine with a drop test every so often. In contrast, a drop test works straight out of the box every time. Well almost, all tests can have interferences or bad read issues sometimes.

Looking at @JamesW’s graphs above, if we were to add in a TDS curve and calibrate to 3000ppm, the entire TDS curve would move up or down to intersect the salt curve at 3000ppm creating something that looks roughly like a cross. At 3000ppm a reading would be quite accurate but as we move away from that point the difference between the curves represents the deviation from and accurate measurement.

Just for giggles here’s a copy of the results from some home testing.

Device/Method	Hanna 3g/L	ACR 4000	ACR 5000
Aquacheck salt strips	3030 +30	4170 +170	5410 +410
Com-100 ACR 5000	3060 +60	4080 +80	5000
Com-100 ACR 4000	3020 +20	4000	4880 -120
Com-100 Hanna 3g/L	3000	3950 -50	4820 -180

 

[JamesW]

The meter reads conductivity in Microsiemens (Micromohs/cm) and converts the value to salt or TDS using the graph or a formula or a lookup table of values.

If the meter is set to salt, then it uses the salt graph.

If the meter is set to TDS, it uses the TDS graph.

It is a different slope, so if the meter can be calibrated that far, it will only be accurate at the intersection/calibration point.

TDS assumes a "442" mixture of dissolved solids.

"442" refers to the combination of salts mixed with deionized water to comprise this standard: 40% sodium sulfate, 40% sodium bicarbonate, 20% sodium chloride.

 

[AUSpool]

That’s a very fancy meter, running off to look up the conversion, on microfish?

Thanks for the graphic of my explanation. From the graph, when calibrated at 6000ppm, and with the meter on 442, the meter would be reading a value of 2800ppm as 4000ppm.

I caught the Kid at the LPS calibrating their fancy salt meter with tap water. Back then most of the chlorinators would have run at 6000ppm. They sold a lot of extra salt doing that.

The conversion factors in my meter are;
KCl/1314 = 1.88
442 = 1.68
NaCl = 2.14

 

[JamesW]

The conversion factors in my meter are;
KCl/1314 = 1.88
442 = 1.68
NaCl = 2.14

Can you explain this better?

Conversion from what to what?

It's a nonlinear function for the graphs, so you cannot have a single conversion factor.

 

[JamesW]

From the graph, when calibrated at 6000ppm, and with the meter on 442, the meter would be reading a value of 2800ppm as 4000ppm.

If it is calibrated at 5,000, then at 3,000 salt, the TDS reading is about 1,600 ppm.

If it is calibrated at 5,000, then at 4,000 salt, the TDS reading is about 3,300 ppm.

So, a TDS meter can be useful if it is calibrated at the same point as the salinity or you can just convert the reading from TDS to salt using the graph or get a meter that is set to salinity.