Hanna HI98309 Ultra Pure Water (UPW) Tester

[TLCarrico]

I have a few generic TDS meters but I am looking for something that will provide a more precise result. Below is my understanding of what Hanna has to offer. I welcome input on whether my calculations are correct and also on which of the two testers you would recommend (or possibly something else altogether). Thanks

Based on my research, it looks like the Hanna HI98309 Ultra Pure Water (UPW) Tester (https://www.hannainst.com/ultra-pure-water-upw-tester-hi98309.html) is the most precise. Its EC Range is 0.000 to 1.999 μS/cm. Assuming the EC to TDS conversion factor is .5 (as I believe it is for their other TDS meters), the TDS reading would max out at just under 1 ppm (.9995 ppm). With an EC Accuracy of ±2% F.S., I think this works out to be ±.02ppm. Thus a reading of 0 TDS could be .02 ppm.

The next best, and maybe the more appropriate TDS meter, is the Hanna HI98308 Pure Water Tester (https://www.hannainst.com/hi98308-pure-water-tester.html). Its EC Range is 0.0 to 99.9 μS/cm. Assuming the EC to TDS conversion factor is .5 (as I believe it is for their other TDS meters), the TDS reading would max out at just under 50 ppm (49.95 ppm). With an EC Accuracy of ±2% F.S., I think this works out to be ± 1ppm. Thus a reading of 0 TDS could be 1 ppm.

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[ryanjohn1]

Can I just ask why you’re wanting it so accurate?

 

[TLCarrico]

Can I just ask why you’re wanting it so accurate?

After finding out that almost all TDS meters have an accuracy of ±2% Full Scale (not % of reading), I became concerned. A TDS meter that has a range of 1000, could have a ±20 TDS level - regardless of what the actual reading is. Thus, using a meter with a 0 - 1000 TDS range, my RODI output could be reading 0, but have an actual TDS level of 20. I know this is not likely, but in a world where most everyone strives for 0 TDS, a ±20 TDS of your reading is not very comforting.

 

[Randy Holmes-Farley]

After finding out that almost all TDS meters have an accuracy of ±2% Full Scale (not % of reading), I became concerned. A TDS meter that has a range of 1000, could have a ±20 TDS level - regardless of what the actual reading is. Thus, using a meter with a 0 - 1000 TDS range, my RODI output could be reading 0, but have an actual TDS level of 20. I know this is not likely, but in a world where most everyone strives for 0 TDS, a ±20 TDS of your reading is not very comforting.

That is not really correct. TDS enters all read zero in air, so the error is not +/- 20 ppm. They are pegged at 0 ppm. The error is proportional to the reading, not percent of full scale, no matter what Hanna claims.

FWIW, high accuracy is neither needed nor useful for an ro/di.

 

[TLCarrico]

That is not really correct. TDS enters all read zero in air, so the error is not +/- 20 ppm. They are pegged at 0 ppm. The error is proportional to the reading, not percent of full scale, no matter what Hanna claims.

FWIW, high accuracy is neither needed nor useful for an ro/di.

I appreciate your response.

I just assumed a manufacturer, Hanna in this case, would err on the side of overselling their product. Publishing the Accuracy of their TDS meters as ±2% of Full Scale certainly does not do that. Most, if not all, of their checkers show the Accuracy as a percent of reading, such as the following for the HI774 Marine Phosphate ULR Checker: ±0.02 ppm ±5% of reading. Can you think of a reason why Hanna would state the Accuracy of their TDS meters as a percent of Full Scale, such as EC Accuracy ±2% F.S. for the HI98309 Ultra Pure Water (UPW) Tester?

Here is a link (https://www.reef2reef.com/threads/not-all-tds-meters-are-created-equal.321024/) to a Hanna post, though a bit dated - Aug 18, 2017, but it is referring to "Our HI98308 Pure Water Tester is specifically designed for ultra pure water, having a maximum range of 99.9uS/cm EC or approximately 50 ppm TDS with and accuracy is +/- 2% f.s."

And then you have a $14.40 HM Digital TDS Meter on Amazon () that shows its accuracy as: +/- 2% Readout Accuracy.

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[RelaxingWithTheReef]

I appreciate your response.

I just assumed a manufacturer, Hanna in this case, would err on the side of overselling their product. Publishing the Accuracy of their TDS meters as ±2% of Full Scale certainly does not do that. Most, if not all, of their checkers show the Accuracy as a percent of reading, such as the following for the HI774 Marine Phosphate ULR Checker: ±0.02 ppm ±5% of reading. Can you think of a reason why Hanna would state the Accuracy of their TDS meters as a percent of Full Scale, such as EC Accuracy ±2% F.S. for the HI98309 Ultra Pure Water (UPW) Tester?

Here is a link (https://www.reef2reef.com/threads/not-all-tds-meters-are-created-equal.321024/) to a Hanna post, though a bit dated - Aug 18, 2017, but it is referring to "Our HI98308 Pure Water Tester is specifically designed for ultra pure water, having a maximum range of 99.9uS/cm EC or approximately 50 ppm TDS with and accuracy is +/- 2% f.s."

And then you have a $14.40 HM Digital TDS Meter on Amazon () that shows its accuracy as: +/- 2% Readout Accuracy.

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If the HM Digital TDS Meter states accuracy simply as +-2% (of reading) it is technically rubbish. This completely fails as the display has a quantization error of 50-100% at 1ppm. In other words, 0.4999ppm or 0.9999ppm water would display as 0ppm depending on how the internal measured value is truncated for the display. So a more appropriate spec for HM may be something like:
+-1ppm +-2% of reading

Reminder – This type of accuracy specification is interpreted as +-1ppm PLUS +-2% of reading.

It's possible Hanna did not want to make this “mistake”, and elected to simply state the accuracy as FS to pull in the quantization error. The HI98309 has a range of approximately 50ppm, so they are essentially stating an accuracy of:
50ppm x 2% = +-1ppm.

For fun, we can grab a Water Quality Conversion Chart, have a guess at a couple things, and dive into the Hanna a little deeper. The EC meter is essentially an ohm meter. The minimum reading for Hanna is 0.1uS/cm (equivalent resistance of 10 meg ohm). Lets assume their ohm meter has an accuracy of +-20%. This means 10 meg water could be read as 8meg to 12meg, and from the chart the water would resolve to a reading between 0.083uS/cm and 0.125uS/cm based on the Resistance Measurement Error.

Next we have the display resolution of 0.1uS/cm. All readings are quantized into 0.1uS/cm buckets, and this creates an additional uncertainty of around +- 0.1uS/cm.

Adding these up, and considering other possibilities of errors, there is probably an overall uncertainty of about +-0.2uS/cm, or +- 0.1ppm. So we would expect a real world accuracy specification something like:
+- 0.1ppm +-2% of reading

This should put the Hanna accuracy at the highest purity of water about an order of magnitude (10X) better than the HM.

There may be some other considerations for what Hanna did when it comes to mid-range accuracy if for some reason there are nonlinearities say at 25ppm, but that is not a concern if you just want to make sure your water is pure.

 

[Randy Holmes-Farley]

FWIW, some high end conductivity meters do not give an error range at all, since it presumably depends critically on how well and how often it is cleaned and calibrated. They give the range and the resolution, but not an expected uncertainty range.

Others do give the +/- full scale uncertainty, such as the one below, despite that being somewhat nonsensical at the lowest end of the range. None read below zero and I have never actually seen anyone report that one gave any nonzero value in air.

http://site.jjstech.com/pdf/Oakton-Instruments/WD_3227_61.pdf