Who should I trust Hanna or KH Director?

[carri10]

Hello.
My Hanna checker, with two month old reagent, is giving me an alk reading of 9.5. This is an average of three readings, range from 9.2 9.7.
My KH director, two weeks after the calibration is giving me 7.7. Recalibrated pH probe and got 7.6.

Which one should I trust?

From a chemistry point of view which test is the more robust?

Thanks for advice.

 

[DanyL]

The KH director *should* be more accurate based on the method it uses to test the water (titration).

However, as an automatic device it also has a lot more variability - pumps are used to take samples as well as to add the reagent.
The pumps calibration may drift, the lines can get momentarily clogged, they can get bubbles or the sample line may fail to return the old water back to the tank before taking a bew sample.

So, on this particular case the Hanna would be more reliable.

That being said, if in doubt - take a third test kit to verify, preferably a manual titration based test like Salifert and you’ll know for sure.

Another option is to test a known sample, like a freshly mixed salt water.

 

[carri10]

Thanks. Might have to get a trustable reference std. problem with getting another kit is I will then ask the same question, but with a choice of 3 results! It’s a little frustrating, as I am (was) a trained biochemist. We had validated, trusted tests. Hobby grade is hobby grade, I suppose.

The KH director, for what it’s worth, has been returning pretty consistent readings. I’ve watched it test to check on air bubbles etc (that was a great 30mins of my life!) and it works well.

 

[JulesH]

I would not sweat it too much. I cross check my KH Manager with a Hanna checker and get up to 0.7 dKH variability. Personally I just keep an eye on the trend with the KH not so much a single value, as long as it is about 8.0 dKH for me.

 

[DanyL]

Thanks. Might have to get a trustable reference std. problem with getting another kit is I will then ask the same question, but with a choice of 3 results! It’s a little frustrating, as I am (was) a trained biochemist. We had validated, trusted tests. Hobby grade is hobby grade, I suppose.

The KH director, for what it’s worth, has been returning pretty consistent readings. I’ve watched it test to check on air bubbles etc (that was a great 30mins of my life!) and it works well.

Well, 2 similar results would most definitely help you determine if the third one is out of whack.

But generally speaking, we’re mostly just looking for consistency, rather than accuracy with the main goal to keep the system stable. So even if one will always show let’s say 1dKH above the other, it wouldn’t really matter much for the corals as long as it’s repeatable and not jumping around or drifting.

 

[ReneReef]

Neither.

Trust a calibrated device.
Get the 7.5 reference solution. Run that three times or until the measurement stabalizes. Adjust the +/- % in the GHL app accordingly. Now you have a calibrated device you can trust.

Repeat every time you have done maintenance on the device. Or if you start doubting the results again.

 

[gbroadbridge]

Hello.
My Hanna checker, with two month old reagent, is giving me an alk reading of 9.5. This is an average of three readings, range from 9.2 9.7.
My KH director, two weeks after the calibration is giving me 7.7. Recalibrated pH probe and got 7.6.

Which one should I trust?

From a chemistry point of view which test is the more robust?

Thanks for advice.

I'd suggest to not worry about it.

You're after a consistent number - the number itself is pretty irrelevant.

Use the kh director with a Salifert kit as a sanity checker.
Toss the Hanna - I did that years ago - the reagent does not last long once opened.

 

[carri10]

So, report back.
I agree that stability is key, rather than a specific number, but it must be around a certain range. (therefore, tester must be reproducble, but does not have to be ultra accurate).

However, the dKH value affects things like equlibrium pH, so we need to have an fairly good idea.

I therefore bought a reference, dKH of 7.5 from GHL and the results were eye opening.

My Hanna tested 6.7 twice and my GHL tested 5.9 against the reference.

My hanna is therefore 11% out. My GHL was 27% out.
Now, given I'd calibrated the pH probe only a few days ago, I was a little shocked it could be so far out of wack. I don't know the inner workings, but struggle to see how a titration with a recently calibrated pH probe could be so wrong.

So, turns out my tank is running at 9.5dKH, not 7.5!

Conclusion - reference stds are a must.

 

[Vinny@GHLUSA]

So, report back.
I agree that stability is key, rather than a specific number, but it must be around a certain range. (therefore, tester must be reproducble, but does not have to be ultra accurate).

However, the dKH value affects things like equlibrium pH, so we need to have an fairly good idea.

I therefore bought a reference, dKH of 7.5 from GHL and the results were eye opening.

My Hanna tested 6.7 twice and my GHL tested 5.9 against the reference.

My hanna is therefore 11% out. My GHL was 27% out.
Now, given I'd calibrated the pH probe only a few days ago, I was a little shocked it could be so far out of wack. I don't know the inner workings, but struggle to see how a titration with a recently calibrated pH probe could be so wrong.

So, turns out my tank is running at 9.5dKH, not 7.5!

Conclusion - reference stds are a must.

Definitely use reference solutions when in doubt. That's your best apples-to-apples comparison.

For what it's worth, I'd double or triple check the water sample tube volume you entered and compare it to how many ml it actually holds. From what I've seen, this is usually an overlooked area of the setup and is going to affect the accuracy of each test, despite its consistency.

 

[carri10]

Thanks. Given what you've said, I won't rely on the calculator, but use a syringe to draw up water and measure that way. Seems a more accurate way to go about it.

By the way, the reference solution has no 'use before' date. How long does it last once opened? Do exposure to atmosphere affect?

 

[Vinny@GHLUSA]

Thanks. Given what you've said, I won't rely on the calculator, but use a syringe to draw up water and measure that way. Seems a more accurate way to go about it.

By the way, the reference solution has no 'use before' date. How long does it last once opened? Do exposure to atmosphere affect?

There's no expiration date as long as the bottle is kept closed and placed in a room with stable temperature.