Alk testing confusion

[rygh]

I am getting a strange mismatch between different equipment.

I had a low-Alk problem caused by a bad doser, but the Alk was not really
going back up like I expected. So I tried a different kit, and it did not match.
I thought it might be a simple reagent issue, so I tested both against
a fresh batch of instant ocean, and they matched there.
So now I am even more confused.

Hanna Electronic tester:
My tank = 7.4
Instant Ocean = 10.1
(I double checked ppm to dkh math)

Salifert test kit:
My tank = 8.3
Instant Ocean = 10.2

From experience and other places, that 10 dkh is expected on IO.

I am used to some difference between testers, but almost 1.0 is really huge.

I ran the test on my tank twice, being very careful the second time. Careful cleaning of equipment,
shaking reagents, the works. Values repeated within about .1.

Reagents for both kits are inside expiration date.

Ideas????
Which one should I believe??

My only theory is that it is some total alkalinity versus carbonate measurement issue.
The electronic versus titration do measure that differently.
I normally do not do many water changes, so perhaps Borates or something are accumulating.

 

[Kensington Reefer]

Tests make me crazy!
If you have a reputable salt mix and preform regular water changes then you can keep the hair on your head!

 

[rygh]

Tests make me crazy!
If you have a reputable salt mix and preform regular water changes then you can keep the hair on your head!

You have a point there.

Maybe it is time to build a fully automated water change system.
Not that it actually needs to be fully automated, but on principle, I would probably have to do that.

 

[xulio]

I haven't used the Salifert but I was having a similar problem with my Red Sea.

In my case it turned out that the dispensing tip for the titration syringe had a small crack. Then some amount of the test solution would end up in the Titrator/ syringe holder instead of the vial. So, I was getting incorrect higher readings. Other than that, Alk testing is the simplest to do (compared to Ca or Mg).