Accuracy of pH test kits...

[rob1116]

I recently bought a pinpoint pH monitor. After playing with it for a month, talking one on one with the manufacturer, and calibrating it with 3 different brands of fluid... I can honestly say it is dead accurate.
Now here's the reason I didn't believe it's reading of 7.6 initially: I have 3 color indicator test kits, each one reading 7.9, 8.2, and 8.3. The 7.9 was the Seatest, the 8.2 was a Red Sea, and the 8.3 was a Hagen.
My pH is now 7.9-8.0, and the test kits read 8.2, 8.4, and 8.6.
Lou at American Marine, (makers of pinpoint monitors) wasn't surprised at all when I called him. He says he gets calls frequently from people that are positive thier probe is malfunctioning, (like I was) and it almost never is. When I asked him why the color indicator tests were so far out, he asked me this: "How many test kits do you have?" ...I said three. "That's what most hobbyists have. Now if you've been adjusting and striving for an 8.4 reading, and one or two of those is giving you that reading, and the other is giving you a much lower one... which one are you going to buy again?"
lol... It's true. I already thought that the Seatest must be the farthest off and wasn't going to buy it again.
Lou says that while most hobbyists pride themselves with having a high pH, (8.4+) the reality is most are running 7.9-8.1 true. And he wasn't trying to save sending me a probe. Through a few simple tests over the phone, he proved to me without a doubt that the probe wasn't 'drifting'... and then told me he would gladly send me a new probe just to make sure, but it would surely read the same.

 

[rob1116]

I'm going to bring a sample of my water into work today and have the lab test it for me.
If my meter is right, the LFS's 200gal reef is running at 7.9, not 8.4 like thier meter suggests.

 

[doublezero]

Rob1116,
When I was teaching undergrad chem labs, we used a pH meter similar to the pinpoint model for a few experiments, we had like 35-30 of them... What I found was that each pH meter needed to be calibrated right before use(for that day) and in the range we were testing... that is if you are looking for a basic reading you should calibrate at pH 7.01 and then 11.01 and then make your readings... It is a linear correlation so those two points give you the mv line at which the meter uses to give you the correct pH. The fact of the matter is that pH electrodes are much more accurate than other pH testing methods when used/handled correctly. color testing kits are subject to lighting and eye interpretation, whereas pH meters rely on calibration points, HTH
also, did you test the pH at multiple times during the day???
#00

 

[rob1116]

Thanks for your advice 00... but I use the monitor on a continual basis. It's in the tank all the time.
Just an update for any that are still following: I did take it to work, and, with both meters calibrated... they read within 0.1 or under of each other. This monitor is obviously accurate... it's my pH that's low!
It's running about 8.1 now... 8.6+ according to some color tests.