PH Controller Accuracy

[g01ngog]

I have a
American Marine Pinpoint pH Controller

and was wondering the accuracy of these things. From what it says, I have a ph of sub 6.0, but my ph test kit says differently. I just calibrated this controller last week.

Other details:
The probe is brand new,
I calibrated using 7.0 and 10.0 calibration solutions.

So anyone else have experiences with the controller?

I'm eventually trying to automate my CO2 injection but since I see that my pH is staying this low, I have strayed away from injecting any CO2 and just sticking to excel doses.

[Cyclesafe]

For routinely measuring acidic solutions, it would have been better to calibrate first with a pH 4.00 standard and then the pH 7.00 standard. My Pinpoint pH controller has been replaced by an Aquacontroller Jr, but in its years of service, I have been confident of its results. Perhaps you should also question your pH test kit?

BTW, a pH controller should be used only as a "fail safe" device, rather than an upper and lower bounds controller. By "automating my CO2 injection" it seems like you might be implying that you're using the Pinpoint as a classic controller - like what it was designed, marketed, and likely bought by you to do. Better practice, I think, is to establish a low, constant bubbles per second with your needle valve, and leave it alone for several weeks so you can see the effect on your plants (and fish). During this period you should continue dosing with Excel so you have unlimited carbon. The Controller can be set up to turn the solenoid off if the pH drops to a level that you're not comfortable with, without concluding that your CO2 level is too high - it may or may not be. A drop checker is helpful here.

The fact that pH is low, by itself, should not dissuade the use of CO2. Low KH would justify some deliberation, however.

Hey, a guy with 3 posts is giving advice to a guy with one post!

[g01ngog]

This is probably one of the most informative posts I've ever read regarding the whole system.

[Cyclesafe]

Well, there's much better stuff on this site from people far more knowledgeable than me. It's well worth a subscription.

[pepetj]

I have had pretty bad experience regarding reliability of at least two American Marine PinPoint instruments.

I purchased two Nitrate Monitors and one Salinity Monitor from Drs Foster & Smith in 2009. Both intended for use with a Nano Reef I kept at the time. This is what I learned from both: No matter how careful I was calibrating the instruments, readings on the same sample widely differ for the Nitrate Monitors. The Salinity Monitor showed capable of reliable readings but it looses calibration quickly so it must be calibrated before each use and then re-check calibration and measure again to be safe.

The reason I purchased the second Nitrate Monitor was to compare how well they performed due to the erratic measurements I was getting with the first unit. I got disparate results both measuring the same sample several times with the same instrument and also when comparing measures taken with each instrument on the same sample both simultaneously (using a bigger beaker) or sequentially.

The name Monitor with which these instruments are marketed is misleading. A monitor should be capable to continuously measure whatever it's intended to measure but the manual discouraged such use and suggested both instruments to be used as any other meter, for punctual, not continuous use. I figure the same applies to the term Controller in yours.

Do you have a good pH meter with three point calibration? Do you have properly stored, valid to due date, reference solutions? If so calibrate it and check the performance of your Controller.

Failure in replicating results led me to question the reliability (how accurately a measured result represents the actual value measured) of my three American Marine instruments. I've decided not to waste my money in such unreliable apparatus. I wouldn't sell them to anyone since that would be scamming someone.

Pepetj
Santo Domingo

[Jim Miller]

As I understand it the problem with pH controllers is that they are measuring pH not CO2 concentration. Other things can cause movement in the pH. That is why the drop checker and personal attention to plant and critter condition are better choices. Even a perfectly calibrated and perfectly reliable pH meter can only measure pH, it can't measure CO2 directly.

jim

[Cyclesafe]

Yes. Even though

[CO2] = 3.0 * KH * 10^(7.00 - pH)

is valid under controlled conditions where only water, CO2, carbonate / bicarbonate ions, and the latter's cation counterions are present (like somewhere at UCLA's Young Hall - or hopefully in a drop checker), it is not valid where other buffering species exist - like in all aquariums. Nitrate, phosphate, sulfate, silicate, as well as many organic acid / base equilibriums exist.

http://en.wikipedia.org/wiki/Alkalinity

Here's an example of the concentration of these "other ions" in my tap water (http://www.sandiego.gov/water/quality/pdf/tbl6.pdf).

Nevertheless the concentration of CO2 is still directly related to pH as one of many variables. You add CO2, the pH drops. This fact can be utilized to use a pH controller to cut off power to the regulator's solenoid if the pH drops precipitously - perhaps due to an uncontrolled injection of CO2. Using it for anything else is questionable.

[Jim Miller]

So useful as a failsafe but not useful as a "setpoint" controller.

And since it isn't truely inherently failsafe itself, I think most would be better served spending their money on a proper CO2 dispenser, e.g. dual stage regulator.

Thanks

Jim

[Cyclesafe]

Agreed. Belt and suspenders.....