The Puffer Forum

Hello... I'm building a 3-channel pH controller and 1-channel ORP controller. I've got the circuit breadboarded out and working... accurate to within +/-0.02 pH. (you should have heard the cheer when I tested it to get this seredipidous result ... I hate analogue electronics).

The idea is, one channel can monitor tank pH, one can monitor calcium reactor pH, and another channel is free to monitor something like kalkwasser pH. An LCD display will scroll between the readings, and there will be three controllable outputs. It will be able to be calibrated to two standards... selectable by the user. I'm incorporating a comms channel for connection to a future data logger / tank controller / PC (and a handheld logging unit with real-time graphs of pH fluctuations... but that's another story).

So far so good.

But I need input from people who have pH and ORP controllers so I can benchmark mine, and make improvements if necessary. You don't have to be familiar with how they work... I'm just after feedback on the controllers you use.

Specifically:

1) How long does your pH probe/monitor take to "steele". Say, if you put the probe in vinegar, and then transferred it to your tank, how long do you need to wait until the reading holds completely steady?

Say my tank is at pH=8.3.. On my prototype, "8.X" is reached within a couple of seconds.... "8.2X is reached within about 20 seconds... and then it takes a further 2 - 3 minutes for the final decimal place to become steady (i.e., up to three minutes later, it could change from 8.26 to 8.27).

Is this performance good? Too slow?

2) Temperature compensation. Many pH meters boast "automatic temperature compensation"... but my view is: What's the point? Our tanks do not fluctuate significantly in temperature, do they? In any case, from what I can tell, most temperature compensation is performed by a temperature dependent resistor housed in the meter's body... not in the probe itself.. which is not useful for compensating for water temperature.

The only problem I foresee with not bothering about temperature compensation, is that calibration is performed with standards at room temperature (i.e., not at tank temperature)... but don't most standards come with a list of pHs corresponding to temperature? In which case, the user could simply enter the pH to be calibrated to -- e.g. 6.99 or 6.98 rather than assuming 7.00.

Does anyone see any problems with me not bothering with temperature compensation?

3) ORP... I was thinking of allowing readings between 0mV and 500mV. Do I need to go higher, say 600mV? Even when cleaning probes with vinegar, dooes your meter ever read higher than 50 mV? Is it genuinely useful to be able to read the Redox of bleach?

4) Anything I'm missing? What could I add that would make these better.



Hoping to send the PCB off for production by next weekend.

Thanks for the help,

John

Hi John,
I use a portable pH "pen" on a daily basis in the spring at our fish hatchery. We buy the cheap pocket or stick type meters because we haven't found that the higher cost ones are any better. Of our meters, we have more trouble with our pH meters than anything else. Some of this may be due to lack of probe cleaning, as I've seen some very crusty ones in use, but we also often receive them working erratically, if at all, new out of the box. For this reason, when we need 2 new pH pens, we order 3 or 4. Of those that equilibrate in buffer, they often produce different readings on an unknown water sample. For example, of 2 meters calibrated to 7.0, one might read a water sample at 8.4x while the other might read 8.8x. The designs seem simple enough - could the production be the problem?

Anyway, the example you gave of time to achieve a stable reading is consistent with what we see when we have a good one. I like to see the display come close within a few seconds and come to within 0.1 within about 20 - 30 seconds. As you noted, it may go up another .01 or .02 after 2 or 3 minutes. I think that is fine. For my purposes, if I can believe the reading to the nearest 0.1, even 0.2, that is close enough. FYI, I use the pH meter to check pH of concentrated oxytetracycline solutions before adding them to tanks with 100K to 2M larval fish. Unbuffered tetracycline is very acidic, so we adjust/buffer it before adding to the tanks.

I can't answer your other questions except to note that the difference between room temperature and tank temp may vary seasonally. Right now, my room air temp is 12 degrees F colder than my tanks.

As for what else to add, I don't know if this is a good practice or not, but I add a small piece of sponge inside the probe cover/cap to hold some moisture around the probe.

Thanks very much for the reply... it is very helpful.

After a bit more digging, I have deided to incorporate auto temp calibration.

With regards to the accuracy, I hope to make it pretty close-- perhaps the problem with the pens is the one-point calibration? To calibrate properly, you really need to measure the change in offset (pH7) and the slope (gradient calculated from two values - pH7 & pH4, or pH7 & pH10)... I have had a few pens which only allowed one-point calibration, and found the same as you.