ORP Measurement

[HotHotHot]

Hi,

I am using an ACII to measure pH, temp, and ORP.

I have read in a couple of places, including this forum that ORP generally rises at night. Also that a higher pH lowers ORP. Correct?

If that's correct, my ORP follows the pH linearly. ie. pH goes up, ORP goes up. pH goes down ORP goes down. ORP also goes up during the day and down at night.

Both probes were replaced in February. The pH probe is calibrated. The ACII is about 4 years old. I can use AquaNotes 3.1 if there are any tweakable parameters. (yes I can supply a serial number)

Is this normal? Is there anything I can check that might cause this? If not, can the unit be serviced to correct this?

Thanks,
Ed

[clp]

pH and ORP and inversely correlated. For every 1 unit change in pH the ORP will change by about 58 mV. Typically ORP will be highest in the morning (pH lowest), and lowest at night (pH highest).
If you turn on pH compensation in the ORP setup section of the controller, then the AquaController will compensate for this correlation. Note that the industry norm is will pH compensation off, so if you turn on pH compensation your ORP readings are not comparable to other non-compensated ORP readings. It sounds like you have pH compensation turned on.

Curt

[HotHotHot]

Makes sense. I'll turn it off and let you know how it goes.

Thanks,
Ed

[HotHotHot]

On a related note, should the temp compensation for pH be on or off?

Ed

[clp]

I recommend leaving temp compensation off for the pH probe for a typical reef tank. The temperature variation in the tank is quite small, and calibration temperature is very close to the tank temp, so temp compensation effect is very small (<.02 pH units).

Curt

[HotHotHot]

Thanks Curt.

I can now report the inverse relationship with pH and ORP. Big thanks.

However, the orp dropped to 109 without pH comp on. I know the true orp is higher than that or most of the livestock would be dead. I clean the probe about once per week with a soft toothbrush.

I messed with the offset and slope to get a reasonable number, although who knows what it really is.

Any suggestions on that.

Also, do you know a cheap place to get quinhydrone? You could make a killing selling premade orp cal fluid.

Thanks,
Ed

[clp]

A quick search on froogle.com brought some sources for quinhydrone. It looks like the cheapest is about $30.

Curt