Different readings between Probes PIII Ex and Expansion Box

I have just purchased an expansion box for my PIII Ex running my reef tank. The expansion box is to monitor water in my breeding setup.

I have had the PII Ex (upgraded to PIII Ex) for about 1 year. The initial probes are the same age (except for the Conductivity probe that I replaced 6 months ago because I dropped the old one).

Both the PIII Ex and the expansion box have the following probes

1. Temp
2. Conductivity
3. Redox
4. pH

For setup and testing of the new equipment I placed the probes in the sump next to my existing probes. They are about 5cm apart.

I have just calibrated all of the probes with the same calibration fluids (old and new probes). About 1-2 hours ago.

Calibrated with the following fluids
Ph with ph 7 and ph 9 Tunze use before date 31.08.2011 and 31.07.2011
Cond with 50mS at 25C GHL no use before date
Redox with 220mV at 25C Aquatronica use before date 09/2011

Fluids were at room temp approx 26C

PIII Ex Probe Readings
pH 1 7.89 ADC1 330 ADC2 113
Temp 1 27.7C ADC1 684 ADC2 312
Cond(S) 1 49.6 ADC1 20 ADC2 414
Redox 1 342mV ADC1 451 ADC2 550

Expansion Box Probe Readings
pH 2 8.02 ADC1 440 ADC2 232
Temp 2 29C ADC1 695 ADC2 323
Cond(S) 2 51.1 ADC1 42 ADC2 447
Redox 2 113mV ADC1 430 ADC2 525

Most of the probes produce a similar result (but with quite a bit of variance) and the readings have been fairly stable for about 1 hour, except for the Redox probe.

The Expansion Box Redox just seems very wrong and is falling on Redox 2 over the hour. It is now at 102mV.

What readings should I trust?

Is there anything that anyone can suggest that would account for the differences and what is going on with Redox 2?

Sorry for the essay, but I wanted to get as much info in as I could so that people have the information that is required.

Thanks in Advance

Maybe you don´t know it, but Redox probes need a some time to burn in (aka some kind of oxyde surface grows on the probe tip). I can´t give you an exact advice, but my probe (installed some month ago) had need approx. two weeks to give stable results. Until that, the value increases slightly day by day. I had not a second one, so I have no compare values.

So I suggest to trust the old probe, have an eye on the new one and calibrate it again in two weeks (or earlier) if the value stay stable. Don´t worry about the value it will go right, but it takes time.

Quote:

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Originally Posted by Wirus

Don´t worry about the value it will go right, but it takes time.

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Thanks for that. Ill give it some time.

I still have them in the test location. The current readings are as follows

PIII Ex Probe Readings
pH 1 7.87
Temp 1 27.3C
Cond(S) 1 47.6
Redox 1 342mV

Expansion Box Probe Readings
pH 2 8.03
Temp 2 28.6C
Cond(S) 2 51.0
Redox 2 86mV

In terms of the redox, Ill give it a few weeks.

It seems that the delta between the PH, Temp and Cond probes is stable. Ill run through a new calibration, of each probe type one after the other and see if they will come out closer.
If they dont get closer, is it reasonable to to adjust the probs to agree?
Or is this a bad idea?

Thanks for any assistance

well I have the same issue with my PH probe on my exbox and P3.
But I never bother testing it since they are differente probes and differenct lifetime. I just assumed it where 'normal' probe differences

did you switch the probes and did the difference stayed the same , if so I might do the same test ?

could it be the the EX box has generic calibratin value or have lost the values for the temp?

Following up on this issue for anyone that experiences the same thing.

I left the probe for approx 1 month then went through a re-calibration.
I then had a match between the two probes.

It appears that there was some time needed for the probes to report reliably (between old and new probes).

Hope this helps if anyone has a similar issue. Also as an asside, its probably worth re-calibrating all new probes after a month of usage as well.