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 Post subject: pH probe calibrationPosted: Wed Feb 01, 2012 7:33 am
 Phidgeteer!

Joined: Sun Feb 03, 2008 2:23 pm
Posts: 84
pH probes have a tendency to drift as they age and become slower in reponse. Aslo, when the slope is outside a certain range, it indicates replacement is required. I need to be able to recalibrate the 1130 from time to time in my software if possible. However, I cannot figure out how to apply the slope and offset to the device. Can detailed calculations used to derive the equation in the user manual please be provided. I tried calculating an amplification factor for the device and going from there but it is not working.

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 Post subject: Re: pH probe calibrationPosted: Wed Feb 01, 2012 10:00 am
 King of the Lab

Joined: Fri Mar 06, 2009 12:42 pm
Posts: 443
One major assumption in this calibration is that the pH probe has deteriorated linearly, which may or may not be the case.

To calibrate, you will need two sets of solutions with an already known pH, preferably ones that aren't too close together.

Take the SensorValue measured at each pH level.
ph1 and ph2 will be the two known pH levels, and SV1 and SV2 will be the measured SensorValues, respectively.

The formula is going to be in the form of y = m*x+b, where y is the pH and x is the SensorValue.

To obtain m:
m = (ph2 - ph1)/(SV2 - SV1)

To obtain b, substitute either the set of ph1 and SV1, or ph2 and SV2:
b = ph2 - m*SV2

And then you can use:
pH = m*SensorValue + b
instead of the formula in the product manual.

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