pH probe calibration

skeety · 02-02-2006, 02:32 PM

When you calibrate your pH monitor...are you calibrating the probe? or the electrical readout?

here's why I'm asking.

I have a pH monitor already in my tank. With my new calcium reactor, I got a pH controller. Later, I'll need both, but for now...can I just unplug the probe from the monitor, and plug it into the controller? If I do this, can I expect a valid reading? or would I have to recalibrate it?

(for now, I'd like to just leave the other probe that came with the controller in it's storage solution)

whatevva · 02-02-2006, 02:52 PM

Quote:

I have a pH monitor already in my tank. With my new calcium reactor, I got a pH controller. Later, I'll need both, but for now...can I just unplug the probe from the monitor, and plug it into the controller? If I do this, can I expect a valid reading? or would I have to recalibrate it?

You definatly need to recalibrate. When you calibrate, your doing a matched set, ie probe/monitor

tdwyatt · 02-02-2006, 03:57 PM

What Butch said.

You re actually calibrating the meter's response to the particular probe's output. Each probe has an output that is close to ablut the same reaction to the circumstances you employ them in, but they will not be exactly the same, and the presence of biofilms, calcium deposits in the glass interface between the KCl solution and the seawater, even seemingly insignificant things like the resistance of the wire between the probe tip and the BNC connector will affect how the probe is read by the meter. The calibration establishes the centering for the meter's reading of the output initially (the 7.000 pH solution) , then establishes a logrithmic slope of a 1x10^+3 difference between the next reding and the initial standard. It is just as important to establish the center of the reading as it is the slope of the difference between the two, as without an accurate starting point, the error induced will be an arithmetic difference between the two expected readings, but if the center is dead on and the slope os off, then the error induced will be off by a multiplicative difference depending on how far the expected reading is from the 7.000 value. It doesn't matter which of the two other buffered standards you use for the second reading, so long as it has the difference between the two of the 10^3 pH value (i.e., pH equals either 4.000 or 10.000 from the centering value of 7.000 used to establish the centering value)

HTH

whatevva · 02-02-2006, 04:22 PM

Quote:

You re actually calibrating the meter's response to the particular probe's output. Each probe has an output that is close to ablut the same reaction to the circumstances you employ them in, but they will not be exactly the same, and the presence of biofilms, calcium deposits in the glass interface between the KCl solution and the seawater, even seemingly insignificant things like the resistance of the wire between the probe tip and the BNC connector will affect how the probe is read by the meter. The calibration establishes the centering for the meter's reading of the output initially (the 7.000 pH solution) , then establishes a logrithmic slope of a 1x10^+3 difference between the next reding and the initial standard. It is just as important to establish the center of the reading as it is the slope of the difference between the two, as without an accurate starting point, the error induced will be an arithmetic difference between the two expected readings, but if the center is dead on and the slope os off, then the error induced will be off by a multiplicative difference depending on how far the expected reading is from the 7.000 value. It doesn't matter which of the two other buffered standards you use for the second reading, so long as it has the difference between the two of the 10^3 pH value (i.e., pH equals either 4.000 or 10.000 from the centering value of 7.000 used to establish the centering value)

Yeah...that's what I MEANT to say...