pH drift

[jmf143]

I calibrate my Hanna HI 991001 meter with 7.01 buffer and then 4.01 buffer per the instructions. If I leave the probe in the 4.01 buffer after calibrating the pH will increase from 4.01 to 4.05 over the course of a couple of minutes, then stabilize at 4.05. If I rinse the probe after calibrating and put it in the 7.01 buffer I get a similar rise to 7.05 over a couple of minutes, then it stabilizes at 7.05.

Does this indicate a problem with the probe, the meter or the buffers, or not a problem at all? The accuracy of the meter is +/- .02 pH at 68 degrees. My readings are done at room temp of about 62 degrees.

Should I simply subtract .04 pH from all of my mash readings?

 

[MikeInMKE]

I have the same issue with my Milwaukee MW102.

 

[mabrungard]

With my MW-101, I observed a couple hundredths drop in the reading when using 7.00 buffer. However, I observed a couple hundredths rise in the reading when using 4.00 buffer. They make a little sense in that the meter seems to be seeking some sort of middle ground between these buffer pH's. These were only minor shifts, so it doesn't appear to be alarming.

 

[ajdelange]

I calibrate my Hanna HI 991001 meter with 7.01 buffer and then 4.01 buffer per the instructions. If I leave the probe in the 4.01 buffer after calibrating the pH will increase from 4.01 to 4.05 over the course of a couple of minutes, then stabilize at 4.05. If I rinse the probe after calibrating and put it in the 7.01 buffer I get a similar rise to 7.05 over a couple of minutes, then it stabilizes at 7.05.

Does this indicate a problem with the probe, the meter or the buffers, ...?

The meter which is really too bad because the fact that the readings level off and stay level means that the electrode is stable. The problem with the Hanna meters is that they decide when to consider a calibration reading stable - not you. With some of the other meters, such as this new Hach many of us are tentatively excited about, you put the electrode in the buffer and wait until you think the reading is stable, then push a button to accept it.

Should I simply subtract .04 pH from all of my mash readings?

Yes as both check readings are off by the same amount in the same direction. If the errors are different you can interpolate between them to come up with the correction for the pH you are reading. For example, if the reading is 0.05 low at pH 3 and 0.04 high at pH 7 that is a span of 0.09 over 4 pH. If you are reading pH 5.20 you are (5.2 - 4)/3 of the way between 4 and 7 and should thus apply a correction of
+ 0.05 - 0.09*(5.2-4)/3

 

[ajdelange]

With my MW-101, I observed a couple hundredths drop in the reading when using 7.00 buffer. However, I observed a couple hundredths rise in the reading when using 4.00 buffer. They make a little sense in that the meter seems to be seeking some sort of middle ground between these buffer pH's. These were only minor shifts, so it doesn't appear to be alarming.

You are adjusting amplifier offset when you tweak the 7 buffer knob and amplifier gain when adjusting the knob for 4 or 10 buffer. These are the two independent parameters (assuming the isoelectric pH is 7) that describe electrode performance. You ought to be able to remove this problem by waiting long enough for the electrode to be completely stabilized before moving from the 7 buffer to the 4 buffer. This can take several minutes - up to 10 even for a relatively new electrode. It is one of those situations where 95% of the change takes place in 10% of the time. Similarly, wait a good long time in the 4 buffer before making the final setting of the gain (slope) control. If a couple of hundredths don't matter to you then you don't have to worry about this. All this is dependent, of course, on the parameters of the electrode being constant over a reasonable period of time. If slope and offset are drifting over minutes you will never get stable, accurate readings but as I recall you found that the electrode was stable.