I know what your are saying, but as near as I can tell, it's very close. I'm not having any problems with them. I don't know why Milwaukee says to only use a 7.01 buffer solution for use in our aquariums. A two or three point calibration would be better, it seems, but I just follow the directions. I have some of all three pH buffer solutions. I might try them the next time that I calibrate my probes to 7.0 just to see what they read.ok, definitely do what the manufacturer recommends. they obviously know best and i completely agree with that recommendation. BUT... if you adjust using only one calibration solution, do you use another one or two 'known' solutions to determine it's working accurately? basically, i should be able to use a pH4 solution and it should read pH4, right? if you want to evaluate the accuracy of measurement, you can measure either a) against a sufficiently more accurate test method or device, or b) the known 'truth' of the thing you are measuring. specifically, using another test kit or pH measuring device as 'truth' is only valid if it has significantly better accuracy than the monitor under test.
"A man with a watch knows what time it is. A man with two watches is never sure."
I have some comments.look at the response of the electrode (aka probe) in the following reference (graph in the lower right).
you'll notice that the pH response is fairly linear. also note that the electrode outputs 0mV at a pH7 regardless of temperature. sometimes when you are measuring a pH7 solution, the output is not 0mV, so you have to provide an offset (or shift the line down or up). this is known as the 'offset' adjustment. you 'offset' the curve up or down.
the electrode will generally output a constant mV per unit pH at a constant temperature (or in other words - a straight line as shown in the graph). sometimes the slope isn't correct and requires adjustment (possible a higher slope or lower slope). in order to do this, you need to use a solution other than 7, like pH4 or pH10 as an example. this is known as the slope adjustment. there is probably a recommended procedure for calibrating the monitor. for example, it will tell you to adjust the offset first before the slope, because if you did the reverse you'd never get it right. this might be the reason people aren't able to get the correct calibration.
to reiterate, it really depends on how the electronics are designed. so, always follow the recommended calibration procedure from the manufacturer.
Snafu,it's true the said reference is from oaktron. the graph is a general one for a typical pH electrode. it has nothing to do with any particular type of monitor or controller. if you pull up a response curve from another electrode/probe vendor you will see a similar profile, roughly 59.2mV per unit pH. i was merely explaining what a probe typically produces and what is meant by offset and slope.
lastly, if it's still not clear... what i have said and will reiterate yet again is that each manufacturer will have a different electronic design and therefore will have different calibration procedures. you should follow the calibration procedures provided by the manufacturer.