Well, I'll give it my best shot (perhaps others here can correct me if I am wrong). My experience with pH meters deals with a Hannah Instruments tester (an absolute Godsend for colorblind people like me), but I imagine this is typical of most pH measuring equipment.
To calibrate a pH meter, I use the following steps.
(1) The device must first be placed into calibration mode, which is probably a slightly different procedure depending on the manufacturer. For mine, I simply hold the power on button for 3 seconds.
(2) The next step is to dip the pH probe into a reference solution. For my meter, I first place my probe into a pH=7 solution until I am instructed by the meter to change solutions.
(3) Next, I dip the probe into a second solution, which can be either pH=10 or pH=4 depending on whether I am planning on measuring pH values above 7 or below 7 respectively. After this time, my probe is calibrated. I should note that this is called "two point calibration" which is a fairly common method, but there are also one point calibration models and perhaps three point models.
The reason pH probes must be calibrated is due to the way a typical pH probe operates. A pH probe is a special glass membrane that is sensitive to differences in pH values. When the pH across the membrane changes, the electrical potential across the glass changes as well. Calibrating the instrument to the probe allows the potential differences across the probe to correspond to actual pH values. In my example, the probe remembers the potentials associated with the two known reference solutions, and can thus calculate the pH of whatever I am measuring by calculating the difference between the reference values and the value of what is currently being measured.
As far as how often the probe needs calibrated, that is a little hard to say. I try to calibrate mine once a month or so, but I am not constantly using it either. One can always place the probe into a known pH solution and see if the readings are still accurate. If not, it is time to recalibrate. I use one of my reference solutions to test my probe, which seems to be an easy way to see how accurate it still is.
Hope this helps a little.
[This message was edited by DaveStall on Thu March 27 2003 at 09:35 AM.]