CO2, pH and KH tests

I've been following the discussions regarding estimating CO2 
concentration using the pH and KH table with interest. I understand the 
importance of getting accurate pH and KH measurements in order to do 
this. I have a question about doing pH test. What I've found is that the 
pH reading of my tank water seems to change over a wide range from the 
time I added the test chemical to the vial and say, 10-30 seconds 
afterwards. When I first mix the water with the reactant by lightly 
shaking the vial for 5 seconds, I get a reading of 6.4. If I continue to 
shake the vial lightly for another 20-30 seconds, the pH reading goes up 
to 7.0.  Now which reading is the correct one. I assume that as I 
continue to shake the vial, some dissolved CO2 leaves the water 
resulting in a higher pH. But when do I know the chemical is well mixed 
with the water and when is shaking too much? The vial is covered as per 
the instructions (I am using a Wardley narrow range kit).

 John Y. Ching (jyching at watnow_uwaterloo.ca)    |
 Pattern Analysis and Machine Intelligence Lab	| 
 Department of Systems Design Engineering      	| 
 University of Waterloo, Canada            	|