[A & J Owen Window Cleaning]

do we need to calibrate them or not. if so what do we do. have had one for 2 yrs and the new one gives a different reading.

[George Gardner]

What reading are you getting mate??

[andyM]

do we need to calibrate them or not. if so what do we do. have had one for 2 yrs and the new one gives a different reading.

I think the answer is plain to see.  
You can buy calibration fluid here: http://www.daqua.co.uk/calibrationfluid.htm
And the meter should have a small screw on the back which can be turned (clockwise/anti-clockwise) to adjust the meter reading while dipped into the calibration fluid.