Thank you for all your replies.

My Ventus 2cT is operated under the manufacturers type certificate in
New Zealand, therefore it has to have certified instruments fitted.
The instruments must pass the 2 yearly calibration checks. At 18,000
ft mine is 320 feet out. The calibration table requires it to be +/-
120 ft at that altitude. At sea level to around 10,000 ft it is good
then the calibration goes way out and come right again through 21,000
ft. (I know they are only certified to 20,000 ft).

Thanks again, I think I may just order a new one and get a calibration
check done before I install it in my glider.