Temperature sensor

[Bizibilder]

I've just had a pc temperature sensor delivered (TEMPerl) (eventually to be used to record the temp in the observatory) but I seem to have a problem in that it gives erratic readings (more often than not 9899°C!!) or no reading at all (no sensor). I have the supplied software "Hid TEMPer" and one I found on the web "Thermol-HID" - both are giving the same problem.

Do I have to calibrate the sensor? If so how?

(Ultimately this is to be used with APT which is where I found out about these sensors.)

EDIT:

I've just discovered that if I hold the sensor in my hand it starts to give sensible readings of around 32-34°C - as soon as I let go its back to 9899°C - surely this can't be right??