My best guess is that you are having problems with the insulation... try using acrylic, thick...
Measuring with the multimeter can lead to problems... normally multimeters impedance is max 4Mohms
You should read DC on the Kelvin water dropper.
A scientific methodology would help you. Take measurements, plot graphics with the results to understand better what you are doing. This way you will be able to make improwemets and also explain better what you exactly need. Its really important.
if you could measure the air speed would be good parameter... but you can relate the rpm to the specific cross-sectional area of the tubing and the tension measured.. to find a relation useable... needed for design parameter.