I'm not certain that this wiould give the required accuracy for raw material moistures (re: a specification limit for incoming inspection), however, this link provides info on some hand held units.
How portable are you talking about? Arizona Instruments (Tempe, AZ) has a small tabletop unit that is excellent. I have used them for years.
Mike Glynn
763-428-6180
This may not be appropriate but moisture sensitive plastic granulate require drying prior to processing. I've seen how effective dew-point meters can be in assessing granulate moisture content. For example, by rigging-up a dew-point meter to a desiccant dryer you can ensure that copolyester materials meet -40C dew point prior to processing. These hand held devices are not cheap.
The other methods sound fine but are they portable?
I suggst you contact the companies that produce material dryers. They should be able to recommend or sell you a tester that works in the range you need.
General purpose hygrometers may have a range of measurment much greater than the moisture levels you will be measuring.
I use alltheweb.com for searching the web.
tomysux - what type of polymer(s) are you interested in determining moisture levels? It has been my experience that handheld moisture analyzers (such as units for determining m% levels in concerte, soil, air, grain, etc.) do not quite fit the plastics applications. This is solely based on the fact that dried resins generally need to have no more than 0.x or 0.0x moisture, based on the manufacturer's spec and your own needs. Handheld units just can't deliver that level of resolution.