Straight acetic acid rubbed in with a cloth is a possiblilty. Start with dry-brushing out everything that you can get out. Hold the item over your kitchen sink and the battery compartment turned downward so that all the debris falls into the sink. Then use the acid but be VERY careful not to get too close to the fumes: that stuff is TOXIC. And, would steel wool really be so out of place here? How much different is this instance from a pan with burned on food? - David Lyga
A pan has burned on food, an exposure meter has electrical/electronic circuitry which will not deal well at all with little bits of steelwool floating around.