temperature sensor 55 and the relative humidity of the corrosive atmosphere is also sensed by a humidity sensor 58 the temperature sensor is preferably of a conventional design such as a national semiconductor corp model lm35 the humidity sensor is preferably of a conventional design such as a minicap 2 model manufactured by panametrics the counter 40 temperature sensor 55 and the humidity sensor 58 provide output signals to a programmable converter circuit 50 the converter 50 is preferably a programmable 80c31 microprocessor operating at 8 3886 mhz a number of conversion steps occur in the converter circuit 50 as further detailed in fig2 and described in detail below the frequency input from the counter 40 is corrected to allow for any deviations from a predetermined standard temperature also in the converter 50 the frequency is converted to either an analog voltage or an analog current signal and the voltage or current signal representing the frequency change is then converted to provide a measurement of corrosion thickness which conforms with the isa standard of corrosion thickness in angstroms corresponding to starting with a clean corrodible surface this is done by applying to the frequency signal appropriate factors depending upon the type of corrodible metal on the crystal 15 the nature of the crystal and the temperature determination of the factors used for these conversions is described below in addition if the relative humidity is above a specific predetermined limit or if the relative humidity has changed by more than a predetermined variation.