I'm looking for very small changes in frequency. As little as 20 Hz. This is why I have a 24 bit counter. The counter is updated once per second, and the new count is always subtracted from the base count. This device is intended to measure accumulated thickness of material deposited on a substrate. The crystal, which is located near the substrate, is also coated and thereby loaded with the material which causes the frequency to decrease. The change in frequency can be related to thickness of coating of the material based on density of material and other factors.