The CupFET pH probe is designed for small volumes of sample (20 microliter). The droplet should cover the sensor (metallic spot) and at least a part of the diaphragm (white spot). Good to realize is that a droplet with such low volume and huge surface is prone to drift into the acid pH values. This is true for buffers as well. Hence, the calibration should be done using the beaker with buffer. Save the data as soon as possible after applying the droplet to the probe.