Calibration caps slide onto the sensing end of a probe providing an easy way to flush it with N2. This makes it easier to zero the sensor and check its responsiveness and calibration. Any tubing with a 6 mm (1/4") i.d. attaches easily to the barbed end of the cap. The different cap diameters and conical shape help to prevent the tip of the sensor from accidentally bumping the bottom, damaging the membrane.