A replacement stopper for an older style CO2 sensor is available: #6 Stopper for Ethanol Sensor (ETH-STOP).

This stopper is a standard 1-hole, #6 stopper that has been over-bored and slit to accommodate the 1.1 cm stem of the older style CO2 Sensor.

The current shipping version of the CO2-BTA sensor (identified by its cylindrical shape and the two red bands attached to the hilt) does not need a stopper because it fits into bio-chambers and respiration bottles without one.

#6 Stopper for Ethanol Sensor (ETH-STOP)