Bacharach OXOR II 19-7037 - Manual (Page 5)

If batteries are accidently installed in the wrong polarity, a positive
temperature coefficient (PTC) thermistor will protect the instrument’s
electronic circuitry. The instrument will operate once batteries are
properly installed and the PTC thermistor is allowed to cool.
3.2 Probe Installation
Install the probe by sliding the end of its hose over the gas inlet port
on the top right side of the instrument as shown in Figure 3.
3
Figure 3. Probe Installation