All new sensors are a little more sensitive than the sensor you replaced. Remember that the instrument is reading in ppb (parts-per-billion) levels. As time goes by, a new sensor will calm down a bit. It is normal for the Halimeter®readings to jump ±10 ppb.
If you are observing the instrument spiking quickly either positive or negative, and returning to “000” quickly after the spike, it is possible that the Halimeter® is experiencing RFI (radio frequency interference). This could be caused by a cell phone, walkie-talkie, pager, etc. If your dental office is close to a radio station, or in a high rise building where large antennas are being used, the instrument is acting like a radio and is picking up the radio waves. It may be possible to mitigate these effects.