Many avalanche transceivers can automatically "revert" from receiving to transmitting (i.e., searching to sending). The concept is, if you're buried by a secondary avalanche while searching, your transceiver should automatically switch back to transmitting so other rescuers can find you. The "Auto revert to transmit" row in the comparison table shows which transceivers have this feature.
Some avalanche transceivers base the decision to return to transmitting after a fixed number of minutes have passed without any user interaction, some transceivers won't revert when they are receiving a signal, some transceivers allow you to choose the number of minutes before reverting, some transceivers base the decision to revert to transmitting after a number of minutes without any significant movement (so they won't revert when you are searching, but will revert if you get buried while searching) and some use a combination of these considerations.
It is worth noting that transceivers that base the decision to revert on whether they are receiving a signal (which makes sense, because you might be on the surface and searching for your buried partner), will not return to transmitting mode if you are buried by a second avalanche and end up next to another buried person.
A significant downside to automatically switching from searching to transmitting is that your transceiver will probably get ripped from your hands if a secondary avalanche does occur. Rescuers will then search for your unattached transceiver while you suffocate. We are not aware of cases where this actually happened, but it is worth considering.
Another significant downside of automatically reverting to transmitting occurs when a rescuer's transceiver begins transmitting without the rescuer's knowledge. In this case, other rescuers may begin searching for the rescuer's transceiver.
BCA (the company that makes the Tracker transceivers) is concerned enough about the downsides of auto-reverting that they require you to enable this option each time you turn on your Tracker. Pieps is also not fond of the feature—they only allow the feature to be enabled via software. Due to its unique proximity switch (which changes between search and send modes), the Pieps Micro's auto-reverting cannot be disabled.
All transceivers that can automatically switch from searching to sending should give a loud warning before reverting so a rescuer doesn't unknowingly begin transmitting during a search (an unfortunately common occurrence during trainings). The Pieps DSP Sport and DSP Pro are probably the best in this regard. Before automatically switching to send mode, they emit a European-siren sound for 30 seconds. After switching, they continually blink "AR" (i.e., Auto Revert) and continue to sound an alarm. The ARVA Axio emits only four beeps.
Another potentially deadly situation might occur if rescuers who are not actively searching with their transceivers (e.g., they are probing, shoveling, etc) turn their transceivers off so their transceivers don't transmit (which might interfere with searches) or receive (which will make unnecessary noise). If these rescuers are buried by a secondary avalanche, they won't be transmitting.
To avoid this situation, the Mammut Barryvox S, Mammut Barryvox S2, Ortovox Diract, Ortovox Diract Voice, and Pieps Pro IPS support a "standby" mode. When in this mode, the transceiver is not transmitting or receiving (so they don't make unnecessary noise) yet they will return to transmitting mode if motionless for a specific amount of time. Most manufacturers suggest that this feature is intended for professional rescuers. The downside may be remembering how to enter the standby mode.
Version 4.2 of the Pulse firmware was updated in 2019 to fix a potential problem with the standby (aka rescue-send) mode. Learn more here.
