I asked Paul Unmack about ARP and off-level operation, to make sure I understand it correctly and gave a correct reply here. He made a couple of further points:
1. The cooling unit is sensitive to the direction of the tilt, i.e. a tilt to one side quickly disrupts the flow of liquid ammonia back to the boiler (because it has to flow uphill), while the same degree of tilt in the opposite direction lets the liquid flow downhill to the boiler and has less impact. It might even help a bit at a very small tilt angle. Tilts to the front or back have relatively little impact on boiler operation until the 6 degree mark. The bottom line is that we can't make broad statements about tilting a fridge or what ARP does or does not do for it. We have to consider the direction of tilt as well as the degree.
2. There are three possible tilt scenarios:
2a. If the fridge is only slightly off-level, or off-level in a less-critical direction, it may not over heat at all, so no worries.
2b. If the fridge is close to the critical point, the return of coolant may slow or become intermittent. This will cause a boiler temperature rise and ARP will catch it, preventing further overheating, and then restart normal operation after a brief cool-down. Another overheat event may occur, but perhaps not for a several minutes or a few hours. Cooling continues in between, as long as the 5 restart threshold is not reached. In this scenario, ARP has made continued operation possible without damage to the cooling unit.
2c. If the fridge is sufficiently off level, coolant return ceases and an event will quickly be triggered. ARP will shut down for a cool-off, and then re-start. But if the fridge remains off-level, another event will SOON re-occur. And another and another. After 5 events, it shuts down with no restart, and the fridge interior begins to warm up because all cooling has stopped. It would have stopped anyway in this scenario, but the ARP has prevented cooling unit damage.
In 2B and 2C, ARP has in essence made it safe to operate the fridge off-level, where safe means no internal, long term, damage to the fridge. So in that sense, ARP gets us back to the manufacturers 3/6 degree level specification.