Remember to check your gauges.

Sometime around 1975 I was involved in a situation similar to this. A dairy had called us up and their approximately 15 boiler horse power water heater would not fire up. Ran on propane. I checked it out and there was no propane to the boiler, none, nada, zip. We go out to the propane tank and it said it had about 50% propane level in the tank. This tank was huge. Probably 20 foot long, and I remember we had to get a ladder to get up to the top to check the gauge. So they have the propane guy come out and I show him how this propane tank that had propane in it wasn't supplying any propane, so he goes out and I show him the gauge said 50% full. He slaps the gauge and it goes to fucking ZERO! Holy Shit! Man, that was embarrassing, but I learned a very good lesson that day. Always check your gauges!