This piece of equipment is one of the most important pieces of equipment in your tools for standpipe operations. It is second only to your hose and nozzles. It's selection and use it just as important as hose and nozzle selection. Unfortunately, it is one of the most overlooked and least understood pieces as well.
By placing this inline gauge on the standpipe outlet you are in essence moving the engine's pump panel to the floor below the fire. As a nozzle man, can you imagine not having an AO or engineer there to set the correct pressure? Thats what you are trying to do if you do not have an inline gauge. More often than not when people complain about nozzle whip it’s due to incorrect pressures. This is especially the case with 2” and 2.5” hose. The gauge is crucial to make sure you are not killing your guys in the line.
Standpipes are low pressure systems. Depending on the year that the system was installed the minimum required pressures are 65 psi and 100 psi. Sometimes due to PRV's, incorrect PDP pressures from the engine, or misinstallation of equipment during the building process, you may have pressures even lower. Look up One Meridian Plaza fire. They had standpipe pressures as low as 45 psi.
A note on use:
To obtain accurate readings, the nozzle must be open fully and flowing water when the "control man" dials in the pressure. The static pressure when the nozzle is closed will read much higher than the flow pressure when the nozzle is open. Also, a trick of the trade is to use a label maker and place the needed pressures for your size hose, at 150', 200', and 250' lays, so you aren't doing the hand method when you should be putting water on the fire.