Just a heads up that the DeVilbiss digital gauges are not serviceable.
I have two of the HAV-555 gauges and one of them started acting up on me. The pressure readings were all over the place and the display was very dim so I assume the battery is dying.
I could't find any information on the battery or how to replace it so I called DeVilbiss and they confirmed it is not serviceable. The only way is to replace the head unit which they admitted was nearly the cost of a new gauge.
These gauges are $60 to $75 each online and only guaranteed for 2 years with the expectation the battery will last 3 years. That means you are buying a high dollar disposable gauge.
I ordered the DeVilbiss HAV-501 non-digital gauge to replace it with.

Agree. I have two which have both started giving erratic readings recently. Looked at serviceability but just replaced with analogue gauges.

Well I cracked on open and here is what I found:




Replaced the battery and applied a couple of drops of super glue to hold the case closed:


So if you have one that is dying you can revive it at least one time. Not sure the case will handle another go around.