I don't agree with the sentiment "fix the problem not look for a better way to measure the symptoms". The statement on its own makes perfect sense but in the aggregate of what the OP has asked it does not apply.
It makes sense from the standpoint of looking for another way to measure IP after finding that there is IP creep or IP is too high, in that case one should definitely fix the problem instead of fiddling with how to better measure the symptom...
...but measuring IP can be used to detect symptoms in the first place. If a 2nd stage is free flowing, one can spend time adjusting it only to find that its not the 2nd stage but instead the 1st stage. Or one can spend time tearing down their 1st stage only to do unnecessary maintenance and replace parts that didn't warrant replacing because the problem is with the 2nd stage.
getting a visual of the IP by connecting a gauge is an excellent way to begin diagnosing problems and can be used from a preventative maintenance standpoint. One can use the IP gauge to detect a problem so it can be addressed before it becomes catastrophic.
One might argue that if one's regulators are maintained properly that this would not be relevant, but the idea of "proper" is only loosely defined. If one is taking their regs in for service which may include an inspection one year and rebuild the following, then during the inspection year one may be paying a technician to connect an IP gauge to the reg. I am not sure how invasive an inspection is but I assume it includes checking IP, checking for overt signs of corrosion, checking the sintered filter for verdigree/rust, checking the hoses for cracks, checking the cracking pressure of the 2nds stages, checking the 2nd stage mouth piece is not deteriorated and the tie wrap holding it on is in good condition, checking the purge button does not stick, and checking all hose connections are not loose. What am I missing? This is all things that someone with basic mechanical intuition can do on their own.
There was reference in the discussion above that most recreational divers don't check their IP....well the OP is talking about a regulator for his pony bottle...most recreational divers don't dive with a pony bottle, so those comments are not really relevant.
The OP is looking for a convenient way to measure the IP of the reg used on his pony bottle and wants to be able to do it in a way that presents the least hassle and the least possible danger to damaging his regulator. From that standpoint the recommendation would be (and it has been stated) to install an inflator hose to the 1st stage and leave it in connected, then use it to check IP. One can get a short hose used for sidemount diving to keep the foot print small or perhaps a normal length one will stowe better under the keeper(s) used around the bottle to keep the 2nd stage and spg hoses tidy when not in use. If adding a hose is not preferred then installing an inflator hose specifically to check iP, when desired would probably be the next least invasive/damage prone way to go about this, followed by removing the 2nd stage.
Of course periodic maintenance should cover issues where IP will be a problem...but problems can develop during the year or 2 year period between regulator maintenace, and some choose to extend that window out even further for various reasons. Using an IP gauge is an inexpensive and easy way to begin gauging the health of one's regulator. Does it give the whole picture? no, but neither does a doctor get the whole picture banging on the patients knee with a reflex hammer, or just listening to their heart....these diagnostic tools are typically used to confirm assumptions or to see if certain signs are present...that is how the IP gauge should/could be used.
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