I have some of the best vacuum chambers there are in a multitude of forms. In a sense any short path head is also a vacuum chamber. I have viton sealed sublimator units that hold deep vacuum to less than one micron and I also have a Scientific bell jar vacuum chamber mounted on a thick steel plate and sealed with polymer rubber. I use a Pirani based graphing vacuum gauge to monitor.
In every case once the vacuum pump ceases to continue evacuating gas molecules any configuration I mentioned above immediately begins to climb irregardless of whether anything is in it or not. This is of course called a leak down phase and monitoring it can indicate problems if they exist. All chambers typically leak down within three minutes or so from one micron to hundreds. Several more minutes and normally they tend to plateau somewhere around 1500 microns. My Pirani based sensor begins losing accuracy up in that range and above but normally a tight scientific chamber will hold at 2000 microns overnight. This is with nothing in them as a sample.
The reason is pretty simple. We cannot prevent the really small gas molecules present in our atmosphere from penetrating the permeable seals used in the polymers at the system joints. Greased joints might be great but at some point in the vacuum system there are polymer seals and hoses used out of necessity. The key is mitigation and limiting the length of polymer hose to just what is necessary and limit joints also to only the minimum needed helps mitigate the problem. Polymers themselves also outgas and all polymers are permeable.
Now when you place anything that by itself evolves gas into a chamber then the laws of physics are what they are. More gas equals more pressure. More pressure over time equals less vacuum unless you are removing on a continuous basis at least the exact amount of gas being evolved plus the inherent leakage present as mentioned above. So why leave the pump on instead of on and off as the pressure cycles up far enough once the pump is off?
Simple. The vast majority of wear and tear under normal usage in an electric motor is the start up cycle. When you fire up a big motor normally you will notice lights on the same circuit will dim a bit. Sometimes a UPS power supply on the same circuit will beep. This is because until the motor spins up to 3450 rpm at 60 hertz the motor windings draw far more current than they do when up to speed. This extra current is converted to heat and also dims your room lights momentarily if they are on the same circuit. While you see that momentary dimming of lights when the pump comes on, what is happening inside the motor windings is that they are getting VERY hot during that time. So they go from cool then to very hot during start up. Metals all expand and contract as they cool and heat and metal windings are simply very, very long metal wires. They expand and contract too with heat changes and they rub against each other slightly as they change dimensions because of the thermal change. This mechanically wears them out over time whereas if they stay at the same temperature then there is no mechanical wear this way. Other parts of the motor also suffer from this heating cycle but you get the idea.
You might also notice circuit breaker problems with repeated restarts for the same reason. Most circuit breakers operate on heat flexing a metal strip. When too much current flows the metal heats up and at a point it trips like a mouse trap because it expands enough as designed to do this. On the first start up the breaker will get hot but not trip. If however it is still hot when the next startup of the motor happens then it might well start to trip. This protects house wiring but would certainly be a real issue to solve.
So to quickly sum up everything in your very nice detailed comment
✔All chambers loose vac, till they even out, unless under constant vac
✔Chambers, while loosing vac, can bring in airborne contaminatents
✔Seals of chambers can off gas into contents within chamber
✔Continuing to vac will stop all the newly purged gas from taking away from chamber pressure
✔Help prevent wear and tear on pump
✔Be a little easier on the circuit breaker
I'm sure I over looked or didn't mention a couple things, I'll surely reread it a few more times, lol.
I'm left with a few more questions if you don't mind
1. Will that gas that was purged from the concentrate somehow re-combined with the concentrate if left in the chamber with out vac over time?
2. Chambers can off gas into are concentrate!?
3. Is it really easier on the pump to leave it on for 3 days, then to turn in on and off twice a day?
If this is so I really screwed up when I returned my oiless Rocker 500 ( Which I could only almost get to -28) for a Yellow Jacket 8cfm 2stage, which is really loud in comparison ( but can get to 29.5, kinda taking a while)