So I’m not trying to untangle this mess again a year from now — here’s what l want to remember about vacuum ratings.
A bar, one that is, is like a thousand millibar. That is one atmosphere, that is 14.7psi. That is a bad vacuum.

Let’s stick with no vacuum for a minute: that’s also 760 Torr (Torr is the same as mmHg — millimeters of mercury). When we talk about mm or inches in pressure readings, we’re talking about how far the atmosphere pushes a column of mercury or water up a tube. If Torr is the same as mmHg, then 760/25.4 = aprox 30 inHg, as in barometric pressure. For what it’s worth, that’s about 400 in H2O.
Think of a U-shaped tube, with the atmosphere applying pressure on one end and the liquid being pushed up the other.
Now, if we start evacuating the atmosphere, we will gradually pull-down the column of liquid.
If I’m talking millibar, the lower I take the number, the closer I’m getting to a perfect vacuum e.g., 10mbar is more vacuum than 20mbar; same for Torr, same for……

….now, here’s where it’s easy to get confused: are we talking vacuum or atmosphere, is right on the gauge more vacuum or left? Hopefully someone has been nice enough to put an A or a V after the reading, or a negative sign to differentiate; but, sometimes you just have to look at the context.
For example: a typical pump for a small pick and place application might have the following on its nameplate:
VAC -.8 bar
CAP 12 CFM (I’ve already converted from m3h)
So, what we have here is an 800 mbar (or 600 Torr, or 6.3 inHg) pump — not much, eh? That’s because it’s not an 800 mbar pump, its a pump capable of pulling atmosphere down 800mbar to 200mbar. See? Even the guys who work with vacuums have to look at stuff a couple of times. So, be careful, or you can make some really stupid assumptions.
For fun, let’s look at a shop-vac to get an idea of where we’re at — typically about 150 Torr (200 mbar) at around 100 CFM. It would appear that it could serve the purpose of our industrial three-phase application above, and maybe it could for awhile — I think the difference might be in the way they describe the horsepower — whereas the industrial pump may have a 1 hp motor on it, the shop vac is rated at 1 hp peak.
Also, it would be interesting to compare curves. I have been told, as a rule of thumb, to look at the 200 Torr — about 22 in Hg — points to get a reasonable idea of what a pump can do.
Here are curves for some bigger industrial pumps

The top curve is for a pump that is rated as:
MAX VAC 28.5 in Hg
MAX FLOW 180 SCFM
That’s a lot. Could I use it to temporarily replace a pump with a smaller vacuum rating? Depending on the application — yes (if I’m not going to distort something). I have been told though that you should not run an oil-filled pump at a high vacuum (that is, at a high flow rate with high vacuum, say — only 400 Torr). This is due to the increased air-flow causing trouble with the oil-air separating function of the filters –too much air flow to allow the oil to separate and fall back sufficiently.
As far as carbon vane pumps, they apparently don’t care. Though I was told in the past that you should never dead-head one of these, I recently spoke with a rep that said you can (due to the fact that most will have a relief) but it is still best not to due to over-heating.
Links to conversion charts and pages here — under “Vacuum”.