Question:  Can I use an air powered vacuum pump instead of the more expensive electric vacuum pump? What's the difference.

The purpose of a vacuum pump is to remove air and moisture from the A/C system. If the goal is to remove moisture, you'll have a hard time with an air powered venturi vacuum pump.

At sea level, water begins to boil and change into a vapor state at 212 degrees Fahrenheit. If we increases the pressure we can raise the boiling point of water. An example of this would be the typical automotive cooling system which uses a 15 lb. radiator cap to maintain a design pressure. In that system, the idea is to raise the pressure and therefore raise the temperature at which the cooling liquid will boil. If we wish to lower the boiling point of liquid, we simply remove the pressure that's on top of that liquid. That's how we boil water out of an air conditioning system. We use a vacuum pump to bring the system to a level of near perfect vacuum so the water will boil off and be carried away as a vapor. It's important to note that ambient temperature has much to do with the point at which liquids will boil under vacuum. The greater the temperature, the fewer microns of vacuum will be required to start the boiling process.  If you've been keeping note, you know that non condensables (air) and moisture are two things you definately don't want in your a/c system. 

The chart below shows how temperature plays a role in the level of vacuum needed to boil water. 

All values are at sea level.  Subtract 1 inch for each 1000 ft. above sea level

Venturi pumps are popular with the do-it-yourself mechanic since cost is often times the biggest factor.  I'll give these little pumps some credit since they can remove a substantial amount of air from the system, provided you have an air- compressor with enough rated CFM to operate the venturi pump long enough to reach the desired vacuum level.  If air capacity to the pump tapers off, so will the vacuum level.  Typical requirements for a venturi pump is  around 4.5 CFM at 90 PSI.

We used a large commercial compressor dialed in at over 150 PSI. with our air vac, and we were able to maintain the suggested air requirements.  The vacuum produced didn't even register on our Yellow Jacket micron gauge.  That's something most electric vacuum pumps can achive in very short order.

Regardless of what sales literature may claim, we believe, under most conditions, you won't be able to boil much water with the vacuum level produced with a venturi pump. Therefore, we feel that air powered a/c vacuum pumps have no place in an automotive service shop.  That is, unless you plan on hacking the thing apart to make some other cool air powered vacuum device.   We have several useful items in the shop that derive their power from the guts of a ventui pump.

 If you plan on servicing A/C systems, and want professional results, invest in a good electric vacuum pump.  It's not uncommon to get ten or more years of service from a good electric pump.  That's a good return on investment.  As an option for  the do-it-yourself mechanic, some tool rental companies now rent electric vacuum pumps on a daily or hourly basis.  I think that would be better than overworking your air compressor for an hour or more, just to get mediocre results.