Well lately my clunky old vacuum pump that was gifted to me has been starting to play up a bit and isn't reaching as good a vacuum any more, so I think it's time for it to be retired with a new, higher flow unit.
I'm looking at pumps on eBay, each with a final vacuum of 5 or 10 Pascals (0.005 or 0.01 kPa), which correspond to better than 99.99% vacuum according to the table here
. Where the prices differ is when it comes to the flow rate (CFM).
Now I'm using a 10L pressure pot so in order to work out the evacuation time I first convert this to cubic feet using google: 10L = 0.353ft^3
. I can then use these figures with the calculator provided on engineering toolbox
to calculate the evacuation time for various CFM pumps. Note that the absolute initial pressure will be atmospheric pressure = 101.4kPa, with a final pressure of 10Pa (0.01kPa).
1CFM = 3.26 minutes
2CFM = 1.63 minutes
3CFM = 1.09 minutes
4CFM = 0.81 minutes (48.6 seconds)
5CFM = 0.65 minutes (39 seconds)
6CFM = 0.54 minutes (32.4 seconds)
7CFM = 0.47 minutes (28.2 seconds)
8CFM = 0.41 minutes (24.6 seconds)
As you can see, the fill time does not correspond linearly to the flow rate/CFM, and once you get above 6CFM the decrease in fill time starts to drop quite a bit. Remember also that these times are best-case and do not account for leaks in the plumbing or lid seal (which effectively reduce the flow rate further at higher vacuum).
Which one is most suitable? Well it comes down to your budget and your resin's pot life. Anything under 2CFM is out of the question as it simply takes too long. 3CFM is acceptable if you're using the same pot for your pressure and vacuum. Me? I'm going to go with the 8CFM pump, which can be had for just over AU$200. For the amount of casting I do it's worth the investment.Disclaimer:I know in the past I've made comments about suitable lower CFM rates but I had been ignorant to the fact that the evacuation rate is logarithmic, making evacuations times longer as the pressure gets closer to complete vacuum. So please take this post as guidance for pump selection.
Oh yeah I can make that....