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 DIY Small Size Air Compressor on the Cheap AuthorGiorgosÂ LazaridisAugust 17, 2012

PAGE 1 of 6 - The air accumulator idea and a few calculations

I had posted once a blog entry with a few Useful tips how to solder random SMD parts. The soldering paste dispenser is a must-have! I want one of these for my workbench, i want it BAD. There is one problem though: The SMD dispense controller costs some \$200, and also requires an additional air compressor, which obviously costs another 150 Euro in the local market (a cheap one).

Regarding the SMD dispenser controller - this is something that i can certainly make, since i have all that it takes - knowledge around pneumatic systems and microcontrollers. I'm quite sure that i can make a very good dispenser controller for less than 50 Euro. But what about the air compressor???

The DIY small capacity Air Compressor idea and some rough calculations
Some time ago, that guy Pike (you probably know him quite well by now) told me that he wanted to make a DIY air compressor. He saw another DIY compressor made with a large fire extinguisher and an air compressor from a refrigerator (or something like that). Nice idea but kinda overkill for my purpose. If i had such a compressor, i would use it, but since i don't have one, i have something else in mind...

Luckily, i managed to find a normal size old fire extinguisher from a friend of mine. I cleaned it, emptied its content, unscrewed its valve and cleaned it inside as well. Additionally, with the help of water and a measuring pot i measured it's internal volume:

 This is the fire extinguisher It was really hard to open it, because it was many years old... Too many... After washing it thoroughly, i measured its internal volume: i is precisely 2.8 liters...

This is the dispenser syringe that i will be using

This is the small syringe that i currently use (by hand)

This pair of batteries weight 4.5 kilos
For my calculations, i had to measure the dispenser syringe as well. I will be using a dispensing syringe with internal diameter 15.5mm. The piston stroke is about 60mm, so i will do my calculations for half this stroke, which is 30mm. From basic geometry we can calculate the area of the piston:

Ad = Pi x r2 = 3.14 x (15.5/2)2 => Ad = 188.6 mm2

I will also need the volume of the syringe at 30mm stroke:

Vd = Ad x h = 188.6 x 30 => Vd = 5658 mm3

Now i need to make an educative guess... How much pressure do i have to apply to the piston for the solder paste to flow through the dispenser? Well, first i made a small experiment. Up until now, i had a small-size syringe which i used to manually put solder paste. Though it does not have the same diameter, i can scale it up to my needs. To determine the force needed for this small syringe, i used a pair of batteries from my UPS. Both batteries weight precisely 4.5 kgm. I first placed one battery on the syringe piston, but the solder paste flow was slow. Then i placed both batteries, and the flow was fast. So, the proper value is somewhere in the middle, i guess around 3.5 kilos.

The small syringe has internal diameter 12.5mm - the area is 122.5 mm2, so the force needed is 3.5 kgf/122.5 mm2. The area of the large syringe is 188.6 mm2, so the force needed is 188.6/122.5 x 3.5 = 5.4kgf.

This is the number that i need to know! Now i can calculate the air pressure inside the syringe needed to achieve this pressure on the piston. I will be using the Pascal’s Law - F=PxA, where F is the force, P is the pressure and A is the area. But first i have to change the units:

5.4kgf = 52.9 N (Newtons)
188.6 mm2 = 1.886 x 10-4 m2

F = P x A => P = F / A = 52.9 / 1.886 x 10-4 => F = 280487.8 N/m2

To convert this number into a more "understandable" unit for me, i will use the conversion formula:

1bar = 100.000 N/m2 => 280487.8 N/m2 = 2.8 bar

Alas! 2.8 bar is a pressure that i can totally understand. I do have the feeling of "bars" from my day-job. All the pneumatic systems of the CNC machines that i service use this unit.

The final question - Is the air capacity enough?
So, why all that fuzz? Simply: I want to see if the 2.8 liters of the fire extinguisher is enough to cover my needs. I start with the air-volume needed to achieve the 2.8 bars inside the syringe. For this calculation i need the average syringe volume calculated before, the 5658 mm3.

I will use the Boyle’s Law: P1 x V1 = P2 x V2. What this means is that if the pressure increases by 2.8 times, the same increment will be observed at the gas volume (any sort of gas). Hence, the air volume needed to achieve this pressure is:

2.8 x 5658 mm3 => V2 = 15842.4 mm3

And how much air volume can my tank (fire extinguisher) accumulate? Since the air is storied inside under pressure, the volume will be much higher than the 2.8 liters that i measured before. The Lowest pressure inside the chamber will be 5 bars, and the highest 9 bars. Using the Boyle’s Law, i can calculate how much air volume will be inside the chamber under different pressures. But first, lets convert the liters to cubic meters:

1lt = 106 mm3 => 2.8 liters = 2.8 x 106 mm3

at 5 bar: V5 = 5 x 2.8 x 106 mm3 => V5 = 14 x 106 mm3

at 9 bar: V9 = 9 x 2.8 x 106 mm3 => V4 = 25.2 x 106 mm3

So, the usable air volume is 25.2-14 => Vu 11.2 x 106 mm3

Eventually, i can now calculate how many times i can use the dispenser without re-filling the chamber (i do not want the compressor to work all the time):

T = 11200000 / 15842.4 => T = 706.9 Times

More than enough, absolutely! So, let's get to work!!!