Booster Pump Efficiency – And More Meanings For PCP!
HAM contributor Bernard Fouché continues his journey into the world of Precharged Compressed Pneumatics (PCP) and filling with a booster pump. This time he concentrates on booster pump efficiency. That is, how many fills can you get from an HPA tank using a booster pump.
As Bernard lives in France, he uses Metric measurements. We show them converted (approximately) to US units for the benefit of our U.S. readers.
This is the second part of Bernard’s story. You can read the first part here.
First – Make A Spreadsheet!
After yesterday’s long preliminary – which I felt was necessary for the reader to understand the author’s backround in air gunning – I can get into more details about HPA booster pump efficiency.
In particular, I’m concentrating on the Altaros booster pump – because that’s what I have. However much of this story is also applicable to similar products – like the Extreme Booster Pump that’s commonly available in the USA..
The first question asked by any person interested in airguns filled from a cylinder is “how many refills can I get from that Particular Configuration, Pal ?”
Even if nothing else than a big cylinder and a hose are concerned, the problem requires some calculations.
I made a Google Sheet available to anyone. It looks like this…
Please do NOT try to alter the sheet. Make your own Plain Copy and play with it afterwards!
If you did not read The Little Prince in its original version, cell B1 allows you to select English instead of French. However since it was never clear to me how to make PV=nRT applicable to imperial units, all the calculations are metric.
Hoses Make A Difference
When I set up that sheet, it quickly appeared that a significant amount of air is wasted by tubing/hoses. The longer the hoses are, the larger the device is, the bigger is the waste of air. This obviously effects booster pump efficiency.
A booster pump has a significant volume that has to be pressurized before delivering any useful service. So one of the first things to assess is the amount of air wasted by simply connecting and pressurizing the device.
Also, since I use the booster pump only at home, I do it with the help of an old, noisy, shop compressor. This compressor delivers the ‘work’ required to move the pistons of the booster.
The booster pump can be used in another way, by pulling the air required to do the work, from the HPA source bottle, the cylinder itself. In that case the booster pump is totally autonomous.
But I don’t refill ‘on the go’, I don’t go for a weekend of airgunning. I go to the range with my airgun already filled and with a complementary buddy bottle. I shoot two to three BR50 targets including many sighting shots. That’s my usual dose of shooting!
Using A Shop Compressor With The Booster Pump
Hence what follows is only about using the Altaros booster pump with the help of a shop compressor, not in complete, stand alone mode.
I did try the stand alone mode, but it was evident that the Pistons Consume Pressure to perform their work. Hey, yet another meaning of PCP?
If I used the stand alone mode at home, I would have to go back to the dive shop more frequently. Having a shop compressor in my garage, why bother using the stand alone functionality?
In order for the booster pump pistons to move, air must be exhausted from the device. If the pistons are moved thanks to air coming from a source different from the main cylinder, the circuit between the main cylinder and the gun is completely closed. Evaluating air consumption should be easier.
Since the booster pump is a complex object with different moving parts, and different volumes at different pressures, it is very hard to get an estimation of booster pump efficiency by calculation alone.
Instead I preferred to take measurements. These are imperfect but they do give an idea of the volume of wasted air, from a real world perspective.
Here is my filling setup:
In this picture, two digital gauges are in line, close to the main cylinder. This was done to check if the gauges were giving similar results, which they did.
Afterwards, I kept one close to the source HPA air tank, and put the second one close to the air gun. However, in that particular test, only the source tank gauge was read because that is the one that matters to understand the air loss when pressurizing the circuit.
Note also that measurements vary between one test and another. The booster pump is a mechanical device with moving parts. When it is pressurized, the pistons may or may not move a bit, changing the volume that gets the filling from the 15L tank.
I did not connect the airgun at that stage. I put a dead head at the end of the hose that is usually connected to the airgun’s tank:
This way, I had the complete piping/hoses that would come into play when filling the tank for real.
I pressurized/unpressurized the whole circuit ten times to get an average of the involved volume. Then, finally I did a refill of the airgun.
By looking at the gauge close to the 15L air tank, I see the difference in pressure between two ‘cycles’. Now I can estimate the quantity of the air lost each time the complete circuit is pressurized.
When the circuit is balanced (more or less: remember there are moving parts that will stop moving at different places, or that will not move at all), the pressure is the same in the tank as in the gauge and most parts of the circuit.
However NOT in the complete circuit!
The end to be connected to the gun, where the air is ‘boosted’ in normal functioning, isn’t in a normal state because there is no work to make the boost during these first tests.
So my readings are incomplete, they miss something. By filling the gun at the end, I will have some understanding of that unknown part.
Here are the values of the 10 pressurization / depressurization of the circuit, the difference in volume when pressure is considered at 1 bar:
|Test||HPA Tank PSI||Air At 1 bar (14.5 PSI)||Loss (liters)|
With different hoses lengths, I would have obtained better (shorter hoses) or worse (longer hoses) numbers.
Now To Fill The Gun!
If you look at the bottle pressure gauge (the one on the right), it shows about 210 bar (3046 PSI).
From previous measurements, I know that the Prophet’s gauge is optimistic by 10 bar, so the displayed 210 bar pressure is more likely 200 bar.
When I did the refill, I did it in two steps. First a climb to 300 bar (4350 PSI) read from the Skemeth gauge on the hose connected to the gun. Then I waited for the bottle to cool down at ambient air temp – which was 24°C (75°F), and completed the refill afterwards.
When the Altaros booster pump is powered from a 5V source, it works by cycling pistons.
Once a cycle is started, it has to finish before the booster comes to a stop. Hence my ‘next stop’ was at 310 bar (4500 PSI). This was a bit over the bottle’s advertised specification but – I felt – still acceptable since such a bottle has a great safety margin built-in.
That makes a 110 bar (1595 PSI) refill, for a total volume of 55L (3356 Cu. In.) for the RTI Prophet’s Carbon Pressurized bottle (0.5L at 1 bar) plus 18.8 liters (1149 Cu. In.) of volume used by the setup, from the numbers obtained with the 10 dry runs.
The estimated consumption is then 73.8 liters (4500 Cu. In.).
However, when reading the difference in pressure at the source tank, I got 84 liters (5126 Cu. In.) for the refill.
So I have a bit more than 10 bar (145 PSI) of additional loss, making the complete loss to be 29 bar (420 PSI). My initial assessment was incorrect but not completely wrong.
Those numbers already give a good picture of booster pump efficiency: whatever volume is your gun’s tank, there is a ‘fee’ in air to pay, about 29 liters.
So when topping a 0.5L liter bottle with 100 bar of pressure, meaning to add 100 x .5 = 50 liters (3050 Cu. In.) of useful air, the ‘fee’ is more than half of the useful volume of air exiting the cylinder!
This is an incentive to fill as much as you can from the moment you have pressurized the compressor circuit.
I will explore that solution in future when I get a ‘tee’ to be able to fill the air into the gun’s tank and a buddy bottle at the same time. I expect to pay the same flat fee of 29 liters and have a better ratio of consumed vs useful air.
To have more accurate numbers, I will empty the gun and refill it again and again. This will occur in the next few days/weeks but I have difficulties emptying the gun just for such a test. When I go to the dive shop, the filling is not immediate, I have to leave the cylinder, and pick it up hours later or the day after.
So I will continue to note the differences in pressure in the 15L cylinder and get a finer picture of the loss. Doing so, I’ll also be able to know what is the minimal pressure I can have in the source tank while being able to fill the gun at 300 bars or so.
Conclusions So Far
At this point I already enjoy using the Altaros booster pump. It is not perfect because of a few details I will describe in a later entry in the Particular Closet for Psychos I’m inaugurating here in HAM, if ever the editor is not reluctant to such an approach.
This booster pump is really a nice piece of machinery, a kind of steampunk design emitting Darth Vador like sounds when it is filling a gun!
It works quickly, it takes the same time installing the gun and connecting the hoses as filling it. One can make multiple shooting sessions in a row, with nearly no waiting in between. There is only the noise of the shop compressor (if used) and it doesn’t last a long time.
The Prophet and such a booster pump setup triggers a significant consumption of pellets because there is no reason to stop shooting, refilling and shooting again!
That is the best 300 bar setup I could have dreamed of. Booster pumps are Pretty Cool Products. Now I really have found the answer to my question. What is the meaning of PCP?