This page last updated: 16 May 2021.In March 2021, during the second year of all things Nikola, my little Jun-Air 6-25 compressor sprung a pin-hole leak from its tank. While this might have been repairable it gave me the opportunity to increase my air supply; the little compressor had never been quite sufficient. I could not find an off-the-shelf compressor that met my needs so I decided to make my own.
For reference, the sizes of pipe threads used below are as
||Major diameter (i.e.
outside diameter of male thread)
||Threads Per Inch
|1/8" BSP, G 1/8, R 1/8
|1/4" BSP, G 1/4, R 1/4
|3/8" BSP, G 3/8, R 3/8
|1/2" BSP, G 1/2, R 1/2
|3/4" BSP, G 3/4, R 3/4
BSPP means BSP parallel (P for parallel), which is the default
for BSP (AKA G). BSPT (AKA R) means that the male version
of the thread has an outside diameter which is tapered (T for
taper), increasing from the size given above slowly along the
length of the thread; this means that a seal is formed purely by
the mating of the threads themselves (with some PTFE tape), no
shoulder on the end of the pipe with a gasket/washer etc. is
required to form an air-tight joint. The female version of
this thread is just BSPP (AKA G) but can be denoted Rp,
presumably to indicate that there is no reliable shoulder to
butt against; the mating of the threads must make the seal.
A compressor consists of a motor driving an air pump to
deliver air into a reservoir tank (so that the motor doesn't
have to run continuously). The reservoir tank is fitted
with an adjustable pressure switch that turns the motor on
when it is at less than the desired pressure (with a little
hysteresis). There is a non-return valve in the pipe
going from the pump to the reservoir tank, so that the pump
can be switched off without air escaping, and the reservoir
tank also has a pressure relief valve to prevent
accidents. Those are all the essential components.
I searched E-bay for a vertically-oriented 100 litre air tank and found the EXTREMELY useful [if you're in the UK] Context Pneumatic Supplies. Not only were they able to supply the tank with fittings (for ~£300), they were able to supply most of the other things I needed as well and were very patient with my voyage of discovery concerning compressor construction. I sent them first the picture on the left. Then I realised that I might want to amp the system up with more motors/pumps and so I sent them the one on the right, swapping the Y-connector for a manifold and adding a couple more non-return valves:
To connect everything up I ordered the following from a
combination of Context
Pneumatic Supplies and RS
Online, total cost about the same as that of the tank
(since many of the items from RS had to be bought in packs):
G 1/4 male blanking cap for unused output.
||Connect pumps/motors 1/8" BSP male output to manifold 1/8" BSP female inputs.||
4 mm pneumatic tubing (~2 metres).
Non-return valves: 4 mm push-fit input, R 1/8 male output (4 off).
4 mm blanking plugs for the above.
||Connect manifold 1/4" BSP female output to tank lower right 3/4" BSP female input.||
8 mm pneumatic tubing (~1.5 metres).
8 mm push-fit to G 3/4 male adapter.
Note: 6 mm, used at the other end (see number 8 below), would probably have been a better choice.
||Connect tank front 3/8" BSP female output to pressure switch 1/4" BSPT male input.||
||Connect tank bottom 1/2" BSP female output to automatic drain 1/4" BSPT male input.||
4 mm tubing (~0.5 metres).
4 mm push-fit adapter to G 1/4 female.
Note: since there was sufficient room for it beneath the tank, I later abandoned this arrangement and just used a G 1/2 male to G 1/4 female adapter (in multiple steps) to connect the automatic drain directly to the ball valve at the base of the tank.
||Connect tank upper left 3/4" BSP female output to filter 1/4" BSP female input.||
||Connect 4 mm push-fit drains from automated drain and filter to collection bottle.||4 mm pneumatic tubing (~3 metres).|
||Connect filter 6 mm push-fit output to user tap.||
||User pressure regulator (i.e. tap).||
||User air connection.||
Note: later, having purchased the very solid Aventics NL2 metal pressure regulator, I abandoned the hose and 6 mm push-fit adapter on the exit side and instead screwed a series 21 female quick-connect coupling directly into the G 1/4 exit hole of the pressure regulator.
And here they all are.
For an initial pressure test I pneumatically connected (a) the
two pumps/motors to two of the input ports of the manifold
through non-return valves (the spare output port being blanked
off and the two spare input ports being blanked off using
non-return valves with blanking plugs fitted just in case), (b)
the manifold output to the lower-right hole of the tank through
a ball valve, (c) the pressure switch to the lower hole in the
front of the tank, (d) the pressure gauge to the upper hole in
the front of the tank, (e) the pressure relief valve to the hole
in the top of the tank and (f) ball valves to the upper-left and
bottom holes of the tank to close them off. Electrically
both motors were connected to the pressure switch and the other
end of the pressure switch to a mains plug, all re-using cabling
salvaged from the two Jun-Air compressors. Assembly
required a heavy duty adjustable spanner (adjustable up to
30 mm in width), a 6 mm allen key, an electrical
screwdriver, wire strippers and the pneumatic PTFE tape (a few
turns on each thread applied in a clockwise direction).
All quite satisfactory: it took around 10 minutes to
pressurise the tank to 8 bar. Next I needed to adjust
the pressure switch to give me 10 bar. From the
The cut-in pressure (normally 6 bar) is set by
adjustment of differential screw B; turn clockwise to
reduce the cut-in pressure. The cut-out pressure is set
by even adjustment of the two screws A (cut-in pressure +
differential = cut-out pressure); turn clockwise to increase
the cut-out pressure. The switch is normally factory-set
for operation at between 6 and 8 bar (approximately 90 to
I found this confusing: screw B is called the "differential
screw" and apparently adjusts the "cut-in pressure", while the
equation in brackets has the "cut-in pressure" plus the
"differential" pressure giving the cut-out pressure, even though
the cut-out pressure is stated as being adjusted by turning
screws A; so do screws A adjust the "differential" (despite the
name given to screw B) or the "cut-out pressure"? I
decided to see if adjusting screws A would adjust the cut-out
pressure by following the three step adjustment procedure shown
in the second picture below, making sure to keep my 7 mm
spanner and fingers clear of the exposed mains
connections. One turn each of screws A increased the
cut-out pressure by about 0.25 bar so it took a few
iterations to achieve my desired 10 bar cut-out, leaving
time for the motors to cool between the later iterations.
When done I checked the cut-in pressure and it was around
8 bar. I did try adjusting it upwards to exactly 8 bar
but that increased the cut-out pressure without moving the
cut-in pressure so I put it back as it was; good enough.
It took 3.5 minutes for the two pumps/motors to
re-pressurise the tank from cut-in to cut-out and
15 minutes to pressurise it fully from empty; could really
do with another pump/motor or two but, as it is, just within the
operating limits for a Jun-Air motor.
With all that sorted it was time for final assembly. To
avoid further clutter in my small loft workshop I fitted the
compressor into half of a fitted wardrobe on the floor
below. Since this wasn't a very open space I purchased a
number of 240 Volt fans, 70 mm x 70 mm x
25 mm, and 3D printed myself
some stands (in PLA, 0.3 mm resolution, each took
3 hours to print; the two parts just shove together) to
mount them in. These were screwed to the floor in pairs behind
each pump/motor, space being available for three
pumps/motors. The tank, pumps/motors and drain bottle were
free-standing; they weren't going anywhere. I purchased a
fused mains switch and some junction boxes, plus some shrouded
mains terminal blocks to connect the suppressor capacitors for
each of the fans in a safe manner. The automatic drain and
the fans were wired such that they would always be on, i.e.
irrespective of the state of the pressure switch. When I
completed the testing above I had found that no water came out
of the hole in the bottom of the tank afterwards so I set the
automatic drain to the maximum periodicity of 30 minutes. I
wanted the user connection at the pressure regulator to be
mounted out of the way on the side of my workbench but I also
wanted the pressure gauge to be easily visible so, rather than
screwing the gauge into the body of the pressure regulator as
intended, I mounted it on a short length of 4 mm tubing
(re-using the push-fit connectors that I had originally intended
to use to connect the automatic drain valve) and a length of
stiff wire that allowed me to adjust its position.
I downloaded a mobile phone app (the Bosch one) which
purported to measure sound level in dBA and, 0.5 metres in
front of the open wardrobe door, with everything running, it
measured 65 dBA. To prevent the floor of the
wardrobe resonating too much I went to my local metals merchant
and had a plate of 5 mm thick mild steel cut (for £16) to
sit underneath the motors/pumps, mounted on a couple of thick
rubber grommets front and back to act as feet, and that reduced
the noise to about 62 dBA. I probably wasn't going
to get much better than that in such an enclosed space. I
later moved it to a different wardrobe so that it wasn't against
a party wall with my neighbour, just in case, so here it is in
its final resting place; interestingly, same sound level