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RCX Controlled Air Compressor Tester
Introduction
There are several air compressor designs available and various claims are made of their efficiency or otherwise. However, these are all subjective assessments. After much thought and several attempts, I've come up with an RCX controlled tester to assess air compressor performance objectively.  Here are presented the details and the test results on a number of commonly used air compressors.
actgen38gamcrp.jpg (43712 bytes)

Overview of the RCX Controlled Air Compressor Tester
Method
The air compressor under test feeds the air to a large pneumatic cylinder through a motorised pneumatic switch. The piston of the cylinder is attached to one end of a lever arm. To the other end of the lever are attached two Lego weighted bricks (part #9936) to serve as the load. Each weighted brick weighs 50 gm, giving a total load of 100 gm.
A felt pen attached to the load end of the arm writes on a moving strip of paper to produce a recording of the arm movement as the piston pushes to-and-fro. The result is a tracing of the particular air compressor which can then be compared with that of another compressor. The recording will indicate both the power and speed of reaction of the pneumatic cylinder.
In an earlier design, the pneumatic switch was operated manually. This wasn't satisfactory as the timing and duration of the switch operation was subject to human variation. To eliminate this, an RCX was used to control all the operations of the air compressor tester in the final design.
The motors of the air compressor, pneumatic switch and strip chart recorder are connected to the ouput ports A, B and C respectively of the RCX. The RCX itself is powered from an AC adapter instead of internal batteries to ensure a stable and constant current source to the three motors.
actlev43gam320.jpg (15697 bytes) actlside48crp300.jpg (15400 bytes)

Detail of the recording mechanism

View from load end
actcside46crp300.jpg (15858 bytes) actback45gamcrp320.jpg (13791 bytes)
View from cylinder end Back view showing strip chart mechanism
Construction
For those who want to replicate the tester, here are the building instructions in LDraw images together with the data files, for the recording mechanism and the motorised pneumatic switch.
The strip chart recorder is based on a design from the 8888 Idea Book but has been rotated so that the paper moves in a vertical plane instead of horizontally. For the paper, adding machine paper is the most convenient but regular ECG paper is recommended if you can get hold of them. Use an oil-based pen if you're using ECG paper.
RCX Control Program

The RCX code is a simple program which does the following:

1. Turn on the air compressor motor.
2. Wait 10 secs for it to charge up.
3. Start the chart recorder motor.
4. Wait 2 secs to get a baseline.

Repeat 4 times:
5. Activate the pnuematic switch for 1 sec to move piston downwards.
6. Wait 3 secs to record the upward arm movement.
7. Activate the pnuematic switch for 1 sec to move piston upwards.
8. Wait 3 secs to record the downward arm movement.
End of repeat

9. Stop all motors.
10. Beep to indicate end of program.  

The number of test cycles can be easily adjusted but 4 or 5 would be sufficient.
Interpretation of the charts
On the chart, it is the upstroke of the tracing that is critical. This is the working stroke where the pneumatic cylinder has to pull up the load of the two Lego weighted bricks.The maximum height of the tracing as well as the speed of achieving the maximum height, reflects the power of the air compressor under test. The downstroke on the chart represents the return stroke of the pneumatic cylinder which is helped by gravity and is thus less important.

To facilitate interpretation, the final tracings were made on regular ECG paper which is ruled in mm squares. This makes it easy to read off the maximum height of the tracing as well as the time taken to achieve full retraction and extension of the pneumatic cylinder. A quick method to compare the performance of the various compressors is to make transparencies of the charts and superimpose them one on top of another.
Results of Air Compressor Tests
Manually operated pneumatic hand pump
In this test, a pneumatic hand pump was operated manually and pumped continuously while the recording was made.
no tank hp_manual-a-crp.jpg (22532 bytes)
with air tank
The top chart is the result when the output of the hand pump was connected directly to the pneumatic valve. The bottom chart shows the effect when an air tank is included in the air supply circuit.

These charts show that the pneumatic hand pump operated manually  is quite effective. The inclusion of an air tank improves the performance producing almost square wave forms. But of course you can't keep on pumping forever...
Effect of charging up an air tank using the hand pump
Normally, the hand pump is used in conjunction with an air tank. The hand pump would be pumped up several times to charge up the air tank. The model is then operated from the air tank. When the air tank is exhausted, the hand pump is operated again. The instruction books show that the hand pump may be pumped 30-35x. More than that, the tubing will pop off the pump outlet.

The following charts show the effect of charging an air tank with different number of pump strokes.
10X hp_stroke-a-crp.jpg (44773 bytes)
20X
30X
35X
Motorised Air Compressors using the Small Pump
Standard air compressor
The following charts show the performance of the standard air compressor (from the Advanced Pneumatics Set) used without air tank and with an air tank.
no tank std_comp-a-crp.jpg (21886 bytes)
with air tank
This simple compressor design works but the performance doesn't quite approach that of the manually operated hand pump.
Mini air compressor
The following tracings were obtained with Michael Powell's mini compressor.
no tank mini_comp-a-crp.jpg (23077 bytes)
with air tank
The charts show this compressor to be quite an effective compressor approaching the wave form produced by the manually operated hand pump.
Double acting air compressor
The following tracings were made using R Hempel's double-acting compressor.
no tank doub_comp-a-crp.jpg (22367 bytes)
with air tank
In performance it is better than the standard compressor but it isn't as good as the mini compressor design in spite of using double pumps.
Motorised Air Compressors using the Hand Pump
Motorised air compressors incorporating the hand pump are becoming to be common. Some have the spring intact, while others have the spring removed.
The following are the tracings produced by a compressor using the hand pump with its spring intact.
no tank hp_spring-a-crp.jpg (25241 bytes)
with air tank
The following tracings were produced by a compressor using the hand pump with the spring removed:
no tank hp_nospr-a-crp.jpg (21345 bytes)
with air tank
As expected, the results show that the motorised air compressor using the large hand pump works better when the spring is removed. However, it is still inferior to any of the motorised compressors using the small pump.
Conclusions
  • The RCX controlled air compressor tester provides an objective means of comparing air compressor performance.
  • Each compressor design produces its own distinctive recording reflecting its performance.
  • The inclusion of an air tank improves the performance which ever type of compressor used.
  • Motorised air compressors incorporating the pneumatic hand pump are inferior to designs using the small pump.
  • The best performer of all the motorised air compressors tested was the mini compressor design by Michael Powell.

Air Compressor Testing Goes Hi Tech

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