Pump part iv: evaluation

This post finds me in the unenviable position of trying to play catch up after a long absence in posting.

In going back over my notes, I will endeavor to get everything up to speed.

When last I posted the pump materials had all been individually finished, then reassembled.

The important thing to conclude the whole affair, is to bench test the pump to predict how it will fare in field use.

I did my evaluation in stages; before mounting the various valve boxes on the trunk I tested with just the exhausters and reservoirs in place, sealing off the open ports with acetate pieces secured with duct tape around them.

pump trunk first test stage

We are attempting to make this collection of wood and cloth pieces as airtight as reasonably possible. Things to look and listen for when doing this kind of testing:

“Lock up”: a well sealed pump will exhaust all of the air from the trunk and reservoirs after just a couple of vigorous pumps. Once that atmosphere is (temporarily) exhausted, the lower pressure inside the trunk will not let you pump open the exhausters, until some atmosphere leaks back into the system. If you can get the pedals to lock up after a couple of pumps, you are off to a good start.

Reservoir suction retention: a visual indicator of how airtight the pump is. After lock up the reservoir springs will begin to force the large pneumatics open, in opposition to the reduced pressure trying to keep them closed.
The duration of time it takes for the reservoirs to completely reset and relax is another key indicator of system tightness.
One minute is the ideal, but can be difficult for an amateur rebuilder to achieve.
Thirty seconds is good; fifteen to twenty seconds fair to minimally acceptable.
Anything less than ten seconds will be not so satisfactory, and will find you pumping constantly to try and maintain suction in the system.

If the retention time is too low, try listening closely over all areas of the pump for audible leaks.
This can be awkward to do while pumping to maintain suction; you may need a helper.
I was satisfied enough with my 30 second time, then I added the valve boxes (cutoff and theme, etc).

Soft function valve box

I did some more comparative testing, as seen in this sort video. I have a pressure gauge set up to measure the differential when I actuate the soft function. It is important to note that both pressure AND flow play a part here, so a pressure gauge alone may not tell the whole story.

checking pressure differential with soft function

For a good idea of how a restored pump is supposed to test on the bench, check out this video from Bruce Newman restorations. 

If you are having trouble chasing down leaks or getting things to work, consult the Reblitz book or the Mechanical Music Digest archives online.
You’ll get there!

With the pump finally done, it is on to the stack next — yippee!