MPS- delving into it's secrets?, Update January 3, 2011 Options

[jk76.914]

I collected a number of MPS over a couple of years. Most held vacuum, some did not. I measured inductance vs. vacuum on those that I could, but then took all of them apart (except one brand new one).

The is a lot of variety in the diaphrams, probably because they get replaced when virtually every MPS is rebuilt. The photo below shows four diaphrams. I'll follow with descriptions and observations...



#1- copper, 2 pleats. This came from a rebuilt MPS that was in a pond or river for some time. There was actually sand in it, along with tiny aquatic snail shells of some sort, and the copper was pretty much green. This diaphram was ruptured. The workmanship is very good, and the alloy is idential to stock (see chart below). The threaded bushing looks like stock in design and attachment.

#2- copper, 3 pleats. This is a stock diaphram from a riveted MPS. It is ruptured.

#3- brass, 2 pleats. This is from a rebuilt MPS. I have three of these diaphrams, all similar. This MPS was freshly rebuilt in a rebuilder's box. Looked like new with fresh paint and plastic cap on vacuum port, but it leaked slightly. You can see why when you look at the ripples in that flange. The O.D. looks like it was cut out by hand with tin snips, and the threaded bushing was taken from a stock diaphram and soldered by hand into place. At the bottom in the picture, you can see that the flange is pretty smooth- that's where I tapped it out with a machinist's hammer on an anvil. I have no doubt that the leakage was from around the O.D. of the diaphram, and that tapping it out would probably fix it. This material (brass, see chart below) is stiffer and thicker than any of the rest. It would add quite a bit of spring tension to the mass-spring-damper system. In general, I rate the workmanship on this as "crappy", though maybe you could get a running car out of it.

#4- stainless steel, 3 pleats. This one is a bit of an enigma. It is identical to stock except that it is made from stainless (see chart below). The workmanship is perfect. It is from a rebuilt MPS. Interestingly, it is the MOST COMPLIANT of all of them, while most steel ones are reported to be stiffer. The other part of the enigma- the aneroid cells from this MPS are also stainless- same alloy. My suspicion is that this is a late Bosch rebuilt. Who else would make stainless cells, because the cells don't fail very often, so there are lots of spares available out there, and the cells are pretty complicated to make...


*** EDITED *** Materials. I measured alloy composition at work using an X-Ray Fluorescence analyzer. This machine is very precise, but it has limits to the range of elements that it can detect. Unfortunately, it cannot detect Beryllium (Be), and it is likely that the stock (at least) diaphram contains Be to harden and strengthen it. You can see a couple of things here though- the stainless diaphram and stainless cells are of exactly the same composition- nickel-chromium stainless steel. The nickel explains why they are slightly magnetic, as many stainless alloys are not. ***



Conclusions (really opinions) - The diaphram was put in there (early VW D-jets did not have one) to provide altitude compensation and to soften transition from leaner (high vacuum) to richer (low vacuum) regions. Early D-Jets had a separate unit with a diaphram and a switch to inform the ECU to richen the mixture at low vacuum. Cost reduction may have been a third reason- eliminating the separate unit, wiring, vacuum hose, etc. Anyway, this switch was either on or off, no soft transition. The MPS had only 2 aneroid cells and no diaphram, and its inductance curve was essentially a straight line from 0" to 25". There was no mixture compensation for altitude with this arrangement.

I'm thinking that the lower the stiffness of the diaphram, the more consistency in the setup and responsiveness, while both of these objectives are met. By maximizing compliance of the diaphram, the springs acting in the system are mainly the coil spring and the leaf springs that act to locate the armature.

My stainless diaphram is the most compliant, but there is another feature of steel (if I remember correctly) that adds to the argument that this is a late Bosch design- steel has a much higher Youngs modulus than copper. The higher the Youngs modulus, the greater the fatigue resistance, and the vastly most common failure mode of the MPS is fatigue failure. Could this have been a Bosch attempt to solve a reliability problem, even as the technology was being superceded by more modern ones? Since the cells are also subject to fatigue, would they have switched them over at the same time?

I am planning on assembling my own personal MPS using the stainless parts, and seeing how close I can tune it to my engine.

There are lots of other parts in the MPS that I've formed opinions about, but I'll hold off for now.

These are my own opinions, which may not be popular, so BLAST AWAY!!

(eye candy below)

[jk76.914]

The Niton XL-800 is also an X-ray fluroescence instrument. Here's an excerpt from a DOE test of the XL-800 dated April 2000:



Note that chlorine is 17 on the periodic table!! That's higher than aluminum (13). AND BERYLLIUM IS 4!! The detection technology is called "proportional count". Newer technology is available now, using "silicon pin diode" detectors. As I understand it, the issue is signal-to-noise ratio. The lower atomic number elements may register, but their peak is lost in noise. The newer technology improves the SN ratio and they become visible. The Thermo-Fischer Scientific unit I tested my samples on (in November '08 BTW) using the newer technology can differentiate down to atomic number 12 (magnesium). There's yet a newer technology that can go lower, but it's moot for beryllium, because it's invisible to X-rays.

So sorry to say, I think we both fell into the same pit. XRF is a fantastic technology, but it's blind to Be.

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Meanwhile, back to the stainless steel idea. I ran some tests over the weekend on my copper and my stainless diaphrams (#1 and #4 in previous pics). I measured inductance vs. vacuum, but with the aneroid cells locked (filled with gorilla glue, clamped at nominal thickness during cure. Photo later tonight). I wanted to see the incremental effect of the enrichment diaphram alone. I had previously tested cells with the diaphram locked, and saw that they were amazingly linear from 0" to about 22", across several sets of cells of wide vintage, including the one stainless set. (post that chart later)

So the two conditions were:

first test, called SS-
1. outer screw set flush with bushing on cell side of diaphram (set before assembly)
2. inner screw set to an inductance of .85H at high vacuum (seated diaphram)
3. full load stop set to inductance of 1.00H at 0" vacuum.

This setting maxes the bias of the diaphram towards the cells, with a total travel of 0.15H. (0.15H is kinda arbitrary, but not far off from Mr. Anders's 0.12H typical travel- I think. Can't find the reference in his MPS Bible at the moment, but I think it's 0.12H)

second test, SS+
1. inner screw set to inductance = 1.00 at 0", full load stop removed for this test
2. outer screw set to inductance = .85H at high vacuum (careful not to turn the inner screw while setting.)

This biases the diaphram position away from the cells as far as possible, but still with the same 0.15H travel.

The two curves represent the adjustment limits of the stainless steel diaphram.

A picture's worth 10000 words:


Comments
1. SS- curve looks like it blends smoothly with the 1.00 upper limit. It really doesn't. If I had the resolution to do 4.5" or even 4.25" it would crash abruptly into the limit.
2. That's quite a range, in both slope and vacuum at which the diaphram begins to lift off the stop. Quite an opportunity for careful tuning if you really got good at this.

Oh, and here's the copper (diaphram #1, from the bottom of the lake) range.... I call it Cu-2P (copper- 2 pleat)-


Note that in Cu-2P+ the diaphram starts to lift off the stop at 7", then stalls at 6", and then resumes a fairly smooth transition the rest of the way to 1.00H. I reran these points several times, and it repeated the same. I suspect that there is a point where the 2 pleat diaphram resists strain, which allows stress to build up, until it pops past that point and then behaves itself. This only occurs for Cu-2P+ because the entire test range for Cu-2P- is beyond this point.

I still want to test the brass diaphram (#3 in picture). I may get a chance yet tonight, but I'll probably post the chart tomorrow.... Like I said, this is getting interesting!

That's enough for now. I've got to drill some holes in my frozen lawn and put reflectors along my driveway. Big storm coming tomorrow.