Compression tester

[WV Mountaineer]

Liquids and gases both have some similar tendencies but they are treated differently with different engineering formulas because by definition fluids are incompressible. We are dealing with gas here which is compressible.

All fluids are compressible. Liquids are considered incompressible at low pressures. A quick google search turned this up in the physics department of a Virginia EDU domain.

Compressible fluids


I'm not going to go any further with this as it's just not worth it to me. Everyone can draw their own conclusion.


A mathematician, physicist, and engineer were all asked the same question. The question was as follows:

I girl sits on one end of the bench and a boy on the other. Every second the boy moves half the distance closer to the girl. How long before they intersect.

The Physicist quickly replied "Never! there will always be some distance between them both"

The Mathematician objected, "No, at time equals infinity they will occupy the same space."

The Engineer just laughed and that upset the Physicist and Mathematician. They barked at him "Ok smart ass, what do you think the answer is?"

The Engineer replied, "In 5 seconds he is going to be close enough for all practical purposes."

 

[tomscott]

...and "for all practical purposes" fluids are incompressible when compared to gases and when dealing with engines. Google "hydrolocked engine" for a practical difference.

As an engineer, for yourself, you should not want to be done with this. You should want to fully understand this. You should not want to put incorrect information out that is wrong. Anyone can make a mistake, and yes, even us engineers make mistakes. You have made one here.

You are hung up on pressure, but the pressure is a result of volume change. Initial volume divided by final volume. You can't escape the physics and the math. Adding the length of hose changes the volume substantially for these small engines, which changes the compression ratio, which in turn changes the pressure reading. That is the whole reason why a proper compression gauge has the check valve at the spark plug hole. They didn't put it there because they like spending money. They put it there because that is what will effect an accurate reading. Can't escape the physics.

That is why you received several frustrated responses from people that have actually built engines; they know from practical experience that you are wrong. Again no offense intended, I just don't want someone to be misled.

 

[Gologit]

oh crap I tried to rep you and failed. It went bad, I am trying to get it fixed....I publicly apologize and will do everything I can to get the negative rep removed. If any mods read this pleas flip my rep to positive. You are all red and you don't deserve that. A voice of reason....

I gave him a good blast of positive rep. Sorry I couldn't change yours back. It happens all the time but thanks for letting us know.

 

[Eccentric]

I gave him some rep too Steve.

 

[tomscott]

Stephen C - Thanks, that is kind of funny about the rep flub! I tried to rep you back for the excellent explanations you provided for this thread, but it was my first ever attempt at giving positive rep; hope I did it right!

 

[naturelover]

Well, intrigued by the thread, so decided to a little "experiment".

Lets see what we can come up with, will the schrader valve placement vary compression readings?

Discussions about equilibrium pressures, compressibility of liquids and gasses, and volumetric compression areas have come up with two distinct thoughts about something as simple (it would seem) as placement of a schrader valve in a compression guage.

One believes that it wouldn't matter about compression, another believes it would.

Arguments were heard and considered from both sides, and while I had my thoughts, I did have an open mind about what the results would be. Will say that some insight may be gained by taking a compression reading without any schrader valve installed.

Anyway, a simple little test is in order. Apologies about the quality of the pictures.

I have two compression testers, two hoses and adapters and such so decided to see what I could come up with.

The baseline test of an Earthquake 3816.

All results were obtained by pulling until the guage stopped rising.

Single hose, schrader valve in.

Around 150psi.

Schrader valve removed.

Here is the setup, verified by 90psi on my compressor guage.

INSERTION of another scientific question and thought of compressibility and areas. Why did my compressor guage read 95-100 psi, but the guage less than 90 psi? Did the 50' of pipe and hose have anything to do with it, or was my guage off? Would I obtain a different reading without that length of pipe and hose inbetween? Would the expansion of the hose under pressure decrease my reading, even though it was at equilibrium with the compressor?

Another experiment for another day... :smile2:

Anyway, this is with a single hose without the schrader valve attached to an adapter that adapted it to the second hose with the schrader valve in it. The part in the middle is from the other compression tester.

And for the results. It was so distinct, that you could even feel the difference in pulling it over.

One could also just take the schrader valve out and plug the guage end of the hose and probably tell the difference, but I did not do this....

Thoughts? Criticisms? Possible sources of error? The adapter connection was ONLY verified by the readings on the compressor, and by the fact that I never heard any leaks when pulling on it, but no water leak test was performed on it.

I took it apart before thinking about that.

Oh, don't take this too seriously, whether my experiments or thoughts are right or wrong, it was just all in fun.

No published scientific papers from me are in my future... :msp_tongue:

 

[maico490]

...and "for all practical purposes" fluids are incompressible when compared to gases and when dealing with engines. Google "hydrolocked engine" for a practical difference.

As an engineer, for yourself, you should not want to be done with this. You should want to fully understand this. You should not want to put incorrect information out that is wrong. Anyone can make a mistake, and yes, even us engineers make mistakes. You have made one here.

You are hung up on pressure, but the pressure is a result of volume change. Initial volume divided by final volume. You can't escape the physics and the math. Adding the length of hose changes the volume substantially for these small engines, which changes the compression ratio, which in turn changes the pressure reading. That is the whole reason why a proper compression gauge has the check valve at the spark plug hole. They didn't put it there because they like spending money. They put it there because that is what will effect an accurate reading. Can't escape the physics.

That is why you received several frustrated responses from people that have actually built engines; they know from practical experience that you are wrong. Again no offense intended, I just don't want someone to be misled.

Totally agree and tried to get this over earlier in the thread. Degree in Mech Eng here but I will admit to always having been baffled by Elec Eng. Stuff you can't see was never my thing.

 

[Qt94b]

Nice people you lot

Just unbelievable how you guys stayed calm and took all the bs from the "engineer". My blood was boiling :angry2: but then it was funny only how it took another engineer to shut him up for good. You guys are really nice people and I truely respect you all for that.

 

[Wagnerwerks]

Just a little note. I didn't read the whole thread, but I upgraded from a harbor freight gauge when the Schrader valve stripped and blew into a previously nice 026 top end while I was pulling the cord. I may have mentioned this a while ago on this very thread, but the otc gauge is very nice and 50 bucks from amazon.

 

[Wagnerwerks]

did it strip out the threads or just unscrew itself? The results are pretty much the same, but one is preventable and the other is not. I hope a few pulls on the recoil didn't result in something bad.

It was stripped. I'm not sure how good the hole was tapped to begin with...

 

[Majorpayne]

Just a little note. I didn't read the whole thread, but I upgraded from a harbor freight gauge when the Schrader valve stripped and blew into a previously nice 026 top end while I was pulling the cord. I may have mentioned this a while ago on this very thread, but the otc gauge is very nice and 50 bucks from amazon.

Which number gauge kit did you get? Did it come with the right fitting for chainsaws? I see they have a kit especially for motorcycles.