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steam engine indicator


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Am new here, and out of my element,so here's my question; I work with steam engines and to tune them we use a tool called an indicator (150 years old ; mechanical) I would like to be able to make an electronic one,  more portable, and more adaptible. What the indicator does is measure the cylinder pressure (from negative pressure to about 200 psi) against piston stroke, or crankshaft revolution degrees. The indicator plots, on a piece of paper, roughly an egg shape so you can see the valve events and make any changes needed. This is like a scope for modern cars. With my dual trace scope I can get the pressure curve, and the distance line but I've got no good way to tie the two together. What I would like to do is use a laptop (with a scope adaptor if needed) to get these plots and store them (record multiple minutes worth, not just one stroke). Also as to the distance sensor something as an optical or magnetic pickup would be nice, for ease of set up. The pressure transducer is no problem, but it does have to be timed to top dead and bottom dead center. Am I asking too much? Thanks for any help or thoughts in this regard. Ron Parola

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Could it be possible to record the pressure and shaft positions as they change over time using the sound card in the PC? There are two channels readily available and the sample rate available there would seem to be sufficient.

It might be that the rate of change of the pressure signal is too slow for the low frequency cutoff of an ordinary sound card to record directly, however, I would think that some kind of voltage-to-frequency conversion (using something like an LM331) could be used to overcome this.

There is also the matter of transmitting the shaft position in some useful form, though a shaft encoder generating a pulse train with some kind of index pulse (of say, twice the amplitude or of opposite polarity) at every TDC and BDC would be a possible way to make this easy to record.

Then the resulting lists of pressure and shaft position values can be plotted and used for further calculations.

Ashtead

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