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Bradyy

Digital Compression Tester ?

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Hello all not long on here but have a few ideas i think we can all benefit from ! ,

Im looking at building a digital compression tester for the rotary's as it seems there is a lack of them around (price availability etc..) and it seems to me that it could be a useful tool for a rotary owner i.e. should be the equivalent of a socket set and wd40

it would comprise of a pressure sensor that screws into the spark plug hole which will then connect into a small box of mysteries and then finally to a laptop or pc .. to be graphed on screen so the peaks and troff's can be viewed etc and then with some maths calculate the rpm etc 

by the looks of it they would cost anywhere between 80-150 euro (still have to sort parts etc etc )

just wondering if there would be much need for a "cheapish" compression tester made available to the community, I would love to hear your thoughts and opinions on this ? 

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Hey dude, 

its a really good idea, and your right theres not alot out there except the mazda tester, i was looking into it last year, but i found thst just biting the bullet and getting a mazda one was the best idea, there calibrated and most accurate, also they normalise the results to the 250rpm standard, 

still though i can imagine building one would be funnand satisfying, also we could compare to check accuracy, 

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6 hours ago, seanmcg said:

Hey dude, 

its a really good idea, and your right theres not alot out there except the mazda tester, i was looking into it last year, but i found thst just biting the bullet and getting a mazda one was the best idea, there calibrated and most accurate, also they normalise the results to the 250rpm standard, 

still though i can imagine building one would be funnand satisfying, also we could compare to check accuracy, 

  • the calibration on each one would be down to the sensor used ,i have two options on sensors so far 
  1. Expensive one : is calibrated to atmospheric pressure (14.7 psi) and is a sealed gauge sensor type which means it always goes from 14.7 psi what this means is that the reading can be varied for each zone on the day through software but generally 14.7 is sea level so it means even somebody up in a higher region can offset there barometric pressure to the sensor and normalise it to 14.7 as for sensor sensitivity its <0.2psi inaccurate 
  2. El cheapo : standard can do a basic reading and should be able to give a decent general value , much like looking at a manual gauge and judging for yourself  

 

  • the 250 rpm standard would can be calculated by finding the dead space of the sensor(hollow space inside ) and removing that from the compression ratio of the rotary in general to calculate each rpm we would subtract the dead space from the measurement and just use the mazda ecu graph to calculate the normalisation values  

    and so to do that we first get the mazda graph ecu graph and we follow the black line for each point :

         compression_chart.jpg

 

  • we then graph the black line into a graphing program  rpm (X) with PSI (y)

        Screen Shot 2016-03-29 at 23.54.39.jpg

 

and then finally we use a Polynomial interpolation to graph a wave through the point we have submitted

       Screen Shot 2016-03-30 at 00.00.55.jpg

 

  • which then gives us our equation for the normalisation to 250 rpm 
  • Pasted Graphic.jpgUntitled.jpg

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52 minutes ago, Bradyy said:
  • the calibration on each one would be down to the sensor used ,i have two options on sensors so far 
  1. Expensive one : is calibrated to atmospheric pressure (14.7 psi) and is a sealed gauge sensor type which means it always goes from 14.7 psi what this means is that the reading can be varied for each zone on the day through software but generally 14.7 is sea level so it means even somebody up in a higher region can offset there barometric pressure to the sensor and normalise it to 14.7 as for sensor sensitivity its <0.2psi inaccurate 
  2. El cheapo : standard can do a basic reading and should be able to give a decent general value , much like looking at a manual gauge and judging for yourself  

 

  • the 250 rpm standard would can be calculated by finding the dead space of the sensor(hollow space inside ) and removing that from the compression ratio of the rotary in general to calculate each rpm we would subtract the dead space from the measurement and just use the mazda ecu graph to calculate the normalisation values  

    and so to do that we first get the mazda graph ecu graph and we follow the black line for each point :

         compression_chart.jpg

 

  • we then graph the black line into a graphing program  rpm (X) with PSI (y)

        Screen Shot 2016-03-29 at 23.54.39.jpg

 

and then finally we use a Polynomial interpolation to graph a wave through the point we have submitted

       Screen Shot 2016-03-30 at 00.00.55.jpg

 

  • which then gives us our equation for the normalisation to 250 rpm 
  • Pasted Graphic.jpgUntitled.jpg

Took the words right out of my mouth bro... :blink::lol:

P.s. I didn't realize you were Asian :ph34r:

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Ok so having done a bit of maths .......:angry:... here is a table for future reference of crank speed to psi on average calculated between A and B Lines from image below and from previous stepped image above and slope 

dealer_compression_chart.jpg


CRANK        PSI
SPEED        FACE

25.00,     13.57
30.07,     16.67
35.14,     18.99
40.22,     20.93
45.29,     23.64
50.72,     26.36
55.43,     29.07
60.51,     31.40
65.22,     33.72
69.93,     35.66
75.00,     37.98
79.35,     39.92
84.78,     42.64
89.86,     44.57
94.93,     46.51
100.00,     49.61
105.07,     51.16
110.14,     53.49
115.22,     55.81
120.65,     58.14
125.00,     60.08
130.07,     61.63
135.14,     63.18
139.86,     65.50
145.29,     66.67
150.36,     68.99
154.71,     70.93
160.14,     72.48
165.22,     73.64
169.57,     75.19
175.36,     77.13
179.71,     78.68
184.78,     80.23
189.86,     81.78
195.29,     82.95
200.36,     85.01
205.43,     85.86
209.78,     86.82
215.22,     88.37
220.29,     89.92
225.00,     90.70
230.07,     92.25
235.14,     93.41
240.22,     94.19
245.29,     95.74

250.00,     96.51
255.43,     97.67
260.14,     98.45
265.22,     99.22
270.29,     100.00
275.00,     100.78
280.07,     101.94

285.14,     102.71
290.22,     103.49
294.93,     104.26
300.00,     104.65
305.07,     105.81
310.51,     106.20

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8 hours ago, Bradyy said:
  • the calibration on each one would be down to the sensor used ,i have two options on sensors so far 
  1. Expensive one : is calibrated to atmospheric pressure (14.7 psi) and is a sealed gauge sensor type which means it always goes from 14.7 psi what this means is that the reading can be varied for each zone on the day through software but generally 14.7 is sea level so it means even somebody up in a higher region can offset there barometric pressure to the sensor and normalise it to 14.7 as for sensor sensitivity its <0.2psi inaccurate 
  2. El cheapo : standard can do a basic reading and should be able to give a decent general value , much like looking at a manual gauge and judging for yourself  

 

  • the 250 rpm standard would can be calculated by finding the dead space of the sensor(hollow space inside ) and removing that from the compression ratio of the rotary in general to calculate each rpm we would subtract the dead space from the measurement and just use the mazda ecu graph to calculate the normalisation values  

    and so to do that we first get the mazda graph ecu graph and we follow the black line for each point :

         compression_chart.jpg

 

  • we then graph the black line into a graphing program  rpm (X) with PSI (y)

        Screen Shot 2016-03-29 at 23.54.39.jpg

 

and then finally we use a Polynomial interpolation to graph a wave through the point we have submitted

       Screen Shot 2016-03-30 at 00.00.55.jpg

 

  • which then gives us our equation for the normalisation to 250 rpm 
  • Pasted Graphic.jpgUntitled.jpg

 

image.jpeg

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I was looking at this a while ago. If you need a hand give me a buzz. Should be easy to make a standalone tester with built in lcd readout. A lot of the testers out there use a standard enough pressure sensor welded to the end of a spark plug.

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1 hour ago, littlefeck said:

I was looking at this a while ago. If you need a hand give me a buzz. Should be easy to make a standalone tester with built in lcd readout. A lot of the testers out there use a standard enough pressure sensor welded to the end of a spark plug.

i was thinking about an lcd readout on each device but .. it will increase the price of each one by nearly a quarter 

Pros v Cons of No LCD

Pro:

  • Smaller (Think Matchbox Sized)
  • Cheaper
  • Fuel Proof
  • Hardwearing
  • Graphing On A PC

Cons:

  • Bigger (Think 2x Iphones Sandwich)
  • More Expensive
  • Not Resistant to Petrol/Oil
  • LCD Easily Damaged
  • Not as Accurate as a Graph on PC 
  • Not able to Share Graphs 


 

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Well the sensor has a <0.002 second sample rate and the computer can communicate at 9600 bits

so roughly i should be able to obtain a sample rate of 100x-150x upwards reliably for each rotor but this is dependant on the rpm obviously the higher the rpm the lower the sample rate for each face / rotor .

so basically i hope to measure on minimum 100 points of data on each compression cycle of each face

 

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That should be easily done with an arduino uno, keypad sheild and a case. Theyre easily bought from china for less than €20. This supports logging to sd card or computer. All we would need is a suitable 0-5v sensor. For a 16 MHz Arduino the ADC clock is set to 16 MHz/128 = 125 KHz. Each conversion in AVR takes 13 ADC clocks so 125 KHz /13 = 9615 Hz. That is the maximum possible sampling rate, but the actual sampling rate will be less due to having to run the lcd module and sd/serial logging. 2-4K sampling should still be possible.

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Agreed ! , Arduino makes perfect sense for me and you, however im more inclined to a computer based solution and let the computer handle the maths as it would have to be preformed for each reading to correct it which has to be kept in ram and not stored  .. and storage of each reading as a graph would be better to read then figures due to a graph being able to display "high spots and low spots" i.e. an early failing apex seal etc etc... i hope to just use a single channel ADC to rs232 to usb on a custom pub and let the computer handle the inputs and output through software plus the arduino would need a 5v dc source 

i want to use such a simple method due to environment , fuel , oil , heat , 

but most of all to be simple and user friendly

 

the most expensive part of this is the sensor however :( plus the wiring and various connectors solder , pcbs , the works ,  

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With the arduino it can store the raw sensor data to sd card during the actual test. This can then be processes afterwards directly on the arduino and normalised values shown for each rotor face.

If you wanted to log directly to a computer instead then the same hardware will work for you. You would have 2 options with this -

 

1) after the test is performed either insert the sd card into the computer and work with the raw analouge test log or retrieve it from the arduino using the in built usb port.

2) while logging the data to sd card, also send the raw ADC data direct to the serial port. The arduinos usb is basically a usb to rs232 convertor. 

I think this would have 1 piece of hardware with both functions. Also if this works we could then upgrade to an arduino mega with 320×240 tft shield and show the graphs directly on the handheld device. I could program the arduino but im not that strong on computer side programming.

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Heres a sensor im thinking of using. Its cheap as chips but should suffice for development purposes.Its a 0-10 bar tester on a linear 0.5V - 4.5V scale and only requires a 5V source. If we push forward to develop this it could be put together including the sensor for a lot less than others on the market

Sensor http://www.aliexpress.com/item/0-1-Mpa-0-145-PSI-Water-Gas-Pressure-Sensor-Air-Compressor-Pressure-Transmitter-G1-4/32471893753.html?spm=2114.13010208.99999999.285.TrB6Xc

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Quick update - Still awaiting the sensor to arrive but have added a basic menu with the option to test a piston engine and also to select the type of rotary engine being tested as the compensation ratio varies on the engine being tested. Still very much a work in progress but getting closer! heres a video to show you the progress.

 

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