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DIY Pressure Transducer

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08 Oct 2018 03:08 #23795 by SailorBob
Replied by SailorBob on topic DIY Pressure Transducer
Ahh, I didn't understand what you said, yeah, the transducer just isn't responding fast enough, need a better quality transducer.

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08 Oct 2018 10:45 #23800 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic DIY Pressure Transducer
The steps are what instrumentation techs call the resolution which is the minimum change ( increase or decrease) in the pressure being measured. These cheap sensors have 8 bit digital electronics in their internal processing, this dosen't really matter much if a a sensor is chosen with maximum pressure in the right ball park for what you want to measure.

"There's always a catch ---- Catch OBD2 ."


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08 Oct 2018 22:50 #23825 by SailorBob
Replied by SailorBob on topic DIY Pressure Transducer
I wouldn't say it doesn't matter - what you need to pay attention to is the Nyquist sampling rate - basically to accurate render the signal, in this case pressure changes, you need a transducer that can respond twice as fast as the signal changes. In dry air at 20*C air pressure changes propagate at the speed of sound, 343.2 m / s , which works out to 3.4 centimeters per millisecond. Or 1.34 inches per millisecond. So for in cylinder pressure readings I'd look for a transducer with at least a one millisecond response time, or really maybe even a little faster than that. Practically speaking, the one I use is rated one millisecond and it seems to produce good waveforms. When paired with 1 khz lpf I get nicely detailed clean waveforms.

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09 Oct 2018 06:37 - 09 Oct 2018 06:47 #23829 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic DIY Pressure Transducer

SailorBob wrote: I wouldn't say it doesn't matter - what you need to pay attention to is the Nyquist sampling rate - basically to accurate render the signal, in this case pressure changes, you need a transducer that can respond twice as fast as the signal changes. In dry air at 20*C air pressure changes propagate at the speed of sound, 343.2 m / s , which works out to 3.4 centimeters per millisecond. Or 1.34 inches per millisecond. So for in cylinder pressure readings I'd look for a transducer with at least a one millisecond response time, or really maybe even a little faster than that. Practically speaking, the one I use is rated one millisecond and it seems to produce good waveforms. When paired with 1 khz lpf I get nicely detailed clean waveforms.


Nothing to do with the sampling rate, in fact the steps are only really noticeable when the change is pressure is slow .
Take for a example a sensor rated at 250 psi ---- a sensor with 8 bit internal processing will only have 255 possible analogue output voltages so it will work in steps of approximately 1 psi.
On a 250 psi sensor a resolution of 1 psi steps is better than 0.5% of full scale is more than is required in any real world application.

But by matching the sensors max full scale range to what is being measure the size of the step change in output voltage is reduced choosing 100 psi sensor for running compression tests will improve the step resolution to 0.4 psi, but won't read high enough pressure for cranking compression tests which normally require around 200psi on older gasoline engines and can be 10 to 20% higher on GDI units.

It is of course fairly easy to smooth out the step changes with either an RC filter on wiring between the sensor and scope or on a scope with inbuilt software filters using a low pass filter but detail will be lost.

"There's always a catch ---- Catch OBD2 ."


Last edit: 09 Oct 2018 06:47 by Andy.MacFadyen.

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31 Oct 2018 17:15 #24311 by Sooknanan
Replied by Sooknanan on topic DIY Pressure Transducer
I believe i may be a victim of a crappy transducer. I bought 3 from amazon, tried the 0-300psi for a cylinder waveform and it came out like this.


Are any of my settings wrong that can cause this or should I just buy the transducer from the ebay link provided on page 1. Thanks in advance.
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31 Oct 2018 17:33 #24312 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic DIY Pressure Transducer
Have you checked the output voltage against shop air -- wondering if you have been given a 3000psi sensor ? A lot of Chinese suppliers aren't too careful about what they send out.

"There's always a catch ---- Catch OBD2 ."


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31 Oct 2018 20:03 #24314 by Sooknanan
Replied by Sooknanan on topic DIY Pressure Transducer
I calibrated it with a fluke pressure tester.

Accuracy is good I believe the transducer output signal is maybe too slow as previously discussed.


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31 Oct 2018 20:07 #24315 by Sooknanan
Replied by Sooknanan on topic DIY Pressure Transducer


Also verified the 5v output from the dc converter is good( checked with the scope)


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31 Oct 2018 20:09 #24316 by Sooknanan
Replied by Sooknanan on topic DIY Pressure Transducer
Pressure Transducer Sender Sensor... www.amazon.com/dp/B0748C27LN?ref=yo_pop_ma_swf


This what I bought


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02 Nov 2018 19:48 #24363 by bruce.oliver
Replied by bruce.oliver on topic DIY Pressure Transducer

Sooknanan wrote: Pressure Transducer Sender Sensor... www.amazon.com/dp/B0748C27LN?ref=yo_pop_ma_swf


This what I bought


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Those are the same ones that I got. I've replaced my homemade transducer with a WPS500 kit. Very happy with it

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03 Nov 2018 01:40 #24368 by Sooknanan
Replied by Sooknanan on topic DIY Pressure Transducer
(null)
Your waveform had the same issues as mine.


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04 Dec 2018 18:53 #25196 by Sooknanan
Replied by Sooknanan on topic DIY Pressure Transducer
Just an update with this PT dilemma. I bought the transducer posted on page 1 from ebay. It was worst than my original, it couldn't read anything under -3 psi so basically there was a flat line at the bottom of the waveform. The problem with the pic I posted before was, I didn't set the look up table to read vacuum pressures. I messaged a youtuber about the transducer he used and he said most new transducers having this same issue. You would have had a better chance of a quality one a few years ago.

So I tried a 0-100psi on running compression to compare to the 0-300 and it was similar.


After some filtering with the 0-300psi I got this. I will settle with this for now since the waveform is still readable.
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10 Mar 2019 16:30 #27728 by tloftis
Replied by tloftis on topic DIY Pressure Transducer
why does the snap on model not have the have the 9volt power and step down converter?

store.snapon.com/Pressure-Transducers-Ki...500-PSI-P636904.aspx

and could we use one of these to use multiple tranducers at one time:?

store.snapon.com/Pressure-Transducers-Ki...Adaptor-P637559.aspx

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10 Mar 2019 16:34 #27729 by tloftis
Replied by tloftis on topic DIY Pressure Transducer
and does anyone know what is inside this unit?

store.snapon.com/Pressure-Transducers-Ki...-Ultra--P737766.aspx

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28 Mar 2019 09:27 - 28 Mar 2019 09:28 #28171 by tloftis
Replied by tloftis on topic DIY Pressure Transducer
I guess for my Snapon Modis I will have to build this:

Description

• Adapts standard meter jacks to EEPV302AL, EEPV302AT and EEPV302AH pressure transducers (not included)

• Provides connection of transducer plug to standard meter jacks, and provides power to the pressure transducers

• Use up to two transducers simultaneously

• View results on VERDICT®/MODIS® Ultra/MODIS® Edge/VANTAGE® Ultra/M2 Meter


I am too cheap to buy one LOL Will have to sourse everything. see attached image
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Last edit: 28 Mar 2019 09:28 by tloftis.

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