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Generic transducer custom probe (Picoscope)

posted Jun 08, 2016 23:46:06 by George
I've been playing with my home made in cylinder pressure transducer for some time now, but I haven't been able to make a custom probe so it can read pressure and vacuum. I tried on another forum but the answers I got were to complicated (maybe on purpose)for a newbie like me,so I tough I ask you gentlemen.

Her's some info on the transducer, my set up, and a capture.

Thank you in advance.




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13 replies
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Tyler said Jun 09, 2016 03:20:08
Wow, not a bad setup! Probably a ton cheaper than a PV-350 or a WPS500, anyway.

So you're concerned that the transducer isn't reading vacuum correctly? I could be totally wrong here, but it sure looks like your waveform is going into vacuum at the right times. How much vacuum I can't really tell (the scales on the left got chopped off).

If you were to attach a compression hose to the transducer and apply light vacuum to the end, would the signal change at all?
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AndyMacFadyen said Jun 09, 2016 04:55:33
That result looks fine with great detail I am guessing your engine has an exhaust with little restriction.
The detail of the ripple on the induction stroke shows it is working properly at below atmospheric pressure.

[Last edited Jun 09, 2016 05:01:25]
"Rust never sleeps"
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George said Jun 09, 2016 06:00:05
Wow, not a bad setup! Probably a ton cheaper than a PV-350 or a WPS500

$5 dollar 5volt regulator, $20 dollars transducer, and my existing compression tester hoses (without the valve).
So you're concerned that the transducer isn't reading vacuum correctly?

Not really, I just want the scope to automatically scale my pressure/vacuum readings (create a custom probe for this transducer), that's why I attached the transducer's specs.
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George said Jun 09, 2016 06:16:39
it sure looks like your waveform is going into vacuum at the right times

This was taken from a Hyundai that was some tooths retarded (I couldn't tell from this waveform, I had to do a ckp-cmp correlation test with my Pico to find out).
Here are the captures:


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George said Jun 09, 2016 06:19:06
If you were to attach a compression hose to the transducer and apply light vacuum to the end, would the signal change at all?

I'll try that tomorrow and post it.

Thanks.
[Last edited Jun 09, 2016 06:19:41]
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George said Jun 09, 2016 23:12:57
Here's the vacuum capture :

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George said Jun 10, 2016 19:44:59
Any help, anyone?
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AndyMacFadyen said Jun 10, 2016 22:48:42
I think you may have transposed the voltages

In very very rough numbers 25 mm hg of vacuum = -0.8 bar or about -12 psi

Y=MX+C
V=MX+C Where C=0.5 Volts and M=(4/300)= 0.0133 Volts per PSI
So at 25mm Vacuum Output voltage = (-12*0.0133) + 0.5 = 0.33 volts

So it looks as if the transducer is useable for our purposes below atmospheric presure as long as you don't expect perefect accuracy.

[Last edited Jun 10, 2016 23:04:11]
"Rust never sleeps"
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Rob Longoria said Jun 11, 2016 18:38:25
Where did you source your Transducer at, I picked one up off the web and got very bad results.
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AndyMacFadyen said Jun 11, 2016 21:22:20
Rob is your problem a huge amount of noise on trace? if the trace looks horrible it could be as simple as you just need a low pass filter. Pro level scopes usually have built in filters controlled by software and other options to fine tune signal aquisition, on cheaper scopes like the Hantek you would need an external hardware low-pass filter.
"Rust never sleeps"
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George said Jun 11, 2016 23:05:38
Where did you source your Transducer at

Bought mine off ebay.
I picked one up off the web and got very bad results

How did you supply the 5 volts required for it to function?, maybe the voltage fluctuated all over the place.
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Rob Longoria said Jun 17, 2016 01:46:45
My problem is lack of detail in the trace. My power supply is this https://www.amazon.com/gp/product/B00A71CMDU/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

I was thinking of using a 1k ohm resistor, I have a homemade one from a fuel rail pressure sensor off an audi but that one only goes to about 150 psi. It works but I would like to do a snap throttle and my pattern gets cut off. I used a 1k ohm resistor for that one.
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AndyMacFadyen said Jun 17, 2016 10:34:39
That power supply should be more than fine, although for anybody starting from scratch to build one there is an alternative called an UBEC that is a tiny low cost very low noise voltage convertor used by drone builders.

Any standard cheap 2.5 mPa industrial pressure sensor/transducer off ebay should do. Some of the Chinese ebay suppliers also sell extra long leads for the sensor as the pig tails on the leads that come with them are too short.


"Rust never sleeps"
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