Pressure transducer

Simon from Clevedon Diagnostician Posted   Latest   Edited  
Discussion
Tooling

Hi everyone I'm looking to build my own pressure transducer for in-cylinder testing and I have found a PT 0-400 bar range but it's output is in mA and not volts , now how do i set scope up for this shall I just use the amp clamp input?? Sorry if it's a stupid question. 

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Brandon from Reading

 

Diagnostician
 

Sorry Simon, I don’t know how I can help. how Many wires interfaces with this device? 

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Simon from Clevedon

 

Diagnostician
 

Hi , it's just a power and ground and a signal output 4-20mA output, power input 8v-32v.

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Mario from Weston

 

Diagnostician
 

I don't see how that's going to end well. If you'd like i can sell you my 5v regulated style transducer setup. It's a custom box, 12v power supply. Two transducer inputs, one vacuum input. Two pressure sensors. I don't use it anymore since I've upgraded. But you also didn't mention what scope it was for. Anyway if you're interested let me know

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Simon from Clevedon

 

Diagnostician
 

Why would it be bad ??

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Mario from Weston

 

Diagnostician
 

Oops didn't see the spec sheet until now. What scope? 

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Simon from Clevedon

 

Diagnostician
 

I have a pico , Snap on zeus, modis, vantage pro, and a bosch kts650.

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Randy from Raleigh

 

Curriculum Developer
 

If you could please send me some specifics about that transducer including manufacturer and part number I'll see if I can clarify that for you. I suspect the output is actually varying voltage but the application or possibly the description documents confuse the raw output of the sensor.

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Simon from Clevedon

 

Diagnostician
 

This is the spec sheet 

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Mike from Mount Pleasant

 

Mobile Technician
 

This is interesting. So does this unit have 3 wires? I'm assuming a power & ground, and then a signal out?

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Dmitriy from Toronto

 

Analyst
 

Hi Simon, sorry to say that but I have some disappointing news, and they are not about 4-20mA issue.

400 bars is about 5800psi. With stated accuracy +/-0.5% of FS (full scale), that is 30psi. You are likely to have issues with accuracy in the region of interest for in-cylinder testing, 0-200psi. Some of the errors may be offset or calibrated out, but be ready for that.

The next issue is response time. It is listed as <=10ms, which is rather slow. WPS500 has that parameter at 0.1ms, while less advanced transducers still deliver <=2ms. It's not that this transducer is not good, it's designed for a different purpose...

4-20mA issue, on the other hand, is easy to deal with. The device needs to be connected to power, e.g., 12V, but grounded not directly, but through a 100 Ohm resistor*. An oscilloscope is connected to that resistor. 4mA will result in 0.4V drop, 20mA will result in 2V voltage drop. Those values will need to be interpreted as 0-400bar.

*For some devices, the resistor should be on the power side, and the voltage drop should be measured using a differential or floating input oscilloscope (e.g., Pico 4425).

**Ground the transducer case when working with fuels.

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Simon from Clevedon

 

Diagnostician
 

Thank you for that explanation , I understand more now. 

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Stephen from Las Cruces

 

Engineer
 

Hi Simon, I'd like to echo everything that Dmitriy has said and just add to and round out the info that he's provided. I would add this; 4-20ma type devices are commonly used in industrial applications (think like a large factory maybe) where there might be a large amounts of electrical noise from all kinds of motors and devices running and or long distances between where the device is and the instrument reading it's output. Because the device's output is current, it is not subject to noise and voltage drops due to long runs of wire; thus affecting its measurement. This is primarily why you'll find these kinds of devices almost as a standard in these types of industrial applications. Another pro to these devices is that you can have more than one instrument reading it's output simultaneously.

If you do come across another such device, I would not be too hesitant to use it. I would just make sure that it has an appropriate range for what you want to do (0-300 psi for in cylinder measurements would be ideal in my opinion) and it has a response time that is adequate. The slowest I would recommend would be no slower than 1ms. Even at that your "horizontal resolution" will be a little choppy. Unfortunately, that is what most of the cheap "eBay" sensors are. But I've found that you can still manage well enough.

Hooking up these types of devices isn't hard, it's just a little different than using a voltage output type. The thing to understand is that the output needs to form a current loop. Current output devices will be found in either 4, 3, or 2 pins. In all cases there are two pins for the supply voltage or what is sometimes referred to as the "excitation" voltage. In your case, you had a 3 pin, and so two of those pins will be for the power supply (excitation) and the current output pin would be wired as such; from the output pin to a resistor, of a conveniently chosen value, and then from the resistor to the negative side of the power supply. You then measure the voltage across the resistor which will correlate to the pressure range of the transducer. The resistor value is rather arbitrary, but a good value that we like to use is 250 ohm. This would give you a voltage range of 1-5 volts and is a little more intuitive. Although, if you're using a Pico, you can just set up a custom probe to account for whatever voltage range you have with the chosen resistor value.

I hope this sheds a little more light on 4-20mA pressure transducer devices. If I had one for personal use (with the right specs), I would use it in a heartbeat.

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Simon from Clevedon

 

Diagnostician
 

Hi yes makes more sense now thank you 

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Christopher from San Antonio

   

Technician
   

I'm actually thinking about doing the same thing, but with a TE M5200 transducer (several options for electrical & hose connections, pressure ranges, as well as supply & output voltage). What are you using for a power supply?

Datasheet link: te​.​com/commerce/Docum…

FYI, it's not on the datasheet, but an engineer at TE told me the resolution is 1KHz. Also, one of the listed applications is for engine test stands.

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Keith from Collinsville

 

Diagnostician
 

I have made my own transducer setup, including a separate 5v power supply that allows for 2 devices(transducer, map sensor, etc.) I have tested about a dozen different types of transducers, from $12 to $200.....

Some important things I looked for were a ratio-metric linear voltage output compound transducer that operated in 5-15vdc range. 

A ratio metric linear voltage output sensor allowed for a voltage to pressure table to be built in the custom probe section of the Picoscope, allowing for pressure scale to be displayed as the y axis of the waveform, much like the WPS500x pressure scale setting in the Pico.

A compound sensor would allow for negative or "vacuum" readings. I found that the majority of less expensive transducers would flatline and not read anything less than a "zero" pressure.

If anyone has any additional info i would like to hear it as well, maybe Bernie Thompson could chime in with a few tips, without giving away any secrets? 

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Simon from Clevedon

 

Diagnostician
 

Thanks for the reply I appreciate all the info people have given .

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Christopher from San Antonio

 

Technician
 

Which transducer did you ultimately go with?

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Simon from Clevedon

 

Diagnostician
 

I think I'm gonna just go with the wps500 and need to cover diesel and petrol 

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