Pressure sensor

[Jean-Sebastien Stoezel]

Hi:

My car came with this pressure sensor (picture attached) and I'd like to use it in parallel with a mechanical gauge for oil pressure measurements. I think this sensor was part of an AIM data acquisition system, though on my engine it originally came as a plug for a cracked block .
Brand is "Measurement Specialities Inc", range is 100psi. There is no model or part number stamped on the id plate.

I went to the manufacturer website. I could not locate exactly what model this is. It seems that regardless of the model, they have different options available. Some can be powered through a variable source (8 to +30V), some have an onboard regulator.

Since it is fitted with an AIM style (Binder) connector, I think I got the pinout right from an online Mychron doc. I plugged the sensor to 5V (I am assuming this is what VRef is on an AIM system) and it seems to respond to small pressure variations (i.e. I blew in it).
Does this confirm this is a 5V powered sensor?

And what is the output scale, psi/V?

Thanks,
JS

[sracing]

Without the MSI part number we will have to assume it is the unit for an AIM system. The Vref is 5V and they are linear. (ie. 0V = 0 psi, 5V = 100 psi, 2.5V = 50 psi. I have used many of these on other car dyno systems and general industrial D/A systems and this one is most popular. However, I would check it with some pressures to make sure.

BTW, most people can blow up to 2 psi so you would have seen a V change of about .1 volt ?

[Jean-Sebastien Stoezel]

Hi Jim:

Thanks for the reply. It simplifies things greatly that this is a linear sensor.
I will have to go back to see how much voltage variation I got while blowing in it, I am using a National Instruments USB acquisition card, via the Measurement Explorer utility. I saw the voltage plot moving as I was blowing in the sensor, though I didn't make any attention to the actually voltage increase.

JS

Without the MSI part number we will have to assume it is the unit for an AIM system. The Vref is 5V and they are linear. (ie. 0V = 0 psi, 5V = 100 psi, 2.5V = 50 psi. I have used many of these on other car dyno systems and general industrial D/A systems and this one is most popular. However, I would check it with some pressures to make sure.

BTW, most people can blow up to 2 psi so you would have seen a V change of about .1 volt ?

[David Ferguson]

AiM used a couple of different MSI sensors. Some of them required 12V and some required 5V. You can certainly tell by the wiring of the binder connector. If you apply 5V, what is the output voltage at 0 psi? It should be either .5V or 1.0V, if you don't see that, then it's either a different input voltage, or it's failed.

The AiM Binder pinout is pin1 = signal, pin2 =ground, pin3 = Vbatt (12V), pin4 = Vref (4.5 or 5v).

[romoman]

The AiM Binder pinout is pin1 = signal, pin2 =ground, pin3 = Vbatt (12V), pin4 = Vref (4.5 or 5v).

Clockwise from the solder side.

[sracing]

BTW, I stand corrected. While they are linear in AIM's case, the 0 psi voltage is about .5. It is linear going up from there.

[Jean-Sebastien Stoezel]

It would be 0V output if it were at 0 PSI I guess. Atmospheric pressure is about 14 PSI something at sea level, hence the "less than 1V" output when in an atmospheric pressure environment.

JS

BTW, I stand corrected. While they are linear in AIM's case, the 0 psi voltage is about .5. It is linear going up from there.

[David Ferguson]

Clockwise from the solder side.

Of the sensor-side connector, which has pins

[Jean-Sebastien Stoezel]

This is slightly confusing to me. The analog sensor outputs about 15psi when the engine is off (this is the atmospheric pressure I guess) while the mechanical gauge I also have installed on the engine displays 0PSI. The mechanical gauge does have graduations starting from 0, so one might think it would indicate 15PSI if this is what the pressure it would measure.

When the engine is running both the analog and mechanical gauge display a similar value, within a few PSI.

I am confused as I thought maybe I'd have to take substract the static reading of the analog sensor (15PSI) to get the actual oil pressure (and match the value on the mechanical gauge).

Does this mean that I might have an actual 0PSI oil pressure when the engine is running, even if the analog sensor is returning 15PSI?

JS

[2BWise]

The analog appears to be measuring absolute pressure which means it includes atmospheric. The mechanical is measuring gage pressure, atmospheric pressure compensated for. So, I would tend to trust the mechnical. Most automotive oil pressure gages already compensate (auotmeter, stack and such)

[Jean-Sebastien Stoezel]

Well this is exactly why it is confusing me. If the mechanical gauge is relative, and if the analog sensor is absolute, why do they output the same value when the engine is running?

JS

The analog appears to be measuring absolute pressure which means it includes atmospheric. The mechanical is measuring gage pressure, atmospheric pressure compensated for. So, I would tend to trust the mechnical. Most automotive oil pressure gages already compensate (auotmeter, stack and such)

[BLS]

Jean,

check the edit below first...
Just looking, I would guess you might have a "gauge" measurement pressure sensor and according to the manufacturer, the low cost US300* might have a voltage output of 0.5 to 4.5 volts. Assuming you have a 0-100psi sensor, this is 0.04v /psi, so at .5 volt you would read 16psi when the aim input is set to a linear scale (0-5vdc = 0-100psi), pretty close to 15psi and nothing is ever perfect.

At 50psi,, the output from the sensor would be: 50 x 0.04 + 0.5 = 2.5 volts, which would read 50psi on the linear 0-5 input of the aim.

Verify what your aim sensor input is set for. It either needs an offset to handle the .5 v zero signal, or a scaling that goes negative, or use the input setup that aim specifies for this sensor.

*just guessing which model, but most of them do not have a true dc voltage of zero volts at zero pressure. There is typically an offset voltage.

Barry

edit:
Hmm, some how my math is in error. Maybe I need a refresher algebra course, or long division, or something...

With the generic aim input of 0-5v selected, when the sensor is at zero pressure and .5 output, and the aim scale set to 0-100, then the aim would read .5/5 x 100 = 10psi, not quite 15. the sensor might be a 1 to 5 v type, in which case the aim would read 20psi.

What is your input set for in the aim? there is an msi 0-100 selection according to the manual. If you have it set to that, then we need to know what aim is using for the input offset and what sensor you have. If you have the msi input selected, then maybe you do have an absolute sensor. if so you can change to the generic 0-5 v input if the aim allows the units to be negative.