Measuring boost pressure

Hi there,

I'm currently working on a 'multimeter' that will display various vehicle parameters on an LCD screen. It will initially be used in a 2001 T4 TDI (ACV).

Request for selection of a suitable pressure sensor for measuring boost pressure.

I am undecided between an absolute pressure sensor (Motorola MPX 4250A) and a differential pressure sensor (Motorola MPX 5500P), and I can't quite decide because I don't know what is more suitable .

When using the 5500P with an open P2 input, the displayed boost pressure is relative to atmospheric pressure (boost pressure minus atmospheric pressure). When using the 4250A, it displays the pressure measured in the LL channel without reference to atmospheric pressure. As far as I know, the ACV operates this way, and the MSG performs a correction for atmospheric pressure.

Which sensor is more suitable and provides more meaningful values?
My idea: Use a 5500P (differential pressure sensor) because it allows for better monitoring of the LD's structure.

Does the absolute pressure value read from the MSG via diagnosis represent the actual pressure in the LL channel, or does it already include the ambient pressure?


Thank you in advance, zaydo.

Beim MPX5500P solltest Du den Messfehler (2,5% FSS) beachtet. Das heißt der mögliche Messfehler beträgt bis zu 0,125bar. Relativ zu 1bar Messdruck beträgt die Ungenauigkeit dann bis zu 12,5%. Das That would be too much for me.
The MPX 4250 is significantly more accurate with a Full-Scale Span (FSS) of 1.5%, resulting in a resolution of 0.0375 bar.
In my opinion, it's better to accept the usual fluctuations in ambient pressure rather than introducing measurement errors. If necessary, you can also install a potentiometer and adjust the offset.
If you're creating a solution using a microcontroller, you can also have the MPX 4250 measure the ambient pressure before starting.

Hi there,

When using the MPX5500P, you should pay attention to the measurement error (2.5% FSS).

That's a good argument against the 5500 series. But ironically, I still have one of those lying around .



If necessary, you can also install a potentiometer and adjust the offset.

It is implemented using a microcontroller. The correction values can be programmed by the user via a button.



If you are creating a solution using a microcontroller, you can also have the MPX 4250 measure the ambient pressure before starting.

Good idea. However, given the differences in altitude, this is obviously not ideal, especially in southern Germany. It doesn't bother me as much because I live in East Frisia, but I'm sure some people would still want one.

Ideally, two 4250 models would be better. But considering the price .


But again, regarding my question about the value that can be queried via diagnosis: Is it an absolute value or one relative to ambient pressure?
The value could then be used for comparison or evaluation, and vice versa.


Thank you, zaydo.

There are also relative/differential pressure sensors with a smaller measurement range, which makes them more precise. However, these are not available from retailers like Reichelt or Conrad.