Why understanding Sensors is important

[Steve Cole]

Guys, forget what may or maynot happen to an ENGINE or ECM this is ONLY about the SENSOR. If you do not have a firm understanding of how a sensor works you are never going to understand how other devices work with them.

MAP sensor is what we are currently trying to work with, later after you understand it, maybe we can move on to other SENSORS ONLY discussions.

MAP = Manifold Absolute Pressure, that means something and all of you have skipped over it or do not understand it, not sure which yet.

[a2wheeler]

This link has some good detail, however it is based on the auto systems.
It has this same Q&A -
What is absolute pressure?
In absolute measurement, the zero point (where the measuring device indicates zero) is an absolute zero pressure. That means no pressure, or in other words, a 100% vacuum.

http://www.fastfieros.com/tech/map_sensors_and_how_they_work.htm

[98fxstc]

The engine creates a vacuum in the intake manifold relative to the outside atmospheric pressure
ie a region of negative pressure
MAP sensors measure absolute pressure
so that 0Kpa (zero pressure) is the absence of pressure
and 100 kPa equates to maximum pressure

[98fxstc]

And we're not talking about IAT sensors yet ?

[akjeff]

Guys, forget what may or maynot happen to an ENGINE or ECM this is ONLY about the SENSOR. If you do not have a firm understanding of how a sensor works you are never going to understand how other devices work with them.

MAP sensor is what we are currently trying to work with, later after you understand it, maybe we can move on to other SENSORS ONLY discussions.

MAP = Manifold Absolute Pressure, that means something and all of you have skipped over it or do not understand it, not sure which yet.

I'll guess that you're referring to the A in MAP, as in Absolute? Breezed right by that. So the MAP measures(or rather puts out a voltage proportional to) absolute pressure. Pressure relative to absolute zero pressure, rather than barometric pressure. Sensors don't really measure anything, as they merely output a voltage, current, resistance. It's up to whatever device this V,I, or R is fed, to intemperate it, as use it as desired.

Jeff

[joe_lyons]

Absolute pressure is from absolute 0 and to below atmospheric pressure.

[akjeff]

Absolute pressure is from absolute 0 and to below atmospheric pressure.

I'll respectfully disagree. Absolute pressure is from absolute zero on up. It can be either above or below barometric.

Barometric pressure is whatever the ambient air pressure is, relative to absolute zero.

Gauge pressure, is pressure(or vacuum), relative to barometric pressure.

Jeff

[Coyote]

 Pressure measured in the TB is below atmospheric.  As the load on the engine goes up, the pressure in the TB drops relative to outside.

[akjeff]

Pressure measured in the TB is below atmospheric.  As the load on the engine goes up, the pressure in the TB drops relative to outside.

My understanding is that as the load on the engine increases, the manifold pressure increases to nearly that of atmospheric? Or another words, as load increases, the amount of vacuum in the intake drops to nearly zero(i.e. atmospheric) But, I've been wrong before!

Jeff

[Coyote]

You know Jeff I do believe you are correct. As the TB opens, the Absolute pressure will rise. Thanks for making me think about it. I had in my mind the old vacuum gauges but that's not the same as the MAP sensor.

[akjeff]

You know Jeff I do believe you are correct. As the TB opens, the Absolute pressure will rise. Thanks for making me think about it. I had in my mind the old vacuum gauges but that's not the same as the MAP sensor.

:up:Doesn't happen often!

[dynaglide]

Guys, forget what may or may not happen to an ENGINE or ECM this is ONLY about the SENSOR. If you do not have a firm understanding of how a sensor works you are never going to understand how other devices work with them.

MAP sensor is what we are currently trying to work with, later after you understand it, maybe we can move on to other SENSORS ONLY discussions.

MAP = Manifold Absolute Pressure, that means something and all of you have skipped over it or do not understand it, not sure which yet.

You're clearly fishing for some particular answer that you're not getting. Instead of having us try to guess the "correct answer" that you apparently already know, how 'bout you tell us what you're trying to say?  After all, isn't the point of this forum for the rest of us to learn, rather than getting frustrated?

[Steve Cole]

Absolute pressure is from absolute 0 and to below atmospheric pressure.

I'll respectfully disagree. Absolute pressure is from absolute zero on up. It can be either above or below barometric.

Barometric pressure is whatever the ambient air pressure is, relative to absolute zero.

Gauge pressure, is pressure(or vacuum), relative to barometric pressure.

Jeff

This is the correct answer, but look at how many people did not know what it is. The whole idea for me is to get you to understand the basics and being able to know what the sensor is and what it is capable of doing. So now that you know WHAT it is, now lets talk about what it can and cannot do. The sensor does not give a rats ass about an engine or what it does. This is why I have stated to limit the discussion to just the sensor. All it does is measure absolute pressure and temperature and output a voltage. Page 3 of the document shows the pressure error and temperature error along with the transfer curve for the MAP portion of the sensor.

So if we assume the sensor is mounted in a chamber and that chamber is at 20 kPa absolute pressure what would the sensor read or tell us the absolute pressure is?

[joe_lyons]

.5ish volts.  Could be off +- 3.9ish KPA

[FLTRI]

So if we assume the sensor is mounted in a chamber and that chamber is at 20 kPa absolute pressure what would the sensor read or tell us the absolute pressure is?

Anywhere between 17.6 to 22.4 without temp error so anywhere from 16.1 to 23.9 with temp error.
Bob

[whittlebeast]

But that is the product sheet.  What is the repeatability of any given one sensor as installed on any given bike?  What about as it rises to 30 kpa?  What if it lowers to 30 kpa?

[joe_lyons]

I confused about this.


It says its the table for temp but says its in kpa? 

[fbn ent]

I used -1.7 for the temp adjustment --- I got 15.9 - 23.90 kpa. Of course that number would come from reading the voltage of the sensor.

[a2wheeler]

At 20 kpa, the chart shows an allowable error of +/- 2.4 kpa.
This matches FLTRI - "Anywhere between 17.6 to 22.4 without temp error"

[FLTRI]

But that is the product sheet.  What is the repeatability of any given one sensor as installed on any given bike?  What about as it rises to 30 kpa?  What if it lowers to 30 kpa?

But the question was what is the reported KPA when the sensor is inside a chamber @ 20kpa.
Answer that,
Bob

[a2wheeler]

That temperature error multiplier chart is confusing. It appears that there is a +/- 1.5 kpa affect on kpa reading across the entire temperature range. However , it calls it a multiplier - could that be 1.5 times the already listed +/- 2.4kpa - ending up with  +/- 3.6 kpa.

[Coyote]

That temperature error multiplier chart is confusing. It appears that there is a +/- 1.5 kpa affect on kpa reading across the entire temperature range. However , it calls it a multiplier - could that be 1.5 times the already listed +/- 2.4kpa - ending up with  +/- 3.6 kpa.

That's the way I read it.

[FLTRI]

That temperature error multiplier chart is confusing. It appears that there is a +/- 1.5 kpa affect on kpa reading across the entire temperature range. However , it calls it a multiplier - could that be 1.5 times the already listed +/- 2.4kpa - ending up with  +/- 3.6 kpa.

I'll bet you're correct...I missed the "multiplier" and took it for +/- 1.5 KPA error.
Bob

[joe_lyons]

1.5 above and 1.75 below is what it looks like

[a2wheeler]

1.5 above and 1.75 below is what it looks like

I agree, that bottom portion looks a bit over 1.5.  Must not be too important as they did not provide a good Y axis scale to read. I'm sure I'll get shot for that "not be too important". LOL