Pressure and Temperature issues at altitude.

Hi,

I'm using the BMP180 pressure module (SparkFun Barometric Pressure Sensor Breakout - BMP180 - SEN-11824 - SparkFun Electronics) in an un-pressurised aircraft at altitude.

The module requires that before calculating the pressure, a temperature reading is taken, this is required to set an internal constant used to calculate pressure.

My question is this, the aircraft is un-pressurised, so the internal and external pressure should be about the same. However the internal temperature of the module is likely to be a lot warmer than the ambient temperature. Is this going to effect the pressure reading, and if so, by how much?

The outside air temperature could be as low as -20 degrees C, and the inside temperature is likely to be around 15 to 20 degrees, I'm concerned that the module won't be measuring the temperature of the air-mass whose pressure I'm interested in!

OhMyCod:
The module requires that before calculating the pressure, a temperature reading is taken, this is required to set an internal constant used to calculate pressure.
I think the internal constants are constant (set in factory calibration) and they do not/cannot be changed.

My question is this, the aircraft is un-pressurised, so the internal and external pressure should be about the same. However the internal temperature of the module is likely to be a lot warmer than the ambient temperature. Is this going to effect the pressure reading, and if so, by how much?
I don't think the temperature reading is used to calculate the pressure so it should not be relevant. The air pressure inside and outside should be the same in an unclosed system.

I agree, pressur inside and outside will be the same.
The temperature reading is likely to compensate the measuring sensor.
As its inside at the same temp as the sensor thats the temp it needs to see.

OhMyCod:
The module requires that before calculating the pressure, a temperature reading is taken, this is required to set an internal constant used to calculate pressure.

My question is this, the aircraft is un-pressurised, so the internal and external pressure should be about the same. However the internal temperature of the module is likely to be a lot warmer than the ambient temperature. Is this going to effect the pressure reading, and if so, by how much?

The outside air temperature could be as low as -20 degrees C, and the inside temperature is likely to be around 15 to 20 degrees, I’m concerned that the module won’t be measuring the temperature of the air-mass whose pressure I’m interested in!

The purpose of reading the temperature is to be able to standarize the pressure. At lower temperatures the air molecules move more slowly so you need more molecule collisions to get the same pressure.

You would probably need to check with someone who knows about flying to find out whether the difference between external and internal pressure matters. You may also need to figure out how, exactly, the sensor measures pressure.

The general relationship between volume temperature and pressure is PV = mRT and temperature must be measured in DegreesKelvin - 0C = 273K. So the difference is between 253K and 293K or about 16%. BUT … I have no idea if this has any relevance to your airplane question.

…R

I think the internal constants are constant (set in factory calibration) and they do not/cannot be changed.

This is true for all but one of the constants, one of them (referred to in the documentation as "b5") varies with temperature. The datasheet says I need to read the temperature first (to set "b5") and then read what it refers to as the 'uncompensated pressure' before converting it to actual pressure using various factory-set constant and "b5".

need to check with someone who knows about flying

I know about flying, but I'm no expert on the atmosphere, and more importantly I don't know how pressure sensors work.

I guess the question can really be summarised as ....

'Does the module need to be at the same temperature as the air pressure being measured?'

OhMyCod:
'Does the module need to be at the same temperature as the air pressure being measured?'

Yes.
Are you saying that the module and temp sensor are seperate ?
I assumed it was an integrated unit.

You have to assume the pressure inside is the same as outside.

Trying to measure outside press in any case is tricky very careful siting is needed or airflow will affect the pressure reading.

Google on pitot tube.

Are you saying that the module and temp sensor are seperate ?

No

I assumed it was an integrated unit.

It is.

What I'm saying is that although the pressure inside and outside an un-pressurized aircraft will be the same, the temperature won't be. Outside air temp may be -20 C, whilst inside temp may be +15C

OhMyCod:
What I'm saying is that although the pressure inside and outside an un-pressurized aircraft will be the same, the temperature won't be. Outside air temp may be -20 C, whilst inside temp may be +15C

When I mentioned asking a flying expert I was really trying to say that this is not an Arduino question.

You need to ask somebody who understands the physics of the atmosphere, how it relates to aircraft in flight, and how you intend to use the pressure information when you get it.

...R

Your question is a good one and for a definitive answer should probably be addressed to the manufacturer of the MEMS sensor. It doesn't matter that the air inside and outside have different temperatures -- the pressure will be the same (as long as air flow effects can be neglected).

However, I think your application will work and in support of that, actually did some experiments a while ago to check. I carried a sensor (an SCP-1000) from indoors to outdoors during the winter (temp difference of about 20 C) and could see no significant difference in the reading after the sensor temperature had stabilized.

Many, if not all, of these sensors have a sensing membrane covering a small compartment that is not completely evacuated. That compartment exerts an internal pressure that depends on the temperature, so to work out the difference between inside and outside pressures, the temperature of the inside comes into play (but not the outside). As Robin pointed out earlier, the pressure of the inside compartment depends on the relation PV = nRT.

I think you should be fine.

Transducers like this are routinley used and considered accurate enough for model rocket altitude competitions.

Boardburner2:
Transducers like this are routinley used and considered accurate enough for model rocket altitude competitions.

There are usually no humans in model rockets.

...R

No , but it did get me thinking.

Provided the sensor is mounted in a box with a small hole, the interior will stabilise at some temperature.
The small amount of air entering or leaving the box should not materialy affect the sensor.

With a rocket the air is continuously leaving and interior temp remains pretty much that of the launch point.

Boardburner2:
No , but it did get me thinking.

I understand where you are coming from.

My concern is that neither of us KNOWS whether any errors due to temperature would matter for @OhMyCod's project.

...R

The pressure sensor you are using gives different values depending the temperature of the sensor. That's why you need to compensate with a temperature reading. You can try this warming or cooling a sensor during measurements.

Your question is a good one and for a definitive answer should probably be addressed to the manufacturer of the MEMS sensor.

I have emailed BOSCH, and got not reply.

I was really trying to say that this is not an Arduino question.

True, but there are some very bright people on this forum (plus a light sprinkling of *&(%$). Some of them may know the answer.

neither of us KNOWS

yes, it all seems to boil down to this, we can all make educated guesses, but know one knows a definitive answer.

My current plan (should probably have thought of this earlier) is to do some experiments. So far the temperature factor "b5" has just been an anonymous constant buried deep in the innards of the pressure function. Next testing session I will see how "b5" varies with temperature, and then look at how big a variance that makes on the pressure reading. Could be that it makes little or no difference.

Thanks for everyones input

Not sure if this is relevant but.

By increasing the dead space , in my case a bit of plastic tube the mixing of the gases is reduced and the sensor remains at a fairly constant temperature.

Sudden changes if big enough however require a settling time due to expansion and compression effects.

As said it really a physics rather than electronic problem.

A couple of comments,
1: Inside static air pressure can be very different from outside static air pressure, sometimes aircraft use a combined pitot static probe, and other times there are static air source holes on the fuselage almost always marked do not paint.
2: Most air pressure sensors are temperature sensitive and require temperature compensation, the better ones have a built in temp sensor for this.
3: Pressure altitude affects true air speed.
4: Outside air temperature also affects true air speed, and outside air temperature is also referenced as "Total air Temperature" which is more involved than just putting a temperature sensor on the outside skin of the airplane.

Bill