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Density Altitude and Grains Per Pound
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Density Altitude and Grains Per Pound

by gmashburn03 on Mon Feb 11, 2019 12:47 pm

I drag race and I want to create a couple sensor inputs that I can record in my engine management system. These two bits of weather information play a key role in tuning our engines and right now we use expensive external products that we have to manually check before we tune the car for each pass.

I would love to be able to put a sensor together that I can mount in the car and have the data automatically record in the engine management system. Something that has like a 0-5v range that I can scale for data. Any ideas on how I can achieve this?

1) Density Altitude
- Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA temperature))
* Pressure = (standard pressure - your current pressure setting) x 1,000 + field elevation
* OAT stands for outside air temperature
* ISA stands for standard temperature

2) Water vapor in the air measures in grains per pound
- This is calculated by correlating relative humidity and temperature

Thank you for your help!

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Re: Density Altitude and Grains Per Pound

by adafruit_support_bill on Mon Feb 11, 2019 1:05 pm

The BME680 sensor from Bosch measures pressure, temperature and humidity: https://www.adafruit.com/product/3660
We have libraries to support this on Arduino compatible and Raspberry Pi microcontrollers.

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Re: Density Altitude and Grains Per Pound

by millercommamatt on Mon Feb 11, 2019 1:13 pm

How important is absolute calibration? Anything you buy from here, you're going to want to calibrate yourself.

Also, not a lot of sensors sold here output raw voltage. You'll have to run readings through a microcontroller and then output your voltage signal with a DAC.

grains per pound! Are you living in the 1800s?

The BME280 does the same thing as the previously mentioned BME680 just without the gas measurements you don't seem to need. These sensors will need calibration for accurate measurements and, in my experience, they have a break-in period and will likely need a recalibration down the road (no pun intended).

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Re: Density Altitude and Grains Per Pound

by gmashburn03 on Mon Feb 11, 2019 4:23 pm

millercommamatt wrote:How important is absolute calibration? Anything you buy from here, you're going to want to calibrate yourself.

Also, not a lot of sensors sold here output raw voltage. You'll have to run readings through a microcontroller and then output your voltage signal with a DAC.

grains per pound! Are you living in the 1800s?

The BME280 does the same thing as the previously mentioned BME680 just without the gas measurements you don't seem to need. These sensors will need calibration for accurate measurements and, in my experience, they have a break-in period and will likely need a recalibration down the road (no pun intended).


Thanks for the reply. You're going to have to forgive my ignorance. I'm not the most educated on all of this stuff when trying to create something yourself.

When you say absolute calibration, are you referring to the accuracy of the sensor or are you referring to something like absolute pressure vs gauge pressure? Or neither? Lol

GPP is very useful in the drag racing world. There are decades of data that help support engine tuneups and it's just what a lot of people go off of. A lot of these engines are making greater than 2,500 HP and when atmospheric conditions change, especially water vapor in the air, it's the difference between a great running engine and a complete meltdown. A lot of these setups are right on the edge of failure to get the maximum amount of power output.

The BME280 seems to be along the lines of what I need. It would be nice if it could interpolate the humidity and temp readings into GPP.

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Re: Density Altitude and Grains Per Pound

by millercommamatt on Mon Feb 11, 2019 4:52 pm

By absolute accuracy mean the absolute truth of the sensor. Many sensors are good at relative accuracy. That is the are better at detecting changes than the absolute condition.

You can by a case of BME280s and ask them the pressure. Without calibration you'll get answers from 1010 to 1020 hPa for a pressure of 1015 hPa (absolute accuracy). But the'll all detect a change in the pressure of 0.05 hPa (relative accuracy). With some work you can get all the sensors to report the same value. If you have a good standard to set them to, you can get decent absolute calibration. Just make sure to re compare to your standard regularly.

For your application, how much error is acceptable? The BME280 may or may not be the sensor for you. Also, will the sensor be exposed to extremes in heat, moisture, or vibration?

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Re: Density Altitude and Grains Per Pound

by adafruit_support_bill on Mon Feb 11, 2019 5:25 pm

Temperature and pressure sensors tend to be fairly stable once calibrated. Humidity sensors in general are some of the most finicky ones out there. If exposed to extremes in heat, moisture or chemical vapors, they can take a while to recover. In my experience, the Bosh humidity sensors are about the best among low-cost humidity sensors.

Industrial grade humidity sensors can handle more abuse, but they are WAY more expensive.

https://learn.adafruit.com/calibrating- ... -calibrate

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Re: Density Altitude and Grains Per Pound

by gmashburn03 on Tue Feb 12, 2019 1:38 pm

millercommamatt wrote:By absolute accuracy mean the absolute truth of the sensor. Many sensors are good at relative accuracy. That is the are better at detecting changes than the absolute condition.

You can by a case of BME280s and ask them the pressure. Without calibration you'll get answers from 1010 to 1020 hPa for a pressure of 1015 hPa (absolute accuracy). But the'll all detect a change in the pressure of 0.05 hPa (relative accuracy). With some work you can get all the sensors to report the same value. If you have a good standard to set them to, you can get decent absolute calibration. Just make sure to re compare to your standard regularly.

For your application, how much error is acceptable? The BME280 may or may not be the sensor for you. Also, will the sensor be exposed to extremes in heat, moisture, or vibration?


Got it. Thanks for clarifying.

Accuracy is important. There is a little wiggle room for error when it comes to weather conditions but it would be nice to get it as close to accurate as possible. We can always use our more expensive external weather stations that we keep in our trailers as a baseline for comparison of data but I am trying to create sensors that can be placed in the car that can be data logged in to the same system as the rest of the engine data for ease of use and not having to flip back and forth between program. and to ensure it always gets recorded. If that makes sense.

They will not be subject to "extreme" temperatures. More than likely it would be placed inside the cabin of the car where it gets (at worst) 115-120* in the summer. No moisture. There will be vibration but I am sure there are ways we can minimize how much vibration those sensors see.

Can you kind of see what I am trying to accomplish here?

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