Measuring Height with Atmospheric Pressure

2023-01-31 | By SparkFun Electronics

License: See Original Project

Guide by MARIAH

Introduction

It can be easy to forget that we are constantly under pressure (although Freddie and Bowie tried to ‎tell us) from the atmosphere around us. Not only that, it’s also counterintuitive: the higher up you go, ‎the less pressure there is, and vice versa. There are a few ways we encounter this in our day to ‎day, like maintaining cabin pressure on a plane at 30,000 feet or water boiling just a little bit faster ‎out here in the Rockies! With this project, we’re going to play around with the atmosphere around ‎us, and do a little bit of math, to create a pressure sensor-based height measuring tool!‎

The powerhouse of this project is the Qwiic MicroPressure Sensor. This sensor has an onboard ‎Honeywell 25psi piezoresistive silicon pressure sensor which gives the ability to measure very ‎minute differences in absolute pressure. We'll take those pressure measurements, apply some ‎math inside the code and translate the change in pressure from a bottom and top reading into a ‎height measurement.‎

Required Materials

Here are all of the parts you’ll need to follow along with this tutorial! You can add what you need or, ‎if you don’t have any of these parts already, you can click to the Wishlist provided and purchase it ‎all in one go!‎‎

‎Qwiic MicroPressure Height Demo SparkFun Wish List

Reversible USB A to Reversible Micro-B Cable - 0.8m ‎
Flexible Qwiic Cable - 100mm‎
SparkFun Qwiic MicroPressure Sensor
SparkFun Thing Plus - ESP32 WROOM (Micro-B) ‎

VIEW QWIIC MICROPRESSURE HEIGHT DEMO ON SPARKFUN.COM‎

‎Hardware Hookup

Luckily, the hardware is pretty easy to hookup; simply connect one end of your Qwiic cable to your ‎Thing Plus, and the other end to your sensor. Boom! You’re done! The parts list below includes a ‎USB cable for power, but this project could also be used with a battery for maximum portability.‎

hardware_1

Software Setup and Programming

To run this project, you’ll need to install the SparkFun MicroPressure Library (for more information ‎on how to install this library, and take advantage of its example sketches, check out our SparkFun ‎Qwiic MicroPressure Hookup Guide!).‎

qwiic_2

SparkFun Qwiic MicroPressure Hookup Guide

‎Get started using your Qwiic MicroPressure breakout board with this hookup guide.‎

Height Demo Code

Once the library is installed, go ahead, and open up a blank sketch. Make sure to select your board ‎‎(SparkFun ESP32 Thing Plus) and COM port before uploading, and to hold down the boot button ‎on your Thing Plus during the upload! The boot button is also what we’ll be using to tell the system ‎when we’d like to take a reading; let us away to the code!‎

Copy Code

//SparkFun Qwiic MicroPressure Height Measuring Demo
//Original Demo by: Dryw Wade, Adapted for Qwiic MicroPressure by: Mariah Kelly

#include <Wire.h>
#include <SparkFun_MicroPressure.h>  // Click here to get the library: https://github.com/sparkfun/SparkFun_MicroPressure_Arduino_Library/archive/main.zip

#define EOC_PIN  -1
#define RST_PIN  -1
#define MIN_PSI   0
#define MAX_PSI   25

SparkFun_MicroPressure mpr(EOC_PIN, RST_PIN, MIN_PSI, MAX_PSI);

// Button pin (using boot button on board)
int buttonPin = 0;

// Interrupt flag for button presses
volatile bool buttonPressed = false;

// Time to wait after a button press before reading the next button press (helps with debouncing)
unsigned long timerDelay = 500;

// Measured pressure at the bottom and top of the measurement
int bottomPressure = 0;
int topPressure;

// Calibrated number of inches per Pascal of pressure difference. Note that it's negative, because lower altitude results in higher pressure
float inchesPerPascal = -3.3;

// Using arrays to take multiple readings and display average
#define numReadings 500
int bottomReadings[numReadings];
int topReadings[numReadings];

// Breaking up array calculations
boolean measuring = true;
boolean showData = true;

void setup() {
// Start serial
Serial.begin(115200); //Be sure that your Serial Monitor is set at this rate!
Serial.println("MicroPressure height demo begin!");

// Initialize the I2C library
Wire.begin();

if (!mpr.begin()) {
    Serial.println("Cannot connect to MicroPressure sensor.");
    while (1);
}

// Configure button pin as interrupt
attachInterrupt(digitalPinToInterrupt(buttonPin), myISR, FALLING);
}

void loop() {
// Wait for button press
if (buttonPressed) {
// Check whether the bottom pressure has been measured
    if (bottomPressure == 0) {
        if (measuring) {
        for (int i = 0; i < numReadings; i  ) {
      // Bottom pressure has not been measured, store this measurement as the bottom
      bottomPressure = mpr.readPressure(PA);
      bottomReadings[i] = bottomPressure;
    }
    measuring = false;
  } 
  if (showData) {
    for (int i = 0; i < numReadings; i  ) {
      bottomPressure  = bottomReadings[i];
    }
    bottomPressure = bottomPressure / numReadings;
    // Print out the bottom pressure
    Serial.print("Bottom pressure: ");
    Serial.print(bottomPressure);
    Serial.print(" Pa        ");
    showData = false;
  }
  measuring = true;
  showData = true;
} else {
  if (measuring) {
    for (int i = 0; i < numReadings; i  ) {
      // Bottom pressure has been measured, this is the top pressure
      topPressure = mpr.readPressure(PA);
      topReadings[i] = topPressure;
    }
    measuring = false;
  } 
  if (showData) {
    for (int i = 0; i < numReadings; i  ) {
      // Print out the top pressure
      topPressure  = topReadings[i];
    }
    topPressure = topPressure / numReadings;
    Serial.print("Top pressure: ");
    Serial.print(topPressure);
    Serial.print(" Pa        ");
    showData = false;
  }
  // Compute the difference in pressure
  int pressureDifference = topPressure - bottomPressure;

  // Compute total height in inches using known inches per Pascal
  int inches = pressureDifference * inchesPerPascal;

  // Compute number of feet in this height
  int feet = inches / 12;

  // Subtract that many feet from the total number of inches
  inches -= feet * 12;

  // Print computed height in ft'in"
  Serial.print("Height: ");
  Serial.print(feet);
  Serial.print("' ");
  Serial.print(inches, 1);
  Serial.print("\"");
  Serial.println();

  // Reset bottom pressure for next measurement
  bottomPressure = 0;
}
// Wait for button to stop bouncing
delay(timerDelay);

// Reset flags
buttonPressed = false;
measuring = true;
showData = true;
 }
}

void myISR() {
// Button just got pressed, set the flag
buttonPressed = true;
}

Now that we’ve uploaded our code, we can find something to measure! I used my desk–including a ‎tape measure for reference–which is about 29 inches tall. Start by placing your system at the lowest ‎point; this is where we’ll take our reading for the bottom pressure (we measure bottom to top, since ‎a lower altitude results in a higher pressure, and vise-versa). The code has a built-in sampling ‎system to help stabilize the sensor readings, so it’ll take a second or two after pressing the boot ‎button to pop up on the serial monitor; it takes our ‘numReadings’ value of readings, which is 500 to ‎start, but feel free to change that up as you see fit! Then, all there’s left to do is the same series of ‎steps for our top pressure:‎

steps_3

reading_4

Now that we have our readings, let’s see how we did!‎

readings_5

Readings (in Pascals) and height calculations (feet & inches)‎

Not too shabby! We’re a few inches off, but that’s to be expected with this project. Now the super ‎fun part is experimenting with how you might be able to tweak it to be even more accurate! I tried ‎lots of different things and managed to get the margin of error down to a few inches, rather than ‎feet.‎

Resources and Going Further

While this tutorial is largely conceptual, I could see a lot of field uses for quick, approximate ‎measurements where another measurement tools might struggle. Since we are using the Thing ‎Plus - ESP32 WROOM the measurement data could be sent over Wi-Fi or Bluetooth for quick ‎reference on an OLED. If that's something you'd like to attempt, below is a tutorial that may help ‎with that process.

‎Thanks for reading and please let us know if you've got a good use case for this type of ‎measurement tool.‎

tool_6