The GY-21P is an interesting module in that it combines a BMP280 sensor and an SI7021 sensor. The on-board BMP280+SI7021 sensor measures atmospheric pressure from 30kPa to 110kPa as well as relative humidity and temperature.
BMP280
Pressure range: 300-1100 hPa (9000 meters above sea level at -500m)
Relative accuracy (at 950 – 1050 hPa at 25 ° C): ± 0.12 hPa, equiv. to ± 1 m
Absolute accuracy (at (950 – 1050 hPa, 0 – +40 ° C): ± 0.12 hPa, equiv. To ± 1 m
Mains voltage: 1.8V – 3.6V
Power consumption: 2.7µA at 1Hz readout rate
Temperature range: -40 to + 85 ° C
SI7021
HVAC/R
Thermostats/humidistats
Respiratory therapy
White goods
Indoor weather stations
Micro-environments/data centers
Automotive climate control and defogging
Asset and goods tracking
Mobile phones and tablets
Size: 1.3*1cm/0.51*0.39″
Features:
Operation Voltage: 3.3V
I2C & SPI Communications Interface
Temp Range: -40C to 85C
Humidity Range: 0 – 100% RH, =-3% from 20-80%
Pressure Range: 30,000Pa to 110,000Pa, relative accuracy of 12Pa, absolute accuracy of 100Pa
Altitude Range: 0 to 30,000 ft (9.2 km), relative accuracy of 3.3 ft (1 m) at sea level, 6.6 (2 m) at 30,000 ft.
Code
I use a variety of Adafruit libraries, took the default examples and made the following out of them
https://github.com/adafruit/Adafruit_Si7021
https://github.com/adafruit/Adafruit_BMP280_Library
https://github.com/adafruit/Adafruit_Sensor
[codesyntax lang=”cpp”]
#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_BMP280.h>
#include "Adafruit_Si7021.h"
Adafruit_BMP280 bme; // I2C
Adafruit_Si7021 sensor = Adafruit_Si7021();
void setup()
{
Serial.begin(9600);
Serial.println("BMP280 and SI7021 (GY-21p) test");
if (!bme.begin())
{
Serial.println("Could not find a valid BMP280 sensor, check wiring!");
while (1);
}
if (!sensor.begin())
{
Serial.println("Did not find Si7021 sensor!");
while (true);
}
}
void loop()
{
Serial.println("BMP280 results");
Serial.print("Temperature = ");
Serial.print(bme.readTemperature());
Serial.println(" *C");
Serial.print("Pressure = ");
Serial.print(bme.readPressure());
Serial.println(" Pa");
Serial.print("Approx altitude = ");
Serial.print(bme.readAltitude(1013.25)); // this should be adjusted to your local forcase
Serial.println(" m");
Serial.println();
Serial.println("SI7021 results");
Serial.print("Humidity: ");
Serial.println(sensor.readHumidity(), 2);
Serial.print("Temperature: ");
Serial.println(sensor.readTemperature(), 2);
Serial.println();
delay(2000);
}
[/codesyntax]
Output
OPen the serial monitor
BMP280 results
Temperature = 27.26 *C
Pressure = 99042.63 Pa
Approx altitude = 191.78 m
SI7021 results
Humidity: 31.04
Temperature: 27.30
BMP280 results
Temperature = 28.43 *C
Pressure = 99044.82 Pa
Approx altitude = 191.60 m
SI7021 results
Humidity: 32.59
Temperature: 28.56
BMP280 results
Temperature = 28.17 *C
Pressure = 99037.41 Pa
Approx altitude = 192.22 m
SI7021 results
Humidity: 32.88
Temperature: 28.46
You can see the temperature output is close between the sensors
Links
Atmospheric Humidity Temperature Sensor Breakout Barometric Pressure BMP280 SI7021 for Arduino