The building that I work in is extremely cold in the winter time. Many Mondays, especially, the area where I work is below 60 degrees. I bought a simple indoor outdoor thermometer, and put the outdoor sensor on the floor, and left the main unit on top of my desk. There is often a 15 degree difference from the floor to the top of the desk.
The building has a unique architectural feature. The first two floors are indented slightly over the third floor, where I sit. I actually have a window seat which, in this case, is a distinct disadvantage. The floor under my desk actually is a slab of concrete exposed to the outside air with little to no insulation. There is a layer of carpet, that is glued to the to the concrete that may be providing some insulation.
I decided to further instrument the situation. I build a matrix of temperature sensors connected to an Arduino. I decided to use the DS18B20 sensors since there are inexpensive, and provide a direct temperature readout with out using the A/D converters. There is a library for using these sensors, and several options for connecting them.
I modified the example to only read the sensors I was interested, and output CSV format to the USB interface. I collected this file, and was able to graph the output. I didn't use a RTC module, so the time output is relative to the start time.
I set up a grid of 6 sensors using yardsticks both for support and to give consistent distance:
The sensors closest to the wall of course gave the lowest readings, but even 3 feet in from the wall showed temperatures about 5 degrees warmer. The ones that were 5 feet from the wall were another 5 degrees warmer. The second set verified the results.
// Include the libraries we need
#include <TimeLib.h>
#include <OneWire.h>
#include <DallasTemperature.h>
// Data wire is plugged into port 2 on the Arduino
#define ONE_WIRE_BUS 2
#define TEMPERATURE_PRECISION 9
// Setup a oneWire instance to communicate with any OneWire devices (not just Maxim/Dallas temperature ICs)
OneWire oneWire(ONE_WIRE_BUS);
// Pass our oneWire reference to Dallas Temperature.
DallasTemperature sensors(&oneWire);
// arrays to hold device addresses
DeviceAddress device[8];
int numSensors=0;
void setup(void)
{
// start serial port
Serial.begin(115200);
Serial.println("Dallas Temperature IC Control Library Demo");
// Start up the library
sensors.begin();
// locate devices on the bus
Serial.print("Locating devices...");
Serial.print("Found ");
numSensors = sensors.getDeviceCount();
Serial.print(numSensors, DEC);
Serial.println(" devices.");
// report parasite power requirements
Serial.print("Parasite power is: ");
if (sensors.isParasitePowerMode()) Serial.println("ON");
else Serial.println("OFF");
// Search for devices on the bus and assign based on an index. Ideally,
// you would do this to initially discover addresses on the bus and then
// use those addresses and manually assign them (see above) once you know
// the devices on your bus (and assuming they don't change).
//
// method 1: by index
for(int i=0; i<numSensors; i++)
{
if (!sensors.getAddress(device[i], i)) Serial.println("Unable to find address for Device 0");
}
// show the addresses we found on the bus
for(int i=0; i < numSensors; i++)
{
Serial.print("Device ");
Serial.print(i);
Serial.print(" Address: ");
printAddress(device[i]);
Serial.println();
}
// set the resolution to 9 bit per device
for(int i=0; i< numSensors; i++)
sensors.setResolution(device[i], TEMPERATURE_PRECISION);
}
// function to print a device address
void printAddress(DeviceAddress deviceAddress)
{
for (uint8_t i = 0; i < numSensors; i++)
{
// zero pad the address if necessary
if (deviceAddress[i] < 16) Serial.print("0");
Serial.print(deviceAddress[i], HEX);
}
}
// function to print the temperature for a device
void printTemperature(DeviceAddress deviceAddress)
{
float tempC = sensors.getTempC(deviceAddress);
//Serial.print("Temp C: ");
//Serial.print(tempC);
Serial.print((int)DallasTemperature::toFahrenheit(tempC));
}
// function to print a device's resolution
void printResolution(DeviceAddress deviceAddress)
{
Serial.print("Resolution: ");
Serial.print(sensors.getResolution(deviceAddress));
Serial.println();
}
// main function to print information about a device
void printData(DeviceAddress deviceAddress)
{
printAddress(deviceAddress);
Serial.print(", ");
printTemperature(deviceAddress);
//Serial.println();
}
/*
* Main function, calls the temperatures in a loop.
*/
void loop(void)
{
// call sensors.requestTemperatures() to issue a global temperature
// request to all devices on the bus
//Serial.print("Requesting temperatures...");
sensors.requestTemperatures();
//Serial.println("DONE");
Serial.print(hour());
printDigits(minute());
printDigits(second());
Serial.print(", ");
// print the device information
for(int i=0; i<numSensors; i++)
{
printData(device[i]);
if (i<numSensors-1)
Serial.print(", ");
}
Serial.println();
delay(10000);
}
void printDigits(byte digits){
// utility function for digital clock display: prints preceding colon and leading 0
Serial.print(":");
if(digits < 10)
Serial.print('0');
Serial.print(digits,DEC);
}
So the output looks similar to:
0:11:12, 28FFD2890117, 70, 28FFF2F10117, 70, 28FF1A040217, 71, 28FFC1F70117, 72, 28FF59070217, 71, 28FFED8D0117, 70
0:11:23, 28FFD2890117, 70, 28FFF2F10117, 70, 28FF1A040217, 71, 28FFC1F70117, 72, 28FF59070217, 71, 28FFED8D0117, 70
0:11:33, 28FFD2890117, 70, 28FFF2F10117, 70, 28FF1A040217, 71, 28FFC1F70117, 72, 28FF59070217, 71, 28FFED8D0117, 70
0:11:43, 28FFD2890117, 70, 28FFF2F10117, 70, 28FF1A040217, 71, 28FFC1F70117, 72, 28FF59070217, 71, 28FFED8D0117, 70
0:11:53, 28FFD2890117, 70, 28FFF2F10117, 70, 28FF1A040217, 71, 28FFC1F70117, 72, 28FF59070217, 71, 28FFED8D0117, 70
0:12:03, 28FFD2890117, 70, 28FFF2F10117, 70, 28FF1A040217, 71, 28FFC1F70117, 72, 28FF59070217, 71, 28FFED8D0117, 70
0:12:13, 28FFD2890117, 70, 28FFF2F10117, 70, 28FF1A040217, 71, 28FFC1F70117, 72, 28FF59070217, 71, 28FFED8D0117, 70
CSV files are easier it ingest, and graph using whatever spreadsheet program one might like.
The night I took a sample, the temperature was down around 10-15 degrees below zero. The graph of the output looks like:
At the lowest, the temperature hit 49 degrees against the wall.
