Floor Temperature

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.