Package Tracker – Thermal Sensor
My biggest motivator for purchasing the Package Tracker was the thermal sensors. We had some problems with recent international shipments. When the product arrived in Australia, it showed signs of exposure to excess temperature. But where? How? We could only guess.
Soon after, I ended up on the Sparkfun website, looking for parts to automate a reliability test, but I stumbled upon the Package Tracker and quickly purchased one along with some stuff for my original product.
My first attempt wasn’t successful. I had no idea about the unit of measure logged in the CSV file. I ran the Package Tracker overnight in my car. I expected that the results would help me understand the operation of the temperature sensor. The next morning, I got this strange looking data.
In the above chart, the time of day is approximate. The saw tooth wave is not at all what I expected. At this early juncture, I had not yet figured out the accelerometers; I was half convinced that my unit was defective. However I had my package tracker enclosed in its small shipping box. In this environment the package tracker doesn’t respond well to its environment.
To get the Temperature Sensor to work, you’ll need to know a few things. For starters, there are two temp columns in the CSV file. Use the SHT Temp column. This logs the temp data from the Sensirion SHT11. The datasheet, section 3. 2, explains the device output. For the way the package tracker configures the device, it outputs 0000 at -40 degrees and each bit corresponds to a rise of 0.018 degrees F. The Package Tracker converts the temperature output to degrees F in the function sht15_read.
To test the accuracy of the temperature sensor, I put it in a bag and dropped the bag into a cup of ice water. The Package Tracker recorded values in the 2500 range, corresponding to 25 degrees F. I could not find an explanation as to why the SHT11 would be offset from true temperature, so I added seven degrees to all of my temperature readings. The ice water experiment looked like this:
You see that I removed the sensor from the ice water after ten minutes.