Smart Temperature Sensor Development

Luke Heyburn Tue, 12/31/2013 - 16:27

I have been tasked with working on our new temperature and humidity monitoring and logging service. We currently have a box of 100 temperature sensors in the office, all broadcasting the temperature of their cardboard home every few minutes. My first challenge was to listen out for this data utilising the accompanying reader hardware and translate it into something readable.

Smart Sensor reader

The Smart Sensor reader; not much bigger than a mobile phone.

With the hardware interfaced and an application displaying the translated data, everything seemed to be working well, though each sensor was sending me a very similar reading. Working in an office environment where the temperature is constantly between 20°C and 22°C I was a little limited when it came to any extreme testing.

Smart Sensor Interface
We did, however, have one common utility that should be well outside of this range: the run-of-the-mill office fridge. It was time for some “extreme” testing!

Smart Sensor Interface

What is particularly interesting about this fridge (in this context at least) is that it also contains a freezer compartment. Both the fridge and the freezer appear be operating normally. The milk in my morning tea has never been lumpy before the expiry date and the freezer compartment had been used for storing groceries on occasion. In the summer months we had purchased two tubs of ice cream for the office, which admittedly did not last very long, but appeared to have been frozen the day after purchase.

Smart Sensor Interface

I placed two sensors in the fridge, one in the main container where we keep the milk and sandwiches and one in the freezer compartment.

I had assigned ‘Fridge’ and ‘Freezer’ tags to two of the sensors in the application so that they were easy to locate and also applied alert thresholds should the readings go over or under a certain temperature. After some research I concluded that a fridge should have a normal operating temperature between 3°C and 5°C whilst a freezer should operate at -18°C, if not just below this. I left the sensors in the fridge overnight and kept the Smart Listener application running in the background on my PC.

The following morning I saw something quite unexpected.

Inside the fridge

The fridge was running at 3.6°C which I understand is fine. However, the freezer was well above the expected -18°C. According to the last reading in the listview it was even warmer than the fridge! I opened up the sensor readings for the last 24 hours and had a look at the data.

Smart Sensor Interface

The red and blue lines represent the maximum and minimum temperature thresholds, while the green line, which is meandering in and out of these thresholds, represents the reading at a particular time.

From this chart it was clear that every 3 hours the freezer would go to a maximum temperature of 7.8°C before gradually dropping back to 1°C. Some more research ensued and I discovered that goods to be frozen that are stored between 5°C and 60°C for more than an hour host the perfect conditions for bacteria to thrive.

Ultimately I am very pleased with this discovery as it means that the sensors are already proving their worth. We are also looking at getting a new fridge freezer.

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