Temperature Sensors

Arduino-Temperature-Sensor

Introduction

Temperature plays an important role in life. We rely on it everyday to monitor our health, food, and electronics to name a few examples. In the old days, temperature was measured with tubes of mercury that would rise and fall based on the surrounding temperature. In the transistor age, temperature sensors are orders of magnitude smaller, occupying areas of less than 1mm^2.

There are different types of temperature sensors available to choose from depending on your application. A few of these sensors types are:

  • Thermocouple
  • Resistance temperature detectors (RTD)
  • Infrared
  • Thermistor

Because the MetaWear boards primarily use thermistors, we will focus on how thermistors work in this post.

How They Work

Thermistors “sense” ambient temperature via changes in resistance. Unlike an RTD, thermistors have a logarithmic relationship between resistance and temperature. This relationship results in high temperature sensitivity for temperatures up to about 40°C but also leads to very poor resolution at higher temperatures.

Temperature vs. resistance of a thermistor

Temperature vs. resistance of a thermistor

 

Resistance, in ohms, is converted to degrees Kelvin by using the Steinhart-Hart equation, a model of semicondutor resistance at various temperatures. It provides better approximation of the relationship than simpler equations and is usable over the entire operational range.

MetaWear Thermistor

All MetaWear boards, except R, come equipped with an NCP15XH103F03RC thermistor by Murata providing a temperature range between -40°C to 125°C. For MetaWear R boards, you can attach an external thermistor to the board and use that as a temperature source. We have a post detailing the setup here.

The MetaWear libraries come fully equipped to help you configure and process data from the temperature sensors. Documentation on the supported features is on the MbientLab site for both Android and iOS.

Sensor Data

The MetaWear firmware reports temperature in degrees Celsius with 0.25°C granularity using a thermistor and 0.1°C with BOSH sensors.

Applications

With the small form factor of the MetaWear board, it can be used in a variety of scenarios. Consider grocery delivery services that are offered by Amazon or Safeway. The MetaWear can be placed in the cooler bags to ensure the groceries remain within acceptable temperatures during transportation. If you’re looking to reduce utility costs for a building, you can place the boards all around a building to determine if the environmental systems are adequately maintaining a set temperature. For the home gardener or farmer, the boards can record the ambient temperatures of the fields to monitor crop temperatures.

It must be noted that while thermistors have a variety of uses beyond these simple use cases, the MetaWear platform is a sensor platform and the use cases of the board must be centered around this idea.

Additional Reading

You can check out this post by Ametherm for more information on other sensor types. U.S. Sensor Corp. has posted about the manufacturing process of their thermistors, providing an insight into how they are made.