Introduction to Temperature Sensors¶
In Simple Hardware devices, we have three sensors/components that we can use to measure temperature.
- Ambient MCU temperature is measured with the MCU - the controlling unit of the device
- Precise MEMS temperature is measured with a dedicated MEMS sensor
- Precise NTC temperature is measured with an external NTC sensor (on case or on a cable)
Here's a simple comparison table:
|Sensor||Precision||Location||Dual sensors supported||Humidity||SimpleLeak||SimplePack 4.0||SimplePack 4.0 Plus||SimpleMeter||SimpleIndustry|
|Internal MCU||2-3°C||Inside the device||❌||❌||✔️||✔️||✔️||✔️||✔️|
|Dedicated MEMS||<0.5°C||Inside the device||❌||✔️||❌||✔️||✔️||✔️||✔️|
|External NTC||<0.5°C||On case or on a cable||✔️||❌||❌||❌||❌||❌||✔️|
Sensor reaction time¶
Please note that the reaction time of temperature sensors can be impacted by the location of the sensor, design of the case etc. For example, the SimplePack 4.0 Plus Precise Temperature and the SimpleMeter both use the MEMS, but the air intakes in the SimpleMeter case allow for a reaction time that's much quicker. This can have a major impact on the validity and informative value of temperature data.
If you're in the process of finding the best solution for your use case, you need to understand in-depth how important temperature data is for your application (main focus/complementary), how precise you need the reported temperature to be and what's your reaction time requirement. This is unique for every use case.
MCU ambient temperature¶
The MCU is the controlling unit of the device and has an internal temperature sensor. The core of the sensor is a circuit which generates voltage proportional to absolute temperature. The precision of this sensor is around 2-3°C.
The temperature data from this sensor can be used in use cases where lower temperature precision and slower reaction time is not an issue (the devices will also be a bit cheaper). Usually used to send temperature as secondary, complimentary data to primary data such as tracking information, door status, leakage detection etc.
Dedicated MEMS sensor¶
The dedicated sensor we use (STMicro HTS221) can measure both temperature with a high precision (<0.5°C) and relative humidity. The sensing element consists of a polymer dielectric planar capacitor structure capable of detecting relative humidity variations. The sensor is capable of operating from -40°C to +120°C. The temperature and humidity output is linear and is calculated from capacitance.
This sensor's main use case is monitoring temperature and humidity in the SimpleMeter device, which has air intakes that allow air circulation and significantly reduce the reaction time of the sensor. However, this sensor is also available in the SimplePack 4.0 and 4.0 Plus and the SimpleIndustry and is used in use cases where you need high temperature precision and don't mind the slower reaction time (as the sensor in internal inside the cases of these devices).
The MEMS sensor is the only way to measure humidity with our devices.
External NTC sensor¶
The external sensor we use is an NTC (Negative Temperature Coefficient) thermistor - a resistor whose resistance is dependent on temperature. When temperature increases, the resistance on the sensor decreases and this allows for very precise temperature value output (precision <0.5°C). The output temperature value is non-linear to resistance, from which it is calculated.
The NTC is used in use cases where measuring temperature with a high precision and a quick reaction time is the main focus. The NTC is exclusively available with the SimpleIndustry device only (if you need a smaller device, we can create SimplePacks with an NTC too, but this would require custom development). The sensor can be located both on the case of the device or on a cable with a length of up to 2 meters. We can even provide a single device with two NTC sensors on two cables. The main use cases are industrial temperature monitoring solutions.
Temperature in our firmware¶
We can report temperature in several ways - temperature can be sent periodically with or without redundant backup data, it can be sent when a temperature threshold is crossed or together with an action-based event (such as movement) and more.