When Cold Chain Reliability Comes Down to a Single Degree, a Precise Temperature Sensor IC CountsSeptember 09, 2020 by Jake Hertz
There are many different types of temperature sensors. But one type is specially equipped to handle the precision and scalability of cold chain management.
One of the biggest challenges in the supply chain of foods and pharma is preserving perishable items. As such, it is important that these items (i.e foods and medicines) are kept well refrigerated during transport, ensuring freshness for foods and efficacy for medicine.
Simplified distribution of food shows the challenges in the cold chain. Image used courtesy of Advantech
This poses a considerable constraint on electrical systems designs that monitor the cold chain refrigeration systems, which we've discussed previously in an article on cold chain monitoring. Variances of as little as two degrees Celsius can be enough to damage the desired goods, rendering them useless.
In this space, design engineers are challenged to create systems that can both measure and control the temperature within a small margin of error. This undoubtedly has led to the development of high-precision temperature sensors in cold chains.
Types of Temperature Sensors
Temperature sensors come in many varieties (both analog and digital), all of which offer certain advantages given the desired application.
Thermocouples are analog temperature sensors that consist of two different metals. Given a temperature difference between these dissimilar metals, a voltage is generated across the device, which can be used to infer temperature. Thermocouples are cheap and low power, but are not very sensitive and don’t exhibit a linear response, requiring significant linearization algorithms to function.
Thermistors are analog temperature sensors that display a change in resistance in proportion to a change in temperature. These sensors are quick to respond and highly sensitive, but require a great deal of power. Because they are non-linear, they also require a lookup table to operate.
Thermocouple working principle. Image used courtesy of Omega Engineering
Logic output temperature sensors are digital temperature sensors that drive a logic high when the sensed temperature crosses some predefined threshold. They offer very accurate sensing capabilities at low temperatures and are often used in place of switches in systems.
Serial output temperature sensors are digital temperature sensors that offer high sensitivity at extremely low power. Communication is normally established between a microprocessor and serial output temperature sensor via I2C or SPI communication.
Temperature Sensors for Cold Chain
Simple analog sensors have been a common option in cold chain management for a long time. However, with today’s advances in semiconductor technology, integrated temperature sensors are considered the best option in such temperature-sensitive applications.
Serial output temperature sensors are arguably the best suited for the stringent requirements of cold chain design. Oftentimes these types of temperature sensors can provide accuracy down to 0.5°C and a low operating current of less than 250 μA.
A daisy chain of temperature sensors. Image used courtesy of Texas Instruments
Another place where temperature sensing ICs are beneficial is when a developer is aiming for cost-efficient scalability. For instance, sensing temperature in multiple locations (like in refrigerators or in reefer containers) can become costly because of the multiple MCUs for all of these measurements.
Using a shared bus, designers can address scalability with digital temperature sensors that share the line, which allows them to be individually addressed in-band in I2C or using chips like in SPI.
New Temperature Sensors From Maxim Integrated
As an example of recent digital temperature sensors purpose-built for cold chain designs, Maxim Integrated released the MAX31889 and MAX31825, which are aimed at improving accuracy, power consumption, and scalability. Built for standalone temperature sensing, the MAX31889 is a digital temperature sensor that displays impressive accuracy and power consumption.
The specs back up the claims, boasting ±0.25°C accuracy from -20°C to +105°C, 68 μA operating current during measurement, and 0.55μA standby current.
MAX31825 application example. Image used courtesy of Maxim Integrated
For scalability solutions, the MAX31825 digital temperature sensor enables up to 64 devices per bus to drastically reduce complexity and the need for multiple MCUs. It also offers some solid specs, like ±1°C accuracy from 0°C to +70°C.
Precision Matters in Cold Chain Management
With the release of these new temperature sensor ICs, Maxim has addressed some of the aforementioned difficulties in cold chain design. These ICs and other integrated temperature-sensor ICs like them provide designers with useful options for both scalable and accurate solutions.
With no signs of slowing down, the cold chain will need more solutions like this in the future to ensure the quality of our food and the reliability of our medicine.