How sensor technology is evolving to meet cold chain needs


With full compliance now in effect for the U.S. Food and Drug Administration’s (FDA) Sanitary Transportation Act, it’s a good time to provide an update on TI’s technology to help meet the evolving needs of cold chain, hazard analysis critical control point (HACCP) and good distribution practices (GDP) requirements. In a previous blog post, Manny Soltero introduced the basics of cold chain and some of TI’s popular sensor solutions to meet your cold chain requirements. Today, I’ll discuss some of the more challenging sensor needs for which TI provides innovative solutions.

While high-accuracy local temperature sensors like the TMP112 are popular in data logger designs, sometimes the application requires measurement off the printed circuit board (PCB) and down a probe — for example, the need to measure the pulp temperature of a fruit shipment, a vial of glycol in a hospital refrigerator, or even the temperature of food at a steam table.

There are three traditional methods to measure the temperature down a probe:

  • Thermistors: While simple from a hardware design standpoint, these devices suffer from nonlinearity issues and increase the software burden on the processor for better accuracy.  
  • Thermocouples: This method provides better linearity but at the cost of more complex circuitry and software. Accuracy is also limited by the accuracy of an additional temperature sensor used as the cold junction reference for the thermocouple.  
  • Resistance temperature detectors (RTDs): Using these devices can provide incredible accuracy and linearity but tend to be expensive and very complex to use in designs.

All three solutions require a compromise and also require calibration to meet traceability requirements, adding significant cost to the overall solution.

As an alternative to solve temperature probe challenges, the LMT01 provides a very simple pulse-count interface requiring no analog-to-digital converter (ADC), references, or calibration. Electrically, the LMT01 only requires two wires in the probe, making it mechanically compatible with existing housing designs. These two wires share power and communication in what is essentially a digital current loop (see Figure 1). The LMT01 will modulate its current consumption between 34µA and 125µA where each 125µA pulse translates to 1 LSB (see Figure 2). All the system needs to do is count the pulses; it’s simple to translate the reading back into temperature with Equation 1:

Figure 1: The LMT01’s two-pin digital current loop topology; no additional Vcc or Gnd pins are necessary

 

Figure 2: The LMT01 modulates current to create a series of digital pulses; each pulse translates to 1 LSB (0.0625°C) of temperature

The LMT01 is 100% tested to meet an accuracy of ±0.5°C from -20°C to +90°C and meets requirements for National Institute of Standards and Technology (NIST) traceability. For deep-freeze applications, the LMT01 is ±0.7°C accurate down to -50°C. For sterilization, the LMT01 can operate up to 150°C with ±0.62°C accuracy.

Although the LMT01 is very useful for measuring temperature at a single location, one of the biggest challenges for cold chain applications is data logging from multiple locations, such as in a reefer container, display refrigerator or cold storage warehouse. It’s not uncommon to measure over a dozen locations across a system, with several meters between sensor locations.

System designers are forced to choose between a hard-wired cable solution or a wireless solution for each location. A cabled solution provides more reliability, but it can be very costly and time-consuming to run individual cables to each location. Wireless systems eliminate the hassle and high installation costs of a cabled system, but come with their own challenges: more complex hardware and software, battery life, and radio-frequency interference.

The TMP107 can provide the reliability and simplicity of a cabled system while dramatically reducing the high costs of wiring to each location. It accomplishes this by altering the wiring topology from a star topology (where two wires run from each location back to the central data-acquisition unit) to a daisy-chain topology (where each sensor communicates to the next sensor in the chain). Running just a single cable can reduce installation times from hours down to minutes.

With copper approaching five-year highs, the TMP107 can provide tremendous savings, not only by using fewer wires but also saving fuel from the reduced weight in refrigerated containers. Each device is uniquely addressable based on its location within the daisy chain and also supports global read and write commands. The TMP107 is NIST traceable with and has an accuracy of ±0.4°C.

The last area I’d like to address is the more stringent requirements of pharmaceuticals for greater accuracy and longer battery life in data loggers. To meet these needs, TI recently announced the TMP116 family, which offers the accuracy of an RTD without complicated, high power-consumption circuitry or the need to calibrate. The TMP116 family is NIST traceable with accuracy options of ±0.2°C or ±0.3°C while consuming only 3.5µA.

As an added benefit, the TMP116 provides 64 bits of electrically erasable programmable read-only memory (EEPROM) that designers can use to store additional traceability data such as product IDs, serial numbers, date codes or additional calibration beyond the data sheet.

Additional resources