## 5 WST Applications
### Cold Chain Monitoring and Traceability  
Perishable food products such as vegetables, fruit, meat or fish require refrigerated transports. Therefore, temperature is the most important factor when prolonging the practical shelf life of perishable food products. Studying and analyzing temperature gradients inside refrigeration rooms, containers, and trucks is a primary concern of the industry. The supply chain management of fresh foods requires fast decisions because goods are forwarded within hours after arrival at the distribution center. Appropriate planning calls for more information than that which could be provided by standard RFID tracking and tracing. Quality problems should be detected as quickly as possible, and alarms should be triggered when temperature gradients cross a threshold. Even if direct access to the means of transport is not possible, online notifications offer new opportunities for improved transport planning. The use of wireless sensors in refrigerated vehicles was proposed by [PERSON] _et al._[28]. The vehicles can host a variety of sensors to detect, identify, log, and communicate what happens during the journey, monitoring the status of perishable products in transport (see Figure 2). [PERSON] _et al._ studied and analyzed intermodal refrigerated fruit transport that integrated wireless sensor networks with multiplexed communications, fleet management systems, and mobile networks [14]. ZigBeemotes were validated for their use under cooling conditions in warehouses, studying the behavior of the motes in fruit chambers. Also, the psychrometric data model was implemented for quick assessment of changes in the absolute water content of air. Thus it was possible to address water loss from the products, and also to detect condensation on the commodities [25]. The fresh fish logistic chain can be also monitored using WST. [PERSON] _et al_. reported a WSN based system that allows. The application is built around a web server and bespoke wireless data loggers operating over a GSM network [17]. [PERSON] _et al_. validated a RFID smart tag instrumented with light, temperature and humidity sensors. The system provide real-time traceability information of the product to the different fish distribution chain links [48]. [PERSON] used active sensors to record spatial temperature profiles [95]. [PERSON] used passive RFID loggers to test the probability to find a certain temperature in a transport, but did not go into spatial deviations [96]. [PERSON] _et al_. showed the use of RFID for temperature tracking in a commercial shipment of pineapples from Costa Rica to the USA. [PERSON] _et al_. monitored 16 refrigerated trucks using semi-passive RFID instrumented with temperature sensors (Turbo Tag) detecting temperature gradients [45]. These kinds of applications can register temperatures during transportation and distribution but the transmission range is less than one meter and they are not able to develop advanced network topologies like the ZigBee devices can do [46]. The accuracy of data loggers is a critical issue in cold chain management. This accuracy becomes even more important if the objective is early detection of temperature changes and gradients. Standards for food distribution allow deviations of \(\pm\,0.5^{\circ}\)C from the set point [97]. [PERSON] _et al_. did a comparison of three different types RFID loggers in a climatic chamber, finding that the percentage of measurements with a difference to the average value less than the deviation \(\pm\,\delta\) was between 66% and 73% [45]. RFID data loggers are available in high quantities, but they require manual handling because of their low reading range. Another disadvantage is that temperature loggers are only available for the 13.56 MHz HF-Range. The major drawback of this band is the limited reading range of about 20 cm. If a gate reader scans items automatically upon arrival at the warehouse, the reading range has to cover several meters.