## 1 Introduction  
Table grapes are very popular for their high nutritional and therapeutic value. However, table grapes deteriorate easily by pathogen infection due to their characteristics of being soft and having a high moisture content, which lead to the quality and safety issues of table grapes that have received worldwide attention [1; 2]. Cold chain logistics is an effective measure to ensure that table grapes are stored in a low temperature environment all of the time by using artificial refrigeration technology when they are in various links, such as processing, storage, transportation, sale, _etc._, and to reduce the quality losses of table grapes [3; 4; 5]. Cold chain management has become crucial, challenging and important. However, the cold chain logistics system is still complex, and the information asymmetry is high, as well. It is urgent to conduct research on the dynamic characteristic of the cold chain environment by intelligent monitoring technology to complete the sensor data acquisition and processing, improve the transparency and traceability of the cold chain logistics and guarantee the quality and safety of the table grapes [6; 7]. Though there are many environmental factors that affect the quality and safety of table grapes, temperature and relative humidity are the main factors that affect the quality and safety of table grapes during the cold chain logistics [8; 9]. The temperature is the key factor that directly affects the respiration intensity of the table grapes and the activity of the enzymes. Suitable temperature management is becoming a very important function of the table grapes' cold chain logistics to extend the quality and storage period [10; 11]. The table grapes' quality will not be maintained in a good condition if the relative humidity is too high or too low, which is conducive to reducing the losses of the table grapes' moisture content as much as possible [12; 13]. It is necessary to monitor in real time, to control the temperature and relative humidity and to analyze the quality to keep table grapes in a suitable refrigerated condition during the whole cold chain logistics. Firmness is an important quality to judge the freshness and softening degree of table grapes, which could indirectly verify the preservation of table grapes [14; 15]. The other quality indicators are mainly the moisture loss [16; 17], decay rate [18], abscission rate [19], total soluble solids (TSS) [20], titratable acid (TA) [21] and sensory evaluation [22]. However, owning to the complex cold chain logistics system, firmness is the easiest quality indicator among these quality indicators to measure using a handheld durometer during the cold chain logistics, while the others need special instruments or environments. Therefore, it is practicable and important to select firmness to evaluate the quality of table grapes in actual cold chain logistics. Traditional methods, such as temperature and humidity recorders and radio frequency identification technology, are the most popular, reliable and accurate ways to monitor the cold chain. However, such methods have high management costs with off-line monitoring [23; 24]. Applying wireless sensor network (WSN) to monitor the cold chain logistics of table grapes, which integrates sensor technology, embedded computing, networking, wireless communication technology and distributed processing together to sense information from monitored objects in the environment and which sends it to the end-user via wireless and multi-hop network, has been an inevitable trend [25; 26; 27]. WSN has been widely adopted and applied in agricultural [28; 29; 30], environmental monitoring [31; 32], industrial [33; 34] and many other important areas.