*3.2. Communication Protocol*

DiverPAD allows diver to store text or image data in the water. In order to communicate those data from DiverPAD to handheld devices (e.g., smartphone), we present a communication protocol for DiverPAD, which is shown in Figure 7. Since the communication function is not smooth in water, an optimization method for fast and accurate information transmission was implemented based on the existing communication standards.


**Figure 7.** Communication protocol of DiverPAD.

As shown in Table 2, some fields of the header and tail are defined as constant values in the data packet of the DiverPAD and are designed to transmit text and image data in addition to the existing sensor data.


**Table 2.** Structure of DiverPAD data packet.

Based on the presented data packet structure, the bidirectional communication between the DiverPAD and a handheld device is performed, and both the sensor information generated in the water and the information about the diver's marine leisure activity are transmitted into a handheld device. In particular, as shown in Table 3, CMD (Command) values in the packet structure are newly proposed in this paper to implement user message transmission about log data request, environment information setting, and data transmission of the DiverPAD (sensor information, text, image), so as to overcome the limitation of DiverPAD's memory capacity.

**Table 3.** Description of Command values.


### *3.3. Design of User Interface (UI)*

As shown in Figure 8a, we show user environment, hardware environment, content management, and technical constraints by applying the UI design process. After wearing the DiverPAD, as a way to recognize the risk situation between marine leisure personnel, the depth and time were set according to the qualification situation, the number of times, and the curriculum. In case of increasing and descending, the update cycle for perceiving the risk situation was set in 1 s increments. The averages of previous unit time (10 s) section and the current unit time (10 s) section were analyzed.

As shown in Figure 8b, if the difference between the maximum and minimum depth for 1 min is within 3 m, the average depth value for the 1 min section is set as the current depth, and if it is within +2~−2 m from the current depth, the swim is displayed. In addition, when a difference in water depth of +2~−2 m or more occurs in the current depth, it is displayed increasing or descending, and when a difference of +10~−10 m or more occurs, a risk situation display screen is designed and implemented.

The above pattern analysis of marine leisure activity is divided into education, emotion, and emergency modes. First, in the education mode, the educational contents of divers are stored based on the water depth and displayed according to the rise and fall. In the emotion mode, the user displays self-written contents using the terminal device buttons. The emergency mode is activated if the control system determines the situation to be an emergency, sending emergency signals to the display. The module was implemented to identify any emergencies by comparing the values using an acceleration sensor, timer, and others. The above emergency situation information (maximum depth, dive time, water temperature, rise alert and visual, nap time, date, time, frequency, and others) is compared with the values of the sensor values and database thresholds.

**(a)** Description of DiverPAD display

**(b)** Examples of DiverPAD display

**Figure 8.** User interface of DiverPAD.

### *3.4. Electrical Insulator Based Capacitive Touchscreen*

DiverPAD adopted novel underwater touch function to conveniently communicate various information among divers in the water. The touch function of DiverPAD was designed as a means of the accurate and immediate communication tools in the process of data sending and receiving. The module to operate underwater touch function consists of touch panel unit and touch input controller. In general, touch function makes it possible to detect the force of touch pen pressing on the surface of LCD or OLED panel.

In this paper, underwater touch function of DiverPAD was implemented to adaptively adjust the force of touch pen pressing according to depth of water. DiverPAD allows a user to freely express intention by inputting through a touch input underwater. Thus, a touch input diver pad comprising a module is configured so that a sensor measurement module having a temperature sensor for sensing underwater temperature and a depth sensor for detecting water depth, as well as touch input and output modules including a touch panel section displaying data by the sensor unit, a battery module for supplying power and control was obtained.

The pressure protection module is made of tempered glass and it is made as thin as possible to lighten the weight and sharpen the design defined as shown in Figure 9.

**Figure 9.** Phased gap adjustment corresponding to the different depth of water.

At the end of the touch input and output modules, a sealing module was installed to prevent the inflow of water and an insulated touch module was attached to the top due to unpredictable water flow and strong pressure of surrounding environment. In addition, the insulation touch module was configured to include an insulating layer between the upper layer and the lower layer as a pressure protection section. Accordingly, when there is no user's touch in the water, the upper layer and the lower layer are separated by the insulating layer so that no touch occurs.

The insulating layer is configured to serve to prevent malfunction of the touch panel unit due to water pressure in water. Based on this, the upper part of the DiverPAD terminal device was divided into an insulation touch module, and a space division divided into a touch space and a reservoir space, so that an empty chamber was formed between the space division and the insulation touch module. In this state, when the user presses and touches the touch space, the insulator is pushed to the reservoir space, and the upper layer and the lower layer come into contact.

The touch panel module senses the point of contact, and thus touch sensing is performed. In addition, when the user releases the touch, the upper layer rises due to the restoring force, and the insulator flows back from the reservoir space to the touch space and returns to the original state.

A method of attaching an adhesive film to improve durability against external pressure was used between the thin film display module and the touch panel module and between the touch panel module and the pressure protection module. The pressure protection module includes a pair of tempered glass, and is configured to withstand strong water pressure by configuring a pressure regulating gas to be injected between the pair of tempered glass. The weight can be reduced by excluding thick tempered glass to prepare for high pressure, and it is possible to prevent fogging inside due to low water temperature due to the external environment and the role of an intermediate buffer layer inside the body.

Based on this, it is possible to conveniently and freely input the contents of the person's intention during underwater leisure activities and to facilitate communication with the other party.

### **4. Experimental Result**

### *4.1. Test Conditions*

To verify the performance in terms of drawing, we defined underwater test conditions. Because it is not possible to measure the data of the same underwater environment due to the flow of water, it is difficult to compare the performance of DiverPAD with those of conventional dive computers.

After two divers wear both DiverPAD and conventional diver computer on their wrist, they checked various underwater information at each of five stages in a range of 0 m to 50 m of underwater and this experiment was repeated four times as shown in Figure 10. We confirmed that there is no difference in the measurement between the two dive devices as demonstrated in Figure 11.

**Figure 10.** Test environment of DiverPAD.


**Figure 11.** Test conditions of DiverPAD.

In addition to data sensing, drawing and writing functions of DiverPAD were confirmed in the water as shown in Figure 12. Although the performance of the device could vary depending on the water pressure, inputs for drawing and writing were successfully processed in a range of 0 m to 50 m of underwater.


**Figure 12.** Drawing tests on DiverPAD.
