## 1 Introduction
### A Review of Existing Wireless LPG Level Sensors for Domestic Stationary Tanks  
All LPG stationary tanks are fitted with a mechanical gauging system (sometimes called a dial gauge) to measure the percentage of LPG in liquid state inside the tank (see Figure 1a). The mechanical float inside the tank rests on the liquid gas and transmits a rotary motion to the stem by means of gears. The float and the stem vary in size (see Figure 1b), as they depend on the capacity (in liters) of the stationary tank [7]. The stem transmits the rotary motion to a diametrically magnetized magnet placed at the end of it and where the external dial gauge is located (see Figure 1c). The magnetic field of the magnet moves the dial needle, indicating the percentage of the LPG level in the tank. Float gauges have numerous moving parts that are subject to wear and can become ineffective at any part of the assembly. The mechanical measurement system only provides an approximate tank percentage and cannot be considered 100% reliable [7]. All proposed electronic solutions for measuring the percentage of LPG level in domestic stationary tanks are based on detecting the direction of the magnetic field produced by the diametric magnet located under the needle dial. This is mainly due to: (1) taking advantage of the mechanical measurement system that all stationary tanks have, (2) no invasive (internal) measurement system being added, nor modifying the existing measurement system, (3) being as accurate as the mechanical measurement system, and (4) when it is small enough, being easy to install under the dial. The different solutions available to estimate the percentage of LPG level in domestic stationary tanks are studied below. Then, a qualitative comparison of them vs. the developed wireless sensor is shown. * Levelgas(r) [8]: This device measures the level of LPG in the stationary tank and displays the data through a mobile application (app). It sends a notification when it detects a low gas level and has the ability to display the battery charge level. It uses Sigfox connectivity, so it only has coverage in major cities. After a trial period, a subscription to the app must be paid for. The sensor displays the data only to the tank user and cannot be shared with the gas refilling plant. It is not specified (N/S) if it uses any cybersecurity strategy. The device performs one measurement per day and  
Figure 1: Mechanical LPG level measurement system in stationary tank: (**a**) the mechanical tank gauging system; (**b**) float gauge assembly; and (**c**) removable, magnetically driven needle dial. uses three AAA-size alkaline batteries (Zn-MnO\({}_{2}\)) with a lifespan of up to one year. Manufacturer: Edison Effect, S.A. of C.V., San Luis Potosi, Mexico. Price: 77.50 USD. * El gaaas(r) [9]: Wireless gas meter for stationary tank. It uses Sigfox wireless connection, so it only has coverage in major cities. It uses the same app as the Levelgas device, so after a trial period, you have to pay a monthly fee to get the app services. The device displays the data only to the tank owner and cannot be shared with the gas refilling plant. It is not mentioned if it uses any cybersecurity strategy. Compatible with stationary tanks from 100 to 3000 L. The sensor performs one measurement per day and uses three AAA-size alkaline batteries (Zn-MnO\({}_{2}\)) with a life span of up to one year. Manufacturer: Logistica Aplicada Tugas, S.A. de C.V., Mexico City, Mexico. Price: 78.00 USD. * iSentinel(r) [10]: With this wireless device, you can measure the level of LPG in your stationary tank from your smartphone. In addition, you can also know how many liters of gas you have refilled. It can look more than one tank in a single app. It uses a Wi-Fi connection and the app is free. The device displays the data only to the tank user and cannot be shared with the gas refilling plant. It is not specified (N/S) if it uses any cybersecurity strategy.