Wireless Water Sensor

An Engineer at Dotstar worked on a project for Scotia Gas Networks to develop a wireless low power sensor with a battery life of several years.

A common problem with gas mains is ingress of water which causes several problems; water can block the pipe if it collects in low-lying areas of the pipe, water can partially block a pipe causing gas pressure variations and increased humidity in the gas can damage customers’ boilers.

Up until then, manual inspection of the gas mains at inspection points all over the network has been the only way to discover if there is water in the pipe and where it is. This is very expensive, inefficient and problems often take a while to find as each inspection point can only be checked every few months.

We were was tasked with developing an autonomous sensor to detect the presence of water in the sampling points and report any incursions of water into the gas network via the GSM mobile phone network. The system had to detect the presence of water as well as the pressure and humidity levels of the gas. The main constraints on the system was the need for a long battery life to greatly reduce the need for engineers to attend the inspection points, and keeping the system very small so it could be inserted in the standard inspection points without modification.

The first part of the project was to design and manufacture a custom PCB for the sensor. In order to meet both the space requirements and battery life, component choice was very important. To minimise power consumption as much of the system as possible needed to be completely powered down when in standby and only powered up for the shortest period possible. Anything else had to be carefully chosen to have minimum power consumption and designed to make sure there was minimal current leakage.

To allow communication over GSM an off the shelf module was used, as this was by far the most power hungry part of the system it was necessary to make sure power could be completely disabled for this part of the system. A very low power ARM processor was used to control the system, while the system was in standby between measurements the processor could power down the rest of the system and enter a low power standby mode until the next measurement was required.

Once the PCB was designed and manufactured the next part of the project was to develop the firmware and software for the system. Firmware was written for the microcontroller on the PCB to power up the sensors intermittently, take readings and then communicate with the GSM module to send these measurements to a central server. The main challenges with this firmware was optimising the code for the times when the system was in low power standby mode, it was important to make sure as many peripherals as possible in the processor were powered down and making sure there was no leakage through any of the processor’s pins. It was also important to make sure that the time taken to take the measurements was minimised to reduce amount of time other parts of the system needed to be powered up.

As well as firmware for the sensor, server-side software was also developed. A database backed web application was created to store the measurements from all the sensors. This provided an API which the sensors could access over the GSM network to upload measurements, this API also allowed other systems to access the collated measurements so the sensors could easily be integrated with Scotia Gas Network’s other systems.

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