November 2005, Issue 184

Water Pressure Sensor
ZigBee-Based NozzleMon Aids Firefighters

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HARDWARE SPECIFICS

The system’s design is simple (see Figure 1). Let’s first examine the nozzle unit (see Photo 1). The MPXH6400A sensor reads the absolute pressure up to 58 psi of air (not water). Thus, the external connection port can’t be directly attached to the fire hose stream, which is a stream of water at up to 200 psi.

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Figure 1—With all the communication network interface complexity managed on the EVB and SARD boards, the system design is simple. The PPU is easy to use. The NU can easily manage other sensors to provide even more information to the pump operator.

 

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Photo 1—The three pieces of the nozzle unit include the SARD board, the MPXH6400A pressure sensor on the right, and the battery. The external connection port is attached to the pressure sensor with heat-shrink tubing to make a sturdy airtight connection.

The tubing leading from the MPXH6400A sensor to the external port was cut to fit and then heated and shrunk in place, forming an airtight, stable connection to the external port. All of the posts supporting the boards and battery inside the box were epoxied in place rather than screwed through the box. With the lid sealed in place, the external port is the only entrance to the box. This isolates the nozzle unit from the firefighting environment.

Output from the MPXH6400A sensor is connected to a free A/D port (AD3) on the MC9S08GT60. A 5-V power source supplies it, so the output voltage can exceed the input limit (VDD = 3 V) on the processor. But the input pressure is limited to less than half the device range, so a voltage divider isn’t needed to protect the A/D input.

Plumbing was needed to step the pressure down and isolate the water stream from the pressure sensor. I tapped into a hose coupling and pointed a brass elbow upstream to give access to the water stream. From there the stream enters a brass nipple where a plunger assembly keeps the water on one side while being supported by a spring on the other. The plunger compresses the air in the spring chamber while the spring counters the pressure, providing the reduction to the working pressure range.

This was built with off-the-shelf parts from the local hardware store, so precise calculations involving the spring constants, air volumes, and other key parameters weren’t available in advance. Instead, a gauge I placed on the high-pressure input to the chamber enabled me to create a look-up table to perform the mapping from output readings to the actual nozzle pressure. Photo 2 shows the completed assembly.

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Photo 2—The nozzle unit is the black box on the right. The brass nipple contains the plunger assembly that reduces the pressure and isolates the water side from the pressure sensor. The gauge gathers data for the calibration look-up table.

Photo 3 shows the inside of the pump panel unit. The off-the-shelf LCD assembly has a 4-bit data path and 3-bit control path from the processor. In addition to those seven lines, the LED and push button switch each take one I/O bit from the processor, using nine of the 10 bits brought out to J107 on the evaluation board. All 10 lines from J107 as well as all 14 bits from the LCD assembly are brought via ribbon cables to a connectorized cross-connect board where connecting wires and resistors are placed. Power and ground lines are also distributed on the cross-connect board. Power is derived from the Freescale breakaway power board supplied with a 9-V battery.

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Photo 3—Starting at the upper left and working clockwise, the components include an On/Off switch, the cross-connect board, the battery (all three on the top), the evaluation board on the bottom, the red push button, and the red LED on the left. The LCD assembly is in the middle.

 

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Photo 4—The front of the pump panel unit includes a display, a red LED, a power switch, and a button for setting the pressure.

Photo 4 shows the front of the pump panel unit. Working pump operators typically wear gloves and aren’t inclined to deal with small controls, so I chose the front panel interface for its simplicity and ease of operation. The power switch is a large toggle. The large pressure set button is easy to push. The bright red LED is visible even in daylight.