January 2005, Issue 174

Microcontroller-Based Nitrox Analyer

OPERATION

The first time the Nitrox analyzer powers up, it enters Sensor Calibration mode, which you can also achieve by holding down the button for several seconds at power-up. Calibration mode allows the AT90S4433 to compensate for deviations from the ideal of the oxygen sensor’s output characteristic.

To perform the calibration, select your approximate altitude above sea level and then expose the sensor to the air. The unit will take an initial reading, and then prompt you to expose the sensor to a known sample of Nitrox. After the measurement, you can specify the actual percentage of oxygen in the reference sample. The unit then computes a slope correction calibration factor to be applied to all future measurements and then stores it in internal EEPROM. The correction factor essentially converts the nonideal sensor characteristic to a reference ideal characteristic on which the AT90S4433 bases its measurements. The sensor slope calibration should be performed any time a different R-17D O2 sensor is used with the unit.

When the analyzer powers up normally, it first prompts you to select the approximate altitude in feet above sea level. This allows it to approximate the ambient atmospheric pressure, which is required to convert the oxygen sensor’s partial pressure measurement to the percentage of oxygen:

Next, you’re prompted to expose the sensor to air, and then the unit takes a measurement. Using this information, an additive offset compensation factor is computed to eliminate sensor and amplifier offset error. At this point, the device displays the percentage oxygen measured by the sensor and updates it every second.

To measure the percentage of oxygen from a Nitrox tank, place the oxygen sensor in the sensor mount, hold the mount to the tank valve, and open the valve just enough to allow a bit of Nitrox to flow through the assembly. The LCD should begin to indicate an increase in the percentage of oxygen.

After the reading stabilizes on a value for about 10 s, the unit displays the calculated percentage of oxygen in the tank. At this point, if you press and release the push button, the analyzer will calculate and display the maximum dive depth at which the partial pressure of oxygen being breathed reaches 1.4 atmospheres, which is otherwise known as the maximum operating depth (MOD). After a few seconds, the LCD will begin displaying the percentage of oxygen currently detected by the sensor. If you don’t press a button for 5 min., the unit automatically shuts down to conserve battery power. To shut down manually, press and hold the button for several seconds.