Closed Circuit Draeger FGGIII

The electronics module was made using a Parallax Basic Stamp 2. Input is handled by a four way operational amplifier and four analogue to digital converters. Outputs are via a tri-colour LED, an LCD panel, and a transistor driver for the solenoid. There are also three push button switches for the setup menus, and a four way rotary switch for operational control.

The main electronics board. The electronics has two modes, setup mode, and dive mode. Setup mode can only be accessed by holding the setup button on the main board during power up. In setup mode you can change the value of the two (high and low) setpoints, you can change the time interval between O2 addition decisions, and you can vary the period that the solenoid fires for. After running through these options, you can opt to either enter dive mode, or go back to the start of the setup sequence. When in dive mode, you can perform a manual calibration using the three trimpots on the electronics board. In dive mode the electronics monitor the output of the three oxygen sensors and decide which two are closest together. It then takes the average of these two and compares the result with the preprogrammed setpoint, which is either your high setpoint or low setpoint depending on the position of the four way switch on the handset. Setpoints can be set anywhere between 0.20 and 2.00 atm PPO2 (yeah I know it's high, but I don't have to use it that high!!) The electronics run from twin batteries, 9v at the monent, but soon to be larger Ah rating. The battery voltage is sensed periodically and displayed on the LCD. The white connector on the left is the RS232 connector for programming from a PC.

The handset showing the Hi PPO2 setpoint in Setup Mode. The value of 130 shown indicates a PPO2 setpoint of 1.30 Ata.

The knob on the right side is a four position switch which selects,

The display in Dive Mode.

The three readings across the top line are the outputs of the three sensors. Beneath this is the calculated average of the closest two sensors, and the current setpoint. The high readings shown are due to the quick calibration for the photographs.