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THE IST GP4000+ COMPUTER
To Set the GP4000+ For EAN (Nitrox)
CNS TOXICITY: PADI recommends that the partial pressure of the oxygen in the air be limited to 1.4 atmospheres or less. NOAA recommends that their divers not exceed 1.6 atm. If a diver exceeds their CNS toxicity level there is a greater chance that the high level of oxygen may cause mild to severe symptoms of oxygen poisoning, the severest being underwater convulsions. They are usually fatal unless someone else comes to the rescue!
Air has 21% oxygen so a person at sea level has 0.21 atm of O2 entering their body. The rest is 0.78 atm.of nitrogen or 78% of the air. If that person were to descend to 33' seawater they would be at double pressure which means the O2 partial pressure would now be 0.42 atm.
If a diver were breathing a 50-50 mixture of oxygen and nitrogen at the surface the partial pressure of the O2 would be 0.50 atm. At 33' that same gas mixture would have a partial pressure of 1.0 atm O2. So you can see that to avoid going over the PADI and Cochran's1.4 (or NOAA 1.6) oxygen pp limits a diver has to know what the percentage of oxygen is in the breathing gas as well as the depth.
Prior to diving the diver should be sure the FO2 is set to 21 if air is the breathing gas. If the diver is breathing Enriched Air Nitrox (EAN) the computer must be set to the oxygen percentage determined after analyzing the gas. The EMC-14 will take that setting along with the depth and constantly report what the CNS toxicity level is. The CNS level is displayed as a percent. If the CNS level reaches 100% it means the diver has encountered a CNS toxicity level greater than 1.4 atm of O2. That is dangerous and the dive should be safely terminated.
In the above underwater screen it is assumed the diver had set the CNS limit to 60% of the allowable 1.4 atm of O2. The diver exceeded the 60% and is now at 62% CNS exposure. The CNS Oxygen Toxicity reading is flashing. At the top the Warning Legend is flashing. Note that the diver is breathing air (F21). Because the depth is so great (115') the PO2 in the air is 0.94 atm. That is about 60% of the 1.6 atm maximum.
OXYGEN TOLERANCE UNITS (OTU): A person, diver or not, is limited to how much oxygen they should be exposed to over a period of time. On the earth we have adapted to a 21% of our breathing gas exposure, so breathing air for long periods of time makes us happy. If the percentage of oxygen is elevated, such as in a hospital cardiac treatment center or underwater at depth, the exposure increases. PADI has a 24-hour period limit of oxygen exposure for humans and that limit must be kept under 100%. If it is not, Pulmonary Oxygen Toxicity may occur leading lung irritation, coughing, burning in the throat and chest, and shortness of breath.
From the PADI DSAT Table, which shows the percentage of oxygen received from various PO2 levels breathed over time, if a person breathed 0.6 atm of oxygen for 720 minutes they would have received 100% of their OTU (maximum allowed in a 24-hour period). That would be similar to a diver breathing air for 12 hours at 66' (720 minutes at 0.21 PO2 x 3 atm). From that same table, breathing 1.0 atm of O2 for 300 minutes would also be 100% exposure. One last example: Breathing 1.5 atm of O2 for 60 minutes would result in 50% of the maximum 24 hour exposure. So, the PADI DSAT Table is a series of calculations of the amount of PO2 a diver is breathing, for how long (in minutes), all within a "moving" 24-hour period.
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OVERVIEW OF THE NITROX PRO AND PILOT/CLASSIC COMPUTERS
FEATURES:
The Nitrox Pro (those with the * are for the Classic) provides the following information to help you control your dive:
It gives the following warnings:
It gives the diver choices:
Additional features:
LCD DISPLAYS
The visual displays on the LCD will change according to the information being delivered. To protect the LCD from scratching you should apply one of the protective sheets that come with each unit, to the LCD surface. It is a removable sticker that can later be changed to a new one, once it comes worn out. The screen examples in the manual are shown in either metric or imperial units, but you should use the display with which you are most comfortable. The diagrams below show how the screen will appear during different aspects of the dive.
DIRECTIONS FOR USE
To activate the computer wet two fingers and touch both NEXT buttons at the same time. At start-up all segments light for 5-10 sec showing unit self-test and adjusting to ambient pressure. If you forget to activate the unit prior to entering the water it will activate itself when it gets wet. However, the computer will assume you are at sea level if it turns on this way. It is wise to activate the computer in the atmosphere if diving above sea level so the altitude will be read by it.
BATTERY TEST:
After the self-test procedure, the computer will perform a battery test. During this test the LCD will display numbers 9 (to 0). If the battery is sufficiently charged the LCD will show all segments and then begin working.
If the battery is not giving full power the computer will run a function that will try to revive the battery. This will take max 10 minutes per cycle, and during this time the LCD display will count down from 9 to 0, if needed. If the battery revival is successful, the LCD screen will show all segments again then begin working. If it is not successful the computer will turn off, and a new reviving cycle is needed. EVEN A NEW BATTERY, WHEN TAKEN IN USE, MIGHT NEED SEVERAL CYCLES BEFORE BEING FULLY OPERABLE, IF IT HAS BEEN STORED FOR LONG AND/OR EXPOSED TO EXCESSIVE HEAT. AFTER THE REVIVAL CYCLES, THE BATTERY IS AS GOOD AS NEW! If, after several revival cycles, the unit still does not turn on, the battery should then be replaced. However, if you do not have a spare battery for immediate replacement, the computer can be turned on and used but the beeps and LCD light will not work. To turn the computer back on in this case connect the 3 function buttons simultaneously and wait for the 9 on the LCD to change to 8. The battery should then be replaced before the next dive.
FUNCTION BUTTONS - The Nitrox Pro has 4 function buttons that allow you to operate the computer:
Go Diving: After activation and the Self Test and Battery Test, the computer goes into Surface Mode. You are now ready to enter the water without doing anything further. However, if the computer is turned on, but not used for 60 minutes it will turn off automatically.
While Underwater:
There is only 1 screen that shows underwater. On that screen the diver simply reads the Dive Time (on the top), the time left until the diver enters the decompression mode (in the middle in the brackets), and the maximum depth (lower left). In the upper right the temperature will be displayed.
On the Nitrox Pro there will be (on the right) a bar graph of the oxygen in the Central Nervous System (CNS), as well as the % of the same. Each bar, of 5, represents 20% of the allowable oxygen before oxygen poisoning may be encountered.
TAP SWITCH
The tap switch operates the LCD light and warning beeps. To turn the functions on and off, tap the computer in the upper left corner of the screen with a hard object or fingertip (avoid scratching the unit though) as described below.
After the Dive:
THE COMPUTER TURNS ITSELF OFF:
After a dive the computer will remain on for 15 minutes. It will then go into an energy efficient calculation mode. This is indicated with a clock icon on the LCD. The computer remains in this mode until the desaturation time is fully calculated down. Connecting NEXT+NEXT will bring the computer back to the Surface Mode.
There are 3 screens in the Surface Mode. The computer will automatically scroll between the 3. Each of the screens shows different data related to the dive.
In the Surface Mode:
These functions are best explained using the Computer Instruction Manual with the computer in hand.
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Here are copies of the pages in the manual for the Pilot and Classic computers:
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The pages from the manual for the Classic/Pilot Nitrox computer:
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