Using the                              
Nitrox VPM Program
in Visual Basic for Applications

Copyright 1999 by Eric Maiken

The program should unzip with Example 2 displayed.

To calculate an ascent schedule, begin in cell C4, and type in a depth.Then, enter the corresponding run-time for the stage in cell D4 . Enter any successive stages in the next lower rows (C5 and D5, then C6 and D6, etc.). The program will read the "Depth" and "Run-Time" columns in this sequence from top to bottom until it encounters a blank row. At his point, it calculates the ascent schedule for the sequence of stages above the blank row.

Each complete entry is assigned a stage number in column B after the program reads it in. An incomplete or incorrect entry should result in an error message with instructions for correcting the problem. Examples of incomplete entries are: a stage depth but no runtime or stage run-time but no depth. Examples of incorrectly formatted entries include entering a deeper-than-previous stage depth in a lower row or entering a stage run-time that is equal to or less than a previous stage's run time. Note that unlike the QuickBasic version, an error message is generated if dive stages are not in order of deepest first. Try testing some pathological entries to see what's going on.

After you complete the entry of the stages, hit the Run button and an ascent schedule should then be output.

Everything is on one sheet, (except for the help form) with no controls on additional forms, etc. A .jpg of the sheet VPM.xls is shown with yellow flags calling out notes that describe the controls and display. There are no ToolTips or help for the controls. Just a little bit of input checking and error control.

Example 3 A dive to 100 feet is made for 40 min on air. Try two different ascent schedules; Ascent 1 is on air; Ascent 2 is on air up to the 20 foot stop, where the switch to oxygen is made. In both cases the last stop is not pulled at 20 feet.

Controls and Display

Below the picture of the VPM.xls sheet, notes on each of the seven flagged items are called out. The gray color sections are areas for input. The blue color sections are for program output. All objects are enabled and unprotected. Examples of input are also given.

(1) INPUT

For each multilevel dive stage, enter the Depth and Run-time in the corresponding gray colored columns. The Stage column is for program output. Each complete depth/run-time entry is automatically assigned a stage number.

(2) Depth and Run-time Columns

The input format for each dive stage is in feet of fresh water and run-time in minutes. Just enter numbers --the units are implied. Run-time is the time since starting the decent. If you think in terms of what your bottom timer would read at the end of each stage, then that's your run-time.

32 feet of saltwater = 33 feet of fresh water. 3.3 feet of anything = 1 meter. For example, a dive to 50 m should be input as 165 feet. See the Making Your Own Changes section below for more notes on conversions.

For multilevel dives, run the program as you add each new dive stage to verify that the first deco stop is shallower than the next shallowest dive stage. You'll need to run the program a couple of times for multilevel dives.

(3) Buttons

The buttons can be activated by either a mouse click or from the keyboard with alt+Underlined character.

Run

After entering depth(s) and run-time(s), Run to calculate the decompression profile.

Clear

All input and output is cleared.

Help

A form summarizing the controls is displayed.

(4) Check Boxes

A mouse click either activates or deactivates each item. Activation (denoted by a check) prior to running a calculation results in the following:

[v] Plot Dive Profile

The stair-step formatted ascent profile that is hidden in the black border of columns H and I is plotted.

[v] Display 1st Iteration

Outputs the first un-converged ascent schedule.

As described in the VPM.xls Algorithm Details section below, the VPM algorithm is different from conventional decompression calculations because the ascent schedule is calculated iteratively. A sequence of increasingly liberal ascent schedules are produced before the final, converged schedule is output. Compare the first and last schedules for some short dives (big difference) and also for long dives (not much difference).

[v] Pull last stop at 20 ft

The last in-water decompression stop is at 20 ft. Note that the ascent schedule outputs two sequential 20 ft stops.

(5) Scroll Bars

Breathing gas settings for bottom mix and two deco mixes are changed by sliding the scroll button. A mouse click on the button may be required to display the proper %O₂ in the text box. The program does calculations using the incremental settings of the scroll bars regardless of the value displayed.

Bottom Nitrox Mix

Set the %O₂ of the bottom mix. You track your END, and O₂ tox. This %O₂ is used for all calculations up to the depth where the Deep Nitrox Mix has a ppO2 = 1.6

Deep Nitrox Mix

Sets %O₂ of all stops between the MOD of the Deep Nitrox Deco Mix and 20 feet. The MOD is automatically calculated in the code.

Shallow Nitrox Deco Mix

Sets %O₂ of 20 and 10 foot deco mix

(6) OUTPUT

The ascent schedule is output in a table in a depth/run-time format.

Differences of one minute in profiles are not significant and result from how the code rounds numbers. As an example, an odd 2 minute stop will occasionally show up among 1 min stops. Also short deep stops often skip a 10 foot increment for the same reason.

(7) Chart

A stair-step formatted plot of the ascent schedule is plotted. Note that the chart plots data in columns H and I.

The chart is built entirely from excel menus. It is not a VBA object.

I've tried to keep the program as straight-forward as possible with the code for the sheet "VPM I-O" serving as a complete module along with minimal input and output formatting and control. Nonetheless, lots of lines are devoted solely to I/O and have nothing to do with the real algorithm. The code is clearest in the QuickBasic form, so download VPM.txt to figure out how it works.