SCR rebreather information.

[Freeflow]

The following tables, spreadsheets and articles are designed to assist in the matching of gas cylinder mixes and flow rates in CMF type SCRs. The tables were specifically designed for the Drager Dolphin, but by adjusting the flow rates many different models can be covered.

To use any of the tables or spreadsheets described below you MUST use a device capable of monitoring the pO2 or loop fO2 of the gas you are breathing. You must also be aware that changing from the manufacturers recommended gasses and flow rates you are increasing the risk of a hypoxic or hyperoxic breathing mix. This is especially relevant when diving with a cylinder mix that is unsafe to breathe at the depth you are planning if a loop flush was required.

Use of any of the information, tables and/or spreadsheets supplied is at the users own risk.

A1 “SCR rebreather matching calculations” gives the background theory behind the spreadsheet S1, and shows the formulae used.

Semi Closed Rebreather diving gas calculations.

A2 “Zorg” gives an insight into how the scrubber works through the eyes of alien scientists who kidnap humans. This is not written by me, and I can’t find the original authors name, if you know it let me know so I can give due credit.

Zorg-scrubbers explained.

T1 “Loop fO2 from vO2 and cyl fO2” are four tables were developed with Steve Sprague that can be used to find the loop percentage of oxygen from a known vO2 and supply fO2. Steve’s tables were adapted for my own range of vO2s, but the original layout is all his. The cylinder O2 % is read along the top, and with the vO2 from the side the resulting loop mix can be found.

Loop % O2 from depth and pO2

T2 “Loop fO2 from pO2 and depth” Probably best used with a gas switching computer so a change can be made to 21% if the loop fO2 drops below the value you had planned.

Tables for loop fO2 with known tank fO2 and vO2

S1 “RB calcs” This is a four section spreadsheet that carries out most of the calculations you will need for gas and jet mixing. The figures in the orange boxes [default value zero] are the only inputs needed. All the results appear in the coloured bands that relate to the jet sizes or the light purple boxes.

Section 1 is used to work out a loop fO2 from a known cylinder fO2 and standard or custom vO2. The standard values are 1.0, 1.5 and 2.0 L/min and the flow rates given are the design for that jet, the default user adjustable vO2 is 1.25. If you have a non-standard unit, a non Drager unit, or know your exact flow rate measurements then you can replace the flow figures where they appear in the coloured bands with your known values. The first table allows comparisons with the various jets and vO2 readings to ensure that you do not get a hypoxic mix. By showing all the jet sizes the ideal one can be chosen.

Just below the loop fO2 table is a calculator for maximum and minimum operating depths. Simply insert the maximum pO2 you wish and your MOD will be displayed. Note that the minimum operating depth is shown only with the custom vO2 entered at the top of section one.

The last table in Section one is an EAD calculator based on the figures already inputted in the tables above, and only the planned depth needs to be entered.

Section 2 is simply the reverse of section 1. Input the desired loop mix and you can check the options for cylinder fO2 and flow rates.

Section 3 is a loop fO2 and vO2 calculator. With the known flow rate and cylinder fO2 the user can note depth and pO2 to get a value for the loop fO2 and the vO2 [this section has been replaced by S2 “RB operating calcs” when multiple readings are taken to save too much repeated key tapping].

Section 4 is used with a known vO2 to calculate the range of mixes that can be used for a given dive. By inputting the maximum acceptable pO2, dive depth, maximum and minimum vO2 values and the flow rate, various values are calculated. The loop and cylinder fO2’s that are available are first, followed by two contingency plans. The first works out the loop fO2 if the cylinder fO2 is set at the lowest end of the ranges [ie for minimum calculated vO2]-a loop fO2 of less than 0.21 [21%] MUST BE AVOIDED to prevent the risk of hypoxia and compromising the calculated nitrogen levels in the breathing loop on a back up plan of 21%.

S2 “RB operating calcs” is simply a pO2 and depth calculator for working out loop fO2 and users vO2. The orange boxes are used to enter the information and the blue boxes give the results. Cylinder fO2 and flow rate need to be entered at the top of the page. For more than one reading the average loop fO2 and diver vO2 are given.

S3 “vO2 from depth and pO2” is used for a rough vO2 figure when the cylinder fO2 and flow rate are used in conjunction with the depth and pO2 readings. This is intended to be printed out and laminated pre dive and then referred to for an instant vO2 figure. It’s not as accurate as the calculations contained in spreadsheets S1 and S2