Question about computer algorithm

[dgangi]

I recently purchased a Sherwood Logic computer and read the manual from front to back. I am an engineer/mathematician and have a natural propensity to know the details of how things work, so I also read the technical specifications about the computer (the fine print in back).

The algorithm utilized by this computer is listed in the manual as:
- Modified Haldanean algorithm using maximum allowable nitrogen levels developed by Merrill Spencer
- 12 tissue compartments
- Repetitive dive database by DSAT/Rogers&Powell
- Altitude algorithm based on NOAA


When was initially shopping for a computer I noticed that just about every one is touted as using a "modified Haldanean" algorithm, so I figured this was the norm. I was curious as to what "modifications" were done to this Haldanean algorithm and if they were all accepted. And after reading this a little more, there is greater detail about the algorithm than I was expecting -- and I anticipate these details vary from computer to computer. Or maybe not.

So can anybody tell me exactly what this algorithm means? Do many other computers use this same algorithm? And is it considered a "safe and acceptable" algorithm?

Thx...Doug

 

[Dr Deco]

Hello Doug:

This is a good question, and it is something that we sometimes see in the “Ask Dr Deco” forum. I will tell you my version, at least, of what this all means.

- Modified Haldanean algorithm

1. There most likely are several ideas of what a modified Haldanian algorithm is. In the original model of John Scot Haldane, the halftimes were actual tissues with 5, 10, 20,40, and 75 minutes. These half times have been adjusted over the past several decades to account for contemporary diving practices. We now see half times for “compartments” that go up to about 500 or 600 minutes. These quite likely are the slow off gassing of a free gas phase ( = bubbles in the tissues).

2. In the original Haldane model, the pressure reductions (supersaturations) were 2 to 1 (if oxygen is counted) or about 1.58 to 1 if only nitrogen is counted for all “tissues.” It has been found that the fast compartments can “tolerate” ratios of more than 4 to 1, and only the slowest have very small supersaturation ratios.

These modifications are found in all tables and deco meters for the past several decades. As you stated, all meters claim to have “modified Haldanian” models. This is a correct statement by the table designers.

- Merrill Spencer

Most systems today use no-decompression limits (NDLs) as determined by Dr Merrill Spencer at the Institute of Applied Physiology and Medicine (IAPM) in Seattle, Washington. [Spencer MP. Decompression limits for compressed air determined by ultrasonically detected blood bubbles. J Appl Physiol 1976 Feb;40(2):229-35.] These were determined by measuring Doppler-detectable gas bubbles rather than using decompression sickness (“the bends”) as an end point. This degree of conservatism is safer for the diver.

- Twelve Compartments

As indicated above, the original tables of JS Haldane were calculated from five “tissue” halftimes.” Contemporary systems use more “compartments” extended beyond the 75 minutes.

- Repetitive dive database by DSAT/Rogers&Powell

This is the series of experiments that I performed while at IAPM for PADI and Diving Science and Technology (DSAT). These studies were published and can be obtained from PADI [Hamilton, RW, RE Rodgers, MR Powell, and RD Vann. Development And Validation Of No-stop Decompression Procedures For Recreational Diving. Diving Science and Technology. (pp. 78 + appendix). February 28, (1994).]

These were multilevel dives and multiday diving. This is laboratory-grade data obtained from recreational divers in the laboratory and open-water trials in Puget Sound (Washington State).

- Altitude algorithm based on NOAA

I believe that this refers to the calculation of atmospheric pressure at altitude by what is known as the “barometric equation.”

Dr Deco :doctor:

 

[dgangi]

Hi Dr Deco -

Wow. I can't believe I got an answer from one of the scientists involved in the theory behind the algorithms my computer uses. That's great! Too bad I don't know enough about physiology as some of your post went a little over my head. Now if you could only relate that to computer science I'd be OK...

So, in a nutshell, it sounds like the algorithm in my computer is quite conservative. If that is the case, that's fine by me.

Thx...Doug