I've attached the MT92 tables, from DiveTables.eu.
Here is a comparison (for air) of the NDLs for those tables, and for several others.
Depth (ft/m) | MT92- Table 3 | US Navy
(old) | US Navy
(new) | PADI RDP
(DSAT) | DCIEM
(1997) | IANTD
(2004) | TDI/Buhlm
(2005) |
40/12 | 165 mins | 200 mins | 163 mins | 140 mins | 90 mins | 125 mins | |
50/15 | 80 | 100 | 92 | 80 | 70 | 75 | |
60/18 | 50 | 60 | 63 | 55 | 50 | 51 | 44 |
70/21 | 35 | 50 | 48 | 40 | 35 | 35 | 28 |
80/24 | 25 | 40 | 39 | 30 | 25 | 25 | 20 |
90/27 | 20 | 30 | 33 | 25 | 20 | 20 | 18 |
100/30 | 15 | 25 | 25 | 20 | 15 | 17 | 16 |
110/33 | 12 | 20 | 20 | 16 | 12 | 14 | 14 |
120/36 | 10 | 15 | 15 | 13 | 10 | 12 | 12 |
130/39 | 8 | 10 | 12 | 10 | 8 | 10 | 10 |
140/42 | 7 | 10 | 10 | 8 | | 9 | 7 |
The USN tables have the longest NDLs (i.e., most aggressive) of any of the tables, while MT92, DCIEM, and the old TDI/Buhlmann tables vie for the shortest NDLs (i.e., most conservative).The RDP and the old IANTD are middle-of-the-road.
MY92 has a Table 7 used for an equivalent air depth, so that Nitrox of various mixtures can be used with the air deco tables.
MT92 also has a Table 8, used for calculating an equivalent depth for a multi-level dive. It is, in fact, simply a table way of calculating a linear average depth. In general, this is an incorrect procedures, as can be shown theoretically and also by comparing results using it to results using correct procedures. It is possible that the differences in the two procedures (correct and incorrect) will be small, but that is not guaranteed and it is not clear how to determine when you will get a "good" answer and when you will get a faulty answer.
- The theoretical argument against using this average depth to determine decompression needs is that it ignores the decompression that takes place on a single level dive when one ascends to the surface. That is, if the dive level is (say) 30m, and you ascend at 10m/min, then you have 3 minutes of ascent time during which you are off-gassing; that off-gassing is included in the table calculation. By ignoring that ascent off-gassing -- which you do by using the average depth procedure -- you think you are in a certain nitrogen status but in fact you are not: you have more nitrogen than you think because you have not done the off-gassing.
- The practical example of the error in the average depth procedure can be illustrated by using the table RDP compared to the eRDPml, which is designed to allow multi-level diving based on the same decompression model as the RDP; they are the same model, just with results presented in different forms. Assume your dive is to 30m/100 ft for 18 minutes (NDL is 20 mins). Your Pressure Group is then P. In that Pressure Group, a second level at 60 ft/18m has residual nitrogen of 39 minutes and NDL of 55 mins, so you think you might have 55-39=16 more minutes you can spend at your new depth. But a multi-level calculation with the eRDPml says you only are allowed 13 mins at that second depth. (This is because you actually have more nitrogen in you than the square-table allows for, since you did not ascend to the surface.) Using the average depth procedure suggested by MT92, your "equivalent depth" is 25m/80 ft. At that depth, your NDL is 30 mins, but you will have spent more than that (either 31 or 34 minutes) by using the average depth procedure of MT92.
Calculating safe profiles for multilevel dives using depth average is not a good practice. You can easly go into deco (violate your NDLs) and not know it.
The only saving grace on using MT92 procedures is that the NDLs of the tables are quite conservative, so violating those NDLs by just a few minutes may not actually put you into deco status if you were using a more aggressive dive model. So, you might be lucky. Luck is no way to plan a dive.