I see where you are going wrong with this. Here is my original equation: MOD = ((ppO2 / fO2) - 1) x DPA. DPA is depth per atmosphere which is derived from the salinity factor you mention. The ppO2 value is not a measured or calculated value, it is an entered value by the user. Typically, for recreational divers its value is 1.4 atm. This value is entered by the diver before the dive. The fraction of oxygen, fO2, is also an entered value as well as the salinity factor. Before the dive begins the MOD is already known. This value gets stored and is compared to the depth (d) which is calculated from the actual measured pressure. If the pressure (p) that is measured from the sensor is in atmospheres then the formula becomes: d = p / DPA. An alarm is given if d > MOD.
The MOD formula is what I use in my spreadsheet and probably what other dive programs use as well. For dive computers there is no need to calculate depth. The measured pressure can be used to calculate a ppO2 which is then compared to the value entered by the user (for recreational divers it is typically 1.4 atm) and if it exceeds this value an alarm is generated.
Nope.
PPO2 is absolutely calculated, continually, during the dive. How else would your alarm work?
PPO2 = FiO2 (entered by the diver) x ATA (measured by the DC).
What you are thinking of is the PPO2 alert value, that is what the diver enters into the DC.
If you use an equation that involves DPA, that only has meaning if you know your depth. So unless you include a salinity factor, you can't know your depth with a DC only.
I mean, we are basically saying similar things, but the reason I brought this up (see upthread) is that an error in entering the salinity factor won't cause you to mistakenly descend to an unsafe PPO2. Because the only purpose of the salinity factor is to correct the depth display.
Yes, you can also use your DPA formula, but that requires adjusting the DPA based on what kind of water you are diving in to get your MOD, which will give you a MOD in feet or meters (for that specific salinity). And then using the SAME SALINITY FACTOR to convert your DC's measured ATA to a depth in feet or meters, so that you can compare the two numbers and see if an alert sounds. That's what I mean by putting it in and taking it out.
OR, you can just take the ATA value that your DC measures directly, multiply it by the FiO2 that you know from analyzing your gas, and if that number is more than 1.4, you ascend. And that simple process does not require a salinity factor.