Salinity and water density

This post is a copy from an answer in the Subsurface support forum but I post it here as well as this point comes up over and over. It is about the setting of some dive computer where you can set the density of water or whether you are diving in sea or fresh water. There is a similar setting in Subsurface, but the default setting in the preferences is to hide it. For a good reason.

Kevin Jones from Vancouver, Canada, CC BY 2.0 https://creativecommons.org/licenses/by/2.0, via Wikimedia Commons

Let me mention once more why this option is turned off by default: There is a good chance that by fiddling with the density you make changes whose effects are not what you intend: The density is the relevant constant that controls the conversion between depth and ambient pressure. That you probably intend. But what is easy to forget: Your dive computer displays depth and reports it in the log that is transferred to Subsurface. But it does not really measure depth, rather it measures ambient pressure and it uses the density to convert that to depth.

It does this conversion because we humans are used to think in terms of depth which is much like a length and we have an idea how much 10m is, much more so than 2bar. 

For most things diving, however, depth does not matter at all, ambient pressure does. This includes gas consumption (as your regulator regulates according to ambient pressure, not to depth) and all deco calculations (because also there partial pressures in their relation to ambient pressure dictate what is happening in your body). Depth only matters when you think about breaking a new world record or worry if the mast of the wreck sticks out of the water or if you buoyo line is long enough. For gas usage estimates and deco calculations, it would be much more honest if your computer displayed ambient pressure in bar rather than depth in m. Only that for the average Joe that would be hard to digest and we have to live with the fact that this conversion is done back and forth by the computer and by Subsurface all the time.

But what really makes zero sense is to use different values of density when translating back and forth. Also your dive computer does not measure the density, this is a setting that you have to make manually. Yes, you could change that setting on your dive computer every time you switch between sea and fresh water to get a more accurate depth display (which as I explained above most likely does not matter at all). But my guess would be you forget to change that setting for at least half of your dives. So my very strong recommendation would be to set it on your dive computer once and for all to any value (maybe according to where you do most of your diving) and set Subsurface accordingly and never ever change it again. The result can be that some of your depth readings are slightly off but at least  your gas and deco calculations will be consistent.

6 thoughts on “Salinity and water density”

  1. OSTC uses percents, Others use 2 or 3 values, Subsurface uses the precise values. Suunto uses Brackish water and doesn’t allow to configure. In case the computer allows this option, would you recommend as uniform value the higher salinity, middle value or which one?

    1. To get really into the weeds, I know at of several dive computers measure pressure, log pressure, and convert the pressure to depth for display purposes. They even transmit pressure values when they send their logs out. When importing those logs into Subsurface through libdivecomputer, the pressure values get converted to depth values.

      Maybe some work could be done to let a dive computer return the raw pressure in the sample values instead of the depth, and then Subsurface could use that value instead of the depth value and the job of converting the pressure to depth would be on Subsurface (using the surface pressure and salinity information) so a user could adjust those values after the fact and it would change the way it displays.

      (I say this having not searched to see if it’s been thought of / discussed before. Maybe it’s been discussed at length before)

  2. A small addition: OSTC (and probably other dive computers) also calculate in fresh water (1kg/l density) with 9.81m depth/bar. This results from the local factor, which is assumed to be 9.81m/s^2. However, all these conversions only have an effect on the displayed depth.

    1. Displayed and logged. When the data is transferred to Subsurface via Bluetooth you also transmit depths, not ambient pressures (which would be the raw data). So, also in the profile data, the density is assumed to be one specific value.

      1. Yes, sorry for that… And it also changed multiple times between “pressure logged” and “depth logged” over the last 10 years or so ¯|_(ツ)_|¯

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