OTU calculation equation

41 views
Skip to first unread message

Andrew Lovering

unread,
Jan 7, 2026, 3:16:51 PM (8 days ago) Jan 7
to Subsurface Divelog
How does Subsurface calculate the OTUs? Using Subsurface data output, If i do the calculations using the UPTD equation (which uses an exponent of -0.5) i get a smaller number than the OTUs calculated by Subsurface. However, if i use the exponent -0.83 as suggested by others (see below) then i get a number closer to what Subsurface calculates based on actual dive data.

I'm not interested in arguing about the merits of each approach here i just need to know what Subsurface uses for the calculations - please and thank you.

UPTD equations source here:

Source of -0.83 exponent

Chris Kuethe

unread,
Jan 7, 2026, 9:34:26 PM (8 days ago) Jan 7
to subsurfac...@googlegroups.com
On Wed, Jan 7, 2026 at 12:16 PM Andrew Lovering <yakin...@gmail.com> wrote:
How does Subsurface calculate the OTUs? Using Subsurface data output, If i do the calculations using the UPTD equation (which uses an exponent of -0.5) i get a smaller number than the OTUs calculated by Subsurface. However, if i use the exponent -0.83 as suggested by others (see below) then i get a number closer to what Subsurface calculates based on actual dive data.

A quick look through the source for "OTU" finds the function "calculate_otu" which says "pow(pm, 5.0/6.0)" .. or about 0.83

I'm not interested in arguing about the merits of each approach here i just need to know what Subsurface uses for the calculations - please and thank you.

UPTD equations source here:

Source of -0.83 exponent

--
You received this message because you are subscribed to the Google Groups "Subsurface Divelog" group.
To unsubscribe from this group and stop receiving emails from it, send an email to subsurface-dive...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/subsurface-divelog/9dcd9257-61a2-40eb-b32c-063a4e680a85n%40googlegroups.com.


--
GDB has a 'break' feature; why doesn't it have 'fix' too?

Robert C. Helling

unread,
Jan 8, 2026, 5:20:39 AM (7 days ago) Jan 8
to Subsurface Divelog
For a number of reasons, the foundations of calculating OTU's are not very sound. I have written a bit about this and explain what Subsurface actually does in my blog: https://thetheoreticaldiver.org/wordpress/index.php/2018/12/05/a-few-thoughts-on-oxygen-toxicity/

Best
Robert

Reply all
Reply to author
Forward
0 new messages