I’ve written about this before.
I went to check my old calculation today at the pool with my trusty old Polar s720i – which, like many old Polars, work in the pool.
I had a few issues with the unit not picking up the HR track all the time but largely it was OK.
5 TRIMPs per 100m
To cut a long story short I said before that 5 TRIMPs per 100m was about right. That was ages ago. Having retried it today I get the same result. A good efficient swimmer not doing a maximal effort might be 1 TRIMP per 25m length. Then again I set my zones conservatively. So 5 TRIMPs per 100m seems about right.
<Edit> It’s April now and I still have this nagging doubt that 5 is too high. So totally arbitrarily I now use 4. The reason being that I came up with a figure of, let’s say, 80 TRIMPs for whatever the swim length time was. It (80) just seemed higher than an equivalently hard 80-TRIMP run session over a similar period. ie an 80 TRIMP run felt much harder.