"Sweat Test"

Heat and effort obviously play a huge role in sweat rate.  You did your test at a higher temperature and effort than what your half ironman will be, so what you have is more of a 40k TT rate.  I did mine over several multi-hour rides in conditions as close to my IM as possible and found a number that worked.  I also suspect that not drinking anything elevated your core temp even higher given the 98 degree temp and possibly caused a greater rate.

First things first, IMO a sweat rate test is more for dialing in and optimizing nutrition for training and racing and not to fight cramping. Currently there is no one known cause for cramping though hypothesis suggest it might be related to dehydration, electrolyte imbalance (sodium, potassium , magnesium, calcium), etc. though some of the evidence also suggest cramping is just a function of exerting above your current fitness level. Hence, even if you do a great job hydrating for instance, if you just exercise above your currently fitness level you might very well experience cramping.

Anyway, your test gives you around 77oz per hour which indeed might be unrealistic, but also remember that we are not seeking to replenish the fluid loss one to one. It is normal to finish a race dehydrated (within reason) and it is not good to finish gaining water weight as it can lead to a series of problems beyond GI issues.

On extreme weather like the one you did your test you might need more fluid indeed but you could start with something like 50-55% of your sweat rate on regular conditions and test this out on training. You will found out how much you need the more you test this on training.

PS. did you do the test outdoors or indoors?

I workin a exercise phys lab and weight loss management lab

I have talked with a few of the dr's here about he hydration test for my self

Mulituple testes for swim bike and run are needed to be done at different variations of weather and temp.

5 min warm up almost to sweat point.
Take your weight
1 hr work out at 80% heart rate
Can intake fluid you just need to be able to subtract the oz out from what you weighed
No urine loses
Post weight taken naked
Find the difference

And that is your weigh loss and how much fluid intake for that piticular environment

Thanks, everyone for the feedback so far.  Yes, the test was done outside in the heat.  Interesting that cramping may not be directly related to hydration/electrolyte balance, just always heard/thought that it was, but certainly I don't keep up with the literature on this subject.  The 50-55% of 80oz is the type of advice I was looking for.  IE, I was hoping someone would say: now start taking 40oz per hour on your long runs/bikes on 70 degree mornings and see how it feels, move up 10oz if its super hot, down 10oz if its cooler, etc... just unsure of the process for "dialing it in"... Certainly I understand that everyone is different and not looking for a hard fast formula, but just generalizations on how to go about figuring this out.

I suppose I never thought of just logging temp/weight loss/fluid intake for all long runs/rides and trying to graph the data points, but that seems like more work than it is worth to a BOP/MOP.  I've never looked at the "nutrition tracker" on BT, will it do this for me?

Going to look at it now.

EW

try looking at this and reading it over

http://resources.runningfree.com/articles.php?article=self-hydratio...

Don't forget that during your races you will be pre-hydrating as well. This will also help offset the loss of 77-80 oz./hour you are losing. Your sweat-test, if done correctly, is done in a quasi-vacuum.

I will be doing my first HIM at Augusta too, and I am a HEAVY sweater! I told my coach that, but I don't think it hit home with him just how much I sweat until recently when we were doing hill repeat in the baking hot sun and I experienced my first bout of heat-exhaustion. I was soaked. I try to tell people how much I sweat by telling them that I look like I just climbed out of a swimming pool when I take off my hockey equipment after a game.