A little hard to say... how thick/thin do you like your oatmeal? How big is the tub? How deep do you intend to fill it?
Overall, I'd say you're in for a treat, so if anything you want to overdo the depth! Last time I made a batch, seemed like the ratio was about 1 volume of oats to 2 volumes of water... then the oats swell a bit, so you probably end up with 3.5 to 4 total volumes once cooked. Sorry I didn't measure the weight...but with all that water, it will be heavy!
You could always experiment a bit for breakfast... try different ratios of water and oats until you get what you're looking for, the multiply that out by the volume you want in the tub. I went ahead and added cream, sugar and a pinch of salt, too... if pouring over your head, or doing a good head dunk, it might as well taste good, too! ...plus the cream and sugar smoothed out the consistency.
I checked the web because oatmeal baths are used to relive wheal-and-flare reactions, you know, hives.
All I can find is to take 2 cups of oatmeal, crush them in a blender and put in shallow bath water. (I think we had to do this for dad in '65 or so.)
In other words, your only option is to check with Candy or someone on how to mix oatmeal, or experiment with a kiddie pool, which should have end-of-season sales going on. I don't think a bath tub and sheets of heavy PVC plastic are gonna work.
I did this 7 years ago and it was by far the most incredible WAM experience of my life! It took ALLLLL day but was 100% worth every minute of prep and clean up.
I used about 36kg of QUICK OATS (30 large containers here in the states). I ground them half a container at a time in a food processor to make a course powder. Then I filled a 5 gallon bucket about halfway with hot water from the faucet and slowly added the oat powder while stirring with my hand/arm. I made it on the runnier side because it thickens up A LOT as it sits.
I had a large wooden crate that I padded the sides and bottom with towels and blankets then lined with a large plastic sheet. I tacked the plastic sheet to the wood so it wouldn't fall in.
I poured the buckets into the plastic lined crate, and I agree with mud_adventures; it was about 2 parts water (maybe a little more) to 1 part oat powder. I think it took about 9 or 10 buckets, so around 180 liters. The crate was a little wider than my shoulders and a little longer than my legs, so it got DEEP. So deep I was able to completely submerge in it. I posted pictures of it here way back in 2011 but I guess they've been archived because I can't find my post.
Almost forgot! I lived on a dead end with woods behind my house, so disposed of the oatmeal bath by carrying scooping the oatmeal back into the buckets and lugging them into the woods behind the house and dumping them on the ground. Good luck!
Love you, too
