Brew Kettle gallon demarks

Brewed 3 - 5 gallon batches today. I do not have demarcations on my brew kettle. I know where the six gallon mark is relative to the handles. Usually end up with very close to 5 1/2 gallons in my carboy. My first two batches were near perfect - hit my volume and OG. The last batch was short almost 3/4 gallon. Not sure if it got cooler outside or if I screwed up my boil time (pretty sure I didn’t). I started my brew day at 9am, high humidity outside, ended at 6pm with lower humidity and a little colder ambient temperature. Has anyone had an experience when evaporation rate changed drastically in a brew day? Anyone have a good technique for marking gallon demarcations on the inside of a stainless kettle. Thanks

Make a stick you can dunk in the kettle. You’ll need to graduate it before and keep track of it…. Also, there is some where a fella made the marks inside their kettle then, used muriatic acid( I think) to make permanent marks… I use a sight glass…. See, many options…. Sneezles61

As @sneezles61 said the easiest way to find a stick, like a yard stick, and graduating it per gallon. If you’re a DIY, here is what you’re looking for.

An accurately marked stick, placed vertically in the center of the kettle, will always provide an accurate volume measure. Kettle markings can be inaccurate, if the kettle is not placed on a level plane.

For my mash tun I use a piece of CPVC (ok with heat) and scored it at each level. Five gallon increments for that but it would work for one. Just use a gallon jug and add one at a time, dunk the piece of pipe and make a cut. I have one of those plastic pipe cutters that made the job quick. Simple and was a left over so free.

I used this etching technique, but I did it on my stainless steel brewing spoon so that it also serves as a measuring stick since my spoon is always near my BK when brewing. Works great, and the technique is very easy to do. I would recommend doing this in a well ventilated area. It does smoke and release some kind of metal oxides of some sort.