I added permenant volume markings to the inside of my brew kettle using a simple electrolytic acid etching technique that I discovered on the Home Brew Talk forum. Here’s how I did it:
1. Mark 10 litre volume increments in the kettle using electrical tape. I measured the water volume accurately by weighing it (1L = 1Kg). The side of the kettle needs to be dry for the tape to stick so I carefully dried it to the water line with paper towel and then line up the electrical tape with the water line.
2. Measure 5 litre markings half way between and mask off the 10 litre and 5 litre markings.
3. Make etching tool from old DC power adapter. I used a Nokia phone charger with 5.3 V/ 500mA output. Connect the negative to a cotton bud, close to the cotton tip. Connect the positive to the kettle, close to etching point, using electrical tape.
4. Prepare electrolytic solution – just mix some salt into vinegar. Ratio is not critical, I just mixed as much as would dissolve.
5. Wet the cotton tip (make sure wire is covered) in electrolytic solution, turn on power and touch the parts of the kettle that you want to mark. There is a faint sizzeling sound and you can see the steel appearance become chalky. Also the cotton tip will start to discolour (brown and green).
6. Remove masking tape.
7. Measure and mask off 1 litre increments. I only did this between the 20 and 40 litre marks as I expect all my measuring requrements to be within this range.
8. Etch the 1 litre markings.
9. Remove tape and clean up. Allow etched areas to passivate naturally in air over 2 week period or you can do a speedy passivation with citric acid.