Struggling to get a simple thiosulphate - hydrochloric rate of reaction practical to work in a handful of minutes, trial and error brings me to this conculsion
0.1M sodium thiosulphate + 1.0M hydrochloric acid + water in equal volumes gives about 6 minutes to obscure a cross at 20oC, or about 2 minutes to max out a turbidity sensor at the same temp.
