Yeast Respiration with Balloon

Our Food Tech teacher came over for some advice - they are investigating factors affecting yeast respiration. They are using a balloon over a plastic bottle containing yeast solution. I'm not being asked to do anything for it but as she was looking for advice I thought I might ask whether anyone on here has any tips. How would you measure how much gas has collected in the balloon given that they don't have any specialist equipment? I suggested measuring the circumference of the inflated balloon. I've suggested using Allinson yeast in the green tub as that's what I've heard works well. Any tips on how they could change pH just using chemicals available to food tech & how much works well. Would just a spoonful of bicarb or vinegar do the job? What differing amounts of sugar show good results? Temperature? I'm guessing iced water, room temperature, warm temp eg 30degC & a hot temp eg, 50 degC - any other suggestions? As I said, I'm not being asked to do any trialling or anything but would like to be able to help with any tips. The teacher will be doing the trialling at home at the weekend.