I think the 24 hour ph test is where they insert a tube through your nose and down into your low esophagus which is hooked up to a monitor that measures the ph in your esophagus. Well that is what my dad had done anyways...maybe they do it differently now. He said the tube was a bit of an annoyance and made him gag a few times. He was diagnosed with severe acid reflux due to his lower esophagus sphincter not working properly. I may be wrong on this as I am no dr.