As mentioned I will discuss pressure today. I have a mechanical engineering background and process pressures is one of the parameters I work with every day.
The intraocular pressure of an eye, IOP, is measured in mmHg which stands for millimetre Mercury column. In the old days barometers were U shaped glass tubes filled with Mercury and the height of the Mercury indicated barometric pressure.
Normal barometric pressure will fluctuate around 765 mmHg, staring at around 750 to maximal about 780 mmHg. Height is very much of influence on the barometric pressure, at 4 km height the pressure will be roughly 450 mmHg.
Imagine diving in a swimming pool to 10 metre depth, any idea how much additional pressure that gives on your body? 750 mmHg!
In my opinion recently not too much research has been conducted on the influence of great heights or depths on IOP. The issue here is the influence of the pressure periocular against the intraocular pressure and how quickly there is a pressure balance.
One article from 1980 I could find on this subject: Diving and Hyperbaric Ophthalmology by the Department of Ophthalmology, U.S. Naval Hospital:
“…..the difference between the two pressures (intraocular and periocular) stays the same despite the greatly increased external pressure of the water.”
We measure IOP with tonometers, readings above 25 mmHg are Glaucoma suspect or can be cause of Glaucoma. This 25 mmHg is identical to 0.48 PSI, normally your car tyres should be around 30 PSI.
What I am trying to say is that we measure very small differences in the IOP, because 18 is good, 25 is high and that is a 0.13 PSI difference. It requires a very good pressure gauge to indicate such low pressures. The unit of measure is very small and it requires good instruments to measure.
Influences such as draft and wind, own breathing and heartbeat will have influence on both contact and non contact tonometry.