Glaucoma open angle. pressure readings taken 4 successive weeks are 20, 26, 28 & 31. why the wide range of readings in only 1 mo. which is right?

Variability: Your eye pressure will naturally fluctuate over time, also at different times of day. Only the reading of 20 could be considered "normal". All the others are quite high and in most circumstances require treatment. You need to rely on your ophthalmologist who knows you best for sound advice on how to treat your pressure and prevent glaucoma damage or progression.

Several options: Untreated glaucoma can give quite variable readings depending upon the state of the eye, the time of day and your hydration. If you are being treated, then all of the numbers you quote are too high. The devices that measure pressure are + or - 1-2 points so that also causes some variation. If your numbers are correct, then the trend is upwards - not a good thing, and needs attention right away.