Leo, once in a while we disagree- this is one of those times. I think a decent, well-maintained pH meter is valuable to any plant, fish, and homebrew hobbiest. Even a pen style job for $30 on eBay is better than nothing or silly pH paper. Yeah, I have the convenience of an awesome lab-grade meter to comare it against, but it is pretty close all the time with only *the maintenance the manual recemmends* (calibration with proper, fresh standard solutions and proper storage for the membrane). But certainly, grossly neglected pH meters, no matter how cheap or expensive will "lie" to you.
I wouldn't call that a HUGE swing at least from the plants' perspectives. And is certainly within a good range for our plants. So Leo and I agree there.
Why? Good question. Could be the metabolite buildup as the others suggest. Or could be that and/or a change in the CO2 concentration as the liquid volume decreases. CO2 DIRECTLY influences pH, and gas concentrations in fluid is closely knit to surface to volume ratio- deeper liquid means gas molecules have farther to go to leave the liquid compared to a shallow liquid in a wide container where gas moves pretty rapdily just because there's not that far to go. An oversimplification, but you get the idea.
-Ernie
Hey Ernie, No offense taken, I know you and I can get pretty good test results out of a $30 pH Pen. But you and I have had years of wrangling pH with professional grade equiptment and really understand the nuances of conditioning a pH electrode and calibrating a slope for the electrode. A couple years ago at a bonsai club meeting I watched an abysmal demonstration on measuring soil pH by a retired M.D., a physician who you would have thought would be able to make the $90 pH pen she was using work, and she so jazzed it up that she did our little bonsai group more damage than help. She was well meaning, I said nothing, as so to not embarras her, but the point is, even people you would think by their education could do it, getting the technique down to get valid pHs out of inexpensive equiptment is really not easy to teach or learn. Good lab technique is almost one of those 'Oral Traditions'. In this I mean most of the best little tricks are not written down, and are passed on generation to generation by our secret cult of Lab Technicians.

Even the PhD and MD researcher bosses don't know our secret Lab Tech Code. I know you haven't showed your boss everything. :evil:
So with that thought in mind I tend to downplay the need for going the technical route. You are right - it can be done, but trying to explain all the technique needed around conditioning and calibrating a pH electrode is exhausting.
Leo, Many articles and many times on this forum it is said that there are some orchids that like a higher or lower ph, depending on their native habitat. Advice is given on how to change the ph to meet those supposed optimums. Are you saying don't worry about this just use drinking water and do not worry about the ph?
This goes also to the point where Ernire and I agree. James Asher published in Orchid Digest in the early 1980's an article where he went to Sumatra, and found Paph glaucophyllum growing on a limestone cliff. He brought along a pH meter with a micro-probe. He measured the pH of the water trickling down the cliff, and then he measured the pH of the water film surrounding the root tip of the Paph glaucophyllum. He took a significant sample (I forget numbers, but it was more than 2) the results were interesting, and the reason Ernie and I both think a pH of 5.5 is not a problem for the s/h solution. The water trickling down the cliff was mildly alkaline as expected (I think ? pH 7.8 ?). The pH of the water on the root tip was I believe 5.6. Memory fails me for the exact number - but Asher and the rest were surprised at first. A mildly acidic pH is ideal for solublizing the over-all greater number of nutrients (see Ray Barkalow's excellent references on his First Ray's website). It turns out Paphs specificly and most plants in general, excrete buffers into the water film around their root tips to bring the pH into their ideal range for optimal nutrient absorption. If your plant is healthy, and has lots of root tips, the plant will modify its own environtment to its preferred range. This is one reason not to over-pot an orchid, also the reason seedlings in community pots grow faster than the same size seedlings in individual pots. Less media per root tip, each plant has to work less hard to buffer the mix and the plant has more energy left over for growing. I would say pH's between 5.2 and 8.8 are mildly acidic to mildly alkaline. This is the range that generally I would not get excited about. Healthy plants will be able to take care of themselves in this range.
Now Corbin asked about pH Calciphyles. This is not well understood, to the potting mix of all my calciphylles I add crushed oyster shell or horticultural lime. Paphs like glaucophyllum are found ONLY on limestone cliffs. Asher's article proves it is NOT soil pH that determines habitat selection for glaucophyllum, I believe the speculation is that either the adult plant itself has a higher need for calcium & magnesium OR the fungal symbiont that the seedlings need to sprout requires a higher presence of Calcium & Magnesium. So the extended thought from this observation from Asher is that the Paphs that are listed as calciphyles, DO NEED the limestone in their mix, but the reason is not for pH adjustment, but rather mineral nutrition.
So I believe, especially when it comes to Paphs, when the dicussion revolves around soil pH, most of the time the real issue is a need for limestone, a source of calcium & magnesium. Sometimes, especially with certain Phrags, it does actually seem to be pH itself as the issue. Phrag extaminodia, really does seem to want a lower pH, less than 6.0 for sure. Phrag fischeri and kovachii do seem to do better at higher pH's, above 6.0 and better with limestone in the mix. But for Paphs I really think the issue is limestone, not pH specifically.
If your municipal water starts you off at less than 800 ppm dissolved solids, is pH neutral enough that you can drink it without burning your mouth. (By this I mean 5.5 to 8.5). It is probably good enough for most Paphs and Phrags. (Note: it is not good enough for Disa, or Nepenthes and other carnivorous) Most of the problems people claim are hard water problems are actually problems with drying the plants out too hard between waterings.
That's my 2 cents. I try to keep my orchid growing non-technical, by being careful about setting up the growing conditions. By this I mean - I use a standard fir bark based orchid mix - which I know will have a mildly acidic pH, I add oyster shell for the brachypetalum, cochlopetalum & other calciphyles. I use a good scientifically designed fertilizer like MSU Orchid Special or some of the DynaGro products, I avoid unbalanced fertilizers like the so called 'balanced' 20:20:20 or the true death in can 'Blossom Boosters' 0:18:0. And I really try hard not to dry my orchids out hard between waterings. I make sure I have good air movement. Cover these bases and as an issue pH will dissappear.