Most orchids like their water to be slightly on the acidic side; a pH meter lets you see just how acidic or alkaline your water is.
Electrical conductivity of water gives you an approximation of the amount of salts dissolved in the water; pure water hardly conducts electricity at all, whilst water with a lot of salts and minerals in it conducts electricity quite well (which means you can use distilled water to put out electrical fires in server rooms, but spilling salt water on electronics usually results in them not ever working again). Most orchids are only ever watered by rainfall, which is pretty close to distilled water in its composition - hardly any mineral salts and a slightly acidic pH.
One generally expects that replicating the conditions orchids usually grow in in the wild will result in success at home; long experience has shown people that this is indeed the case.
There is of course another source of salts which we generally consider "good" for our plants - fertilisers, which are made up of various salts and minerals, which of course affect the composition of the water. Years of advice has suggested that fairly weak concentrations of fertilisers on a fairly frequent basis ("weakly, weekly") are the best bet for orchid cultivation. Too much fertiliser can be worse that too little (just as more orchids are killed by over-watering than drying out!).
I hauled the meter out of its box this morning and quickly did some tests. As I expected, Grahamstown water did not fare well:
Grahamstown Tapwater: pH 8.20, EC 669μS/cm - quite alkaline and rather salty (for orchids).
Aquelle spring water: pH 7.2, EC 72 μS/cm - not too bad!
Distilled water: pH 7.05, EC 3μS/cm - pretty good, as you'd expect.
Experience has shown that different kinds of orchids will tolerate different levels of dissolved salts in their water before it essentially has the same effect as you drinking seawater - i.e. it dehydrates you and is no good at all, despite your best intentions.
I found a reference on the Eastern Province Orchid Society's website which suggests the following thresholds:
400μS/cm: Cattleya, PaphiopedilumThat means that even without fertiliser added, Grahamstown tapwater is unsuitable for all but the most commonly and widely grown orchids (Phalaenopsis and Cymbidium). And I suspect Disa are real pansies and need a lot lower than that! Buying distilled water and/or spring water to water your plants seems a little excessive, so I'm going to test some other possible sources, particularly spring water. Given that we live in a rented second story flat, there isn't really any feasible way of harvesting rainwater, and Reverse Osmosis units require plumbing modifications, unfortunately!
600μS/cm: Odontoglossum, Coelogyne, Lycaste, Oncidium, Masdevallia, Miltonia
800μS/cm: Phalaenopsis, Cymbidium
Next I'll have to test the various fertilisers we use to see how they affect the conductivity and pH and just how much we should be using... I've also ordered some calibration and storage solutions for the meter to make sure it's accurate and stays that way!