When you say bark chips, is this the only medium in your mix? We like to use a bit of coconut hust (holds water) in our mixes here. Regardless of your mix though, to answer your question: Try the weight method of determining if your orchid needs water. First, find one of your plants that you are 100% sure has dry medium. Now, lift it or weigh it if you have a small scale. Feel the dry weight. Next, run water through the pot - flushing the medium very well. Be sure to water heavily and not just splash the medium. Orchid roots will start to take in water after a few seconds, but not instantly. Now, re-weigh the pot or lift it again and feel the difference. You can compare the two weights to know when it is time to re-water. Hope this helps.