I had an idea. You can get fetal heart monitors which amount to an electronically enhanced stethoscope. For day 18 candling, I believe one of these will be more than powerful enough to pick up a heartbeat in an egg. I think I'd keep the volume down a bit at first because a cheep could be painful. I'm thinking it may need a cone formed on the reciever to enhance pickup. Silicone caulk will work, and it's cheap. I went to look for mine and I can't find it anywhere. If anyone has one lying around, I'd like to hear about your results. If this works, I think the price of the things on ebay may increase a bit in the short term.