Our friends have a TB stud a nd use them, I think they generally work but arent 100% reliable. Certainly a lot of money for just one mare I think.
They have a fantastic cat that stays with the mares when theyre close and comes to get them when a mare goes down-impressive huh?!!
We have a wyke one which we have found to be very reliable.
We have a mare that shows no signs that she is about to foal (does not bag up, run milk, get restless etc etc) and in the past we have stayed up checking her and watching the cctv and has nearly caught us even after checking her 10 minutes earlier. With the foal alarm (which works of a sensor on a neck strap), we get a good nights sleep and get a few minutes notice to check the cameras. Also has a good range on it. Didn't think I would get to trust it but after the last few births with this mare, would recomend it.
Luckily Amy will be on CCTV when she's close to foaling. I've also been allowed to change my hours of work to an 10.30am start, which will make life so much easier!