In reply to Misha:
I was at the start of Clogwyn Left Hand and tried a "blob" on the wall because I thought the sensor might be buried and stuff on the wall might freeze better because it was free of snow. I was surprised that the turf wasn't frozen enough to hold a deep "stick". I know the guy who supervised the sensor project and he's a local climber so I guess he made sure the sensor was placed in an appropriate location.
I'm definitely going to stay on non-turf routes unless the sensor shows the turf is below zero at 5cm. I know I might be able to get away with a warmer turf reading if things cool down in the 100m between the bottom and top, but I really don't want to take the risk of breaking an ankle when a placement rips, trashing the route, or getting the whole crag banned.