by Sir Ray
Actually there is another way to build a 'Tell-Tale', although I haven't seen one like this per se:
Simply have a light beam (laser or infrared) set at the desired height a fair distance from the bridge and if an overtall vehicle breaks the beam, activate the warning devices.
This system is in use at a car wash I go to, to adjust the height of a overhead dryer system (you can see it move up and down as it follow the contours of your vehicle) - admittedly it's spanning only 8-9 foot as opposed to a 40foot roadbed (+ shoulders?), so a laser to maintain coherency would be good - of course, infrared is good to prevent false positives in the event of smoke, fog, etc..
Actually, I suppose there are infrared lasers (since infrared is just another frequency band of light), so that buys you the best of both worlds.
Simply have a light beam (laser or infrared) set at the desired height a fair distance from the bridge and if an overtall vehicle breaks the beam, activate the warning devices.
This system is in use at a car wash I go to, to adjust the height of a overhead dryer system (you can see it move up and down as it follow the contours of your vehicle) - admittedly it's spanning only 8-9 foot as opposed to a 40foot roadbed (+ shoulders?), so a laser to maintain coherency would be good - of course, infrared is good to prevent false positives in the event of smoke, fog, etc..
Actually, I suppose there are infrared lasers (since infrared is just another frequency band of light), so that buys you the best of both worlds.