From Yorktonaircraft.com on there Services.
REABE Quantity Hopper Gauge
Now STC’d in Canada for both Thrush Aircraft and Air Tractor. Users of this system absolutely love it! This system measures hopper fluid levels to 0.001 of an inch then converts and displays the quantity in gallons (or litres). The display also knows if the airplane is flying or on the ground – adjusts the quantity displayed to reflect attitude changes. Boom pressure – positive and negative- is also displayed. Sun light readable display!
This kit will enhance your safety margin and remove some of the fatiguing guess work.
And in the Farm Air page we get a mention.