The frame material was not a random choice. We were looking for a solution that would last outdoors for years without maintenance and that the customer would not have to pay extra for.
Construction
bhScale is one whole unit. You place it under the hive and the scale stands there, with no self-movement while you set the hive down. The cabling is routed in a milled frame, so there is very little room for cable damage. The cut edges are treated with paint. The frame is made of 15 mm birch plywood with a phenolic film.
The 15 mm thickness is not random. It gives the frame stiffness and ensures stable geometry under load. At the same time, plywood is slightly flexible: under load it adapts gently to a support that is not perfectly level. The load cells are therefore loaded according to the real distribution of hive weight, not according to how perfectly you placed the scale.
Plastic parts, the feet and the boxes with electronics and batteries, are positioned so that they are exposed as little as possible to UV radiation and heat. The feet are hidden under the frame, 2 cm from the edge. The electronics and batteries are on the rear side of the scale, and in the standard position behind the hive they are in the shade for most of the day.
We recommend installation on the flattest surface possible. Up to a 1% slope, measurement remains accurate and the hive stands safely on the anti-slip surface. With a greater slope, the weight on the sensors is distributed unevenly and measurement accuracy decreases.
Material in the field
Plywood behaves as thermally neutral. It follows ambient temperature swings more slowly than aluminium.
In spring, when temperature moves from below zero at night to above zero during the day, the frame does not pass these swings directly to the load cells and joints.
It is not a huge effect, but with a device that is supposed to stand outdoors for years without intervention, every joint that is not stressed unnecessarily will matter one day.
The plywood frame does not hold water. The phenolic film on the surface is water-resistant and the plywood does not rot even after years outside.
An open aluminium profile works structurally like a tray. It catches rainwater, melted snow and dirt. When it freezes, the water expands and has nowhere to go. Joints and load-cell mounting are gradually stressed by repeated freezing and thawing cycles.
An enclosed aluminium design solves this problem, but at the cost of a welded frame, more material and higher weight, which directly affects the price. And aluminium's thermal conductivity stays the same in both cases. On dark surfaces the effect becomes even stronger.
After years
The first bhScale scales went to customers in 2019. Most of them are still in use today without any intervention in the frame construction. The phenolic film still holds, the plywood has not lost stiffness, the ABS plastic parts are positioned so that they catch only a minimum of UV radiation. The joints are fine.
Long-term durability is not only a matter of material. It is also a matter of whether the thing can be repaired when something happens. Because we build the scale ourselves, spare parts are always available and we can offer service even after years.