Air-bag pressure sensing
An air suspension carries the axle load on air bags, and the pressure inside a bag is proportional to the load it is carrying. Put a pressure sensor on the air line, calibrate it against a certified scale at two known weights, and you have a weight read-out for that axle group. It is elegant, because the measurement already exists in the vehicle and you are only reading it.
What it is good at
It is inexpensive, it installs without touching the load path, and it can be retrofitted to an existing trailer in an afternoon. On a fleet of air-ride trailers it is the cheapest way to stop guessing, by a wide margin.
Where it goes wrong
It only works where there is an air bag, which rules out mechanically suspended axles and most steer axles. The reading depends on the suspension being at its normal ride height, so a vehicle on a slope, on soft ground or with the parking brake binding the suspension can read wrong while looking perfectly plausible. Bushings, sticking valves and a levelling valve that is slow to settle all introduce error, and every vehicle has to be calibrated individually because no two suspensions are quite the same.
Load cell sensing
A load cell is a strain gauge in a housing: it deforms very slightly under load and reports that deformation as a force. Placed in the load path — between the suspension and the frame, or in the mounting — it measures what the axle is actually carrying rather than inferring it from something else.
What it is good at
It does not care what kind of suspension is underneath it, so it works on spring suspensions as well as air. It is less sensitive to ride height, and because it is measuring the quantity you want rather than a proxy for it, there is one less step in which to go wrong.
Where it goes wrong
Installation is a real job, because the sensor has to go into the load path rather than onto an existing air line, and that costs money and vehicle downtime. Load cells drift with temperature and with age, so calibration is still required. And a cell that is damaged in service is a harder thing to notice than a leaking air line.
What is true of both
Neither is certified for trade. Onboard scales help an operator decide whether a trailer is near the limit, needs redistribution or should be weighed. When the weight must be proved to enforcement or a customer, the certified ticket is still the document that does it.
Both need calibrating per vehicle against a certified scale, and both need recalibrating after suspension work. And both share the same structural limit: they measure the load that is already on the trailer. By the time an onboard scale tells you a group is over, the freight is aboard, the trailer is probably sealed, and the fix is a re-load rather than a placement decision.