Onboard Scales Question ...

We have Air Weigh in-cab scales (digital with calibration menu) for tractor [we also have either gauges or digital for trailers]. Have any of you ever verified issues that might be related to altitude? I can't find any detail an altitude compensation for these devices but a web search seems to indicate they are supposed to have this feature but I'm unsure how it might supposed to work.

I happened to notice while running across Wyoming I-80 my tractor readout was showing about 900-1000 lbs heavier with same fuel load (34,900) then it did in Vermont. So, to be safe, I scaled on a Cat scale at Wamsutter, and my real current weight was the same as my Air Weigh weight in Vermont, but it indicated 34,960 on the Wyoming scale when the Cat scale showed 33,900). I had just recalibrated this system a week or so ago in Dallas after loading, en-route to Albany. Loaded in Vermont and all seemed good. And was verified with Wyoming scale ticket.

Anyone verified altitude issues or temperature issues that were significant with bag pressure systems? (the temperature was maybe 20 degrees colder in Wyoming than in Vermont)

 

All I know is my gauge reads almost 1000 lbs higher in wamsutter, WY than it did in St. Albans VT. Both times while sitting in a level scale platform. Same fuel, no ice either end, and it was properly calibrated to correct a 240 lb difference a week earlier at 700 ft MSL

And No. There was not a "load shift". Everything was exactly where it was when it was loaded.

I guess the moral of my story is that sometimes you can trust some of them and sometimes you can't.

 

Nope. I'm always careful to make sure I'm where truck won't roll with no parking brake or pedal application.

I haven't done an empty calibration in over 12 months. Maybe I need to start over from scratch. Will update thread as I can.

 

I was kind of thinking in terms of the effects of altitude on air pressure in bags and on the pressure sensor. For an aircraft altimeter to be accurate, it must be set to the known real barometric pressure at the target airport [that is affected by high and low pressure systems that come and go]

Like when you close an empty water bottle at 8,000 feet then take it to seal level, it's all crushed in on itself. But again, this may not even be a factor and maybe I just need to be more diligent in entering known empty weights more often then I have been. Altitude might affect the accuracy but probably very little. I suspect I have other issues going on.