How are load cells calibrated?

[Packard V8]

I have neat old Australian-made valve spring tester which is quite a ways out of calibration. Since it's a closed load cell, connected via a small copper tube soldered to a presumably mechanical gauge mechanism, two questions:

1. How were they originally calibrated? Is there a medium under pressure inside the cell?
2. Since it's closed, how/why would/could it go out of calibration?

jack vines

 

[SwissMetric]

Can you post a pic? Recalibration may or may not be possible, impossible to tell without knowing more details.

Is what you're referring to a fluid-filled transmitter with a basic mechanical manometer? They're still used now even with a separate MEMS pressure transmitter though in many cases direct measuring load cells (with strain gauges, piezo cells are rather for specific uses like vibrations, as single or multiaxial accelerometers) are a better choice, as especially pressure transmitting fluid leakages can lead to problems and now strain gauges are no longer more expensive than pressure transmitters.

All systems need some sort of factory calibration and also purely mechanical manometers age, some can be recalibrated, some not, like capillar thermostats.

Some load cells are very inexpensive but unless the amplifier is integrated (e.g. with a 4-20 mA or 0-10 V analog output, and/or some serial bus interface) you'll need external interfacing electronics.

Even some cheap Chinese load cells are surprisingly precise when new but I don't know really how well they age compared to some HBM (now HBK) or Mettler-Toledo load cell.
Load cells intended for weighting purpose are usually calibrated according to OIML requirements but are often used also for other applications.

 

[PCustoms]

@SwissMetric

I'm getting the feeling this isn't an electronic load cell.

 

[SwissMetric]

Obviously purely mechanical.

I meant it more generally, if it can be replaced by a load cell or if it's just about calibration. For now we've no idea if it can be adjusted, it's mostly about the offset, I sort of doubt that the linearity can be influenced but than I mostly know modern instrumentation.
When troubleshooting it's mostly about TSMGO, I'm not a museum curator.

 

[TurnipTruck]

Pic of mechanism? The vast majority of mechanical pressure gauge movements have adjustments for zero, span, and linearity (assuming the Bourdon tube has not been damaged by vibration, overpressure, or impact). The medium in my experience is usually butane, which can leak out.
Modern load cells like for truck scales are digital Wheatstone bridge strain gauges and can stay in calibration for years of warranty. Older load cells are analog and require calibration more frequently.

 

[Packard V8]

Yes, definitely mechanical; the load cell connects to the dial readout gauge via a very small diameter copper tube.

FWIW, applying heat to the load cell with an electric gun raised the resting set point of the dial by forty pounds. When cooled, it returned to zero.

jack vines

 

[no704]

Need picks.