Pressure to weight

[abfer]

There are some weighing systems in some machines operated by hydraulic pistons and are quite precise. They calculate the weight by converting the measured pressure from the pistons via pressure sensors. Does anybody know or have an idea how the system works? How do they convert pressure to weight?

 

[itsmoked]

The area of the piston times the pressure acting on that piston gives force.

PSI = Pounds per square inch
or
PSI = lbs/inch[sup]2[/sup]

Piston Area = inch[sup]2[/sup]

PSI x Piston Area = lbs/inch[sup]2[/sup] x inch[sup]2[/sup]= lbs

This is the force the piston will be providing. If it is positioned vertically then it is also the weight.

With certain conditions met. The main one being there is no stiction present.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[abfer]

Hmm how do they know the piston is vertical and on what reference?

 

[itsmoked]

Since it is a sine function I suspect any small angular displacement from true vertical makes little difference in the actual weight measurement error.

What reference? Local gravity.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[abfer]

I mean vertical to what? Earth's surface? Since machine can be on different grounds, it may be tricky. So the operator should find a flat surface then.

 

[IRstuff]

That why McMaster carries levels of all sorts. It's up to the user to ensure proper installation.

TTFN

FAQ731-376

 

[itsmoked]

Yes abfer. As well as the system can be, it should be vertical if you want weight measurements. but like I said you could be off a few degrees and it will still work well.

Perpendicular to the planet's surface. Which is in-line with the force of gravity.

You would want to calibrate for better accuracy.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[suviuuno]

We recently designed similar systen using loadcells. We use beam type loadcells that respond only to vertical load. We compensate offset angle from two axis via inclinometer. This compensated angle is mg*sin b, where b is the offset angle. If your system is stationary, this works fairly well. Vibrations cause accuracy problems ie.bouncing of reading. Therefore filtering is necessary.

 

[JimCasey]

Sounds like you are describing a dead-weight tester.

Her is a link:

http://www.ashcroft.com/products.cfm?doc_id=352

Hydraulic pressure applied against a precisely known piston area is used to lift precisely known weights. All these parameters are NIST traceable and are useful as an absolute reference standard when calibrating pressure instruments.

 

[JimCasey]

The Ashcroft tester referenced in the previous post has a paragraph regarding levelness in the instructions:

"3.3.4 Levelness
The deadweight tester must be level to function properly.
The unit may be leveled by placing a bubble type level on
top of the piston plate and revolving it slowly. Shims may
be used between drip pan and bench to level the piston
plate. The unit is level when the position of the bubble
does not change within the glass as it is rotated on top of
the piston assembly."

Small angular deviation will, as mentioned earlier, cause an effect as the sine of the angle, but any side-load component (cosine) may cause binding of the plunger and introduce a hysteresis error. Careful leveling will avoid this error.