Weigh Vessel Conversion

The current application involves a steam jacketed vessel (2000 lbs deadweight), 3 Mettler Toledo load cells (2mV/V), each with a 2500 lb capacity, a summing box and a Panther display.

The Panther display is going to be replaced with a IND110 converter with the 4-20 mA signal reporting to a 12 bit analog card.

The maximum capacity of the vessel is 800 lbs of water.

My normal approach would be to scale the logic so that 4-20 mA equates to 0-7500 lbs (0-4096 counts). Since the active weight spans from 0-800 lbs, this means that only about 10% of the total range is used and the best resolution achieved will be around 2 lbs/count.

The IND110 calibration mode allows the 'zero load' to be set to 4 mA, which would be the vessel deadweight (2000 lbs).
It can then be spanned to set the 'full load' of 2800 lbs to 20 mA.

Now, if I scale the logic so that 4-20 mA equates to 0-800 lbs (0-4096 counts), there will be a much improved resolution.

Is this correct or am I missing something?

gphillips said:

My normal approach would be to scale the logic so that 4-20 mA equates to 0-7500 lbs (0-4096 counts).

Click to expand...

Why is this "normal"?

gphillips said:

The IND110 calibration mode allows the 'zero load' to be set to 4 mA, which would be the vessel deadweight (2000 lbs).
It can then be spanned to set the 'full load' of 2800 lbs to 20 mA.

Now, if I scale the logic so that 4-20 mA equates to 0-800 lbs (0-4096 counts), there will be a much improved resolution.

Is this correct or am I missing something?

Click to expand...

This is correct, although I would set the full-scale 20mA to higher that 800lbs, say 900, or make it a nice round number, 1,000lbs.

I scale pressure transducers to their full range and assumed that I would have to do the same with load cells.

Yes, I am new to load cells and quickly figured out that I could not simulate a 7500 lb load.

gphillips said:

I scale pressure transducers to their full range and assumed that I would have to do the same with load cells.

Yes, I am new to load cells and quickly figured out that I could not simulate a 7500 lb load.

Click to expand...

Unfortunately you will find the steam pressure in the jacket will cause unreliable readings as the ratio of dead weight to live weight is so high. Inrushing steam adds load onto load cells , if the dead weight ratio to live weight ratio is low these fluctuations may not be be noticeable, but in your application where the dead weight is higher than the live weight, steam pressure fluctuations will affect weight readings.