Ken,
I believe you are correct.
Mike,
There must be a significant time-delay between readings. In most cases, the PLC can read the internally converted/stored analog value faster than the analog card can update that value.
If a PLC has a 10-mSec scan-time and the analog card takes 35-mSec to update the analog reading then it takes 3-1/2 scans to get the next update. Between updates the value will remain the same. Multiple readings of that value, between updates, will give the impression that the value has stopped changing... at least for three scans.
The typical scheme to ensure reading a valid update is to add a delay equal to at least two analog card update periods. In this example, that would be at least 70-mSec.
However, even that doesn't ensure that the reading can be properly interpreted.
Just as an extreme example...
If the pump transfers 8-ounces per second, and scan-time is 10-mSec, then that would be 8-ounces per 100 scans => .08-ounces per scan.
If the delay between readings (as determined by the analog update period) is 70-mSec then the expected weight change per reading would be .56-ounces.
If the maximum weight is something like 500-pounds, then that would be 500-pounds x 16-ounces per pound => 8000-ounces.
The analog card has to be able to handle the entire weight range. If the analog card is capable of 8000 discrete indications, then the card has a resolution of 1-oz per count. Since the weight is changing at the rate of .56-ounces per update, then the analog card will not be able to indicate a change until the weight has changed by at least 1-ounce.
That means that there must be an additional delay between readings. In this case, the delay must be increased to approximately 140-mSec.
The values used in this example might be way out of line. However, I used them simply to make the point that the resolution of the analog card and the expected rate of change plays a role in determining what the delay should be.