Monthly Archive: June 2007

Jun
29

Slow Down

We wind up our series on measurement and valve dynamics with the timely question do we want a large time constant anywhere? A large time constant in the measurement or valve slows down what the controller can see and manipulate, respectively. A large time constant in the process slows down the effect of disturbances at …

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Jun
23

So Smooth

Is smooth good? We like smooth trend charts but is that what is really going on in the process? Do we want a smooth talking measurement or the straight story? My first clue dates back to a startup of a world class intermediate plant when smooth temperature recordings were traced back to sand in thermowells …

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Jun
14

If I had Just 12 Wishes for pH Control Systems

If I had just 12 wishes for practices to improve pH control systems they would be: 1. Review and improve electrode design (glass thickness, shape, and formulation and reference type and electrolyte) 2. Check and improve electrode location for dead time (transport delay) and velocity 3. Review and improve calibration practices 4. Check and improve …

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Jun
08

Deadtime’s Secret Identity – Part 2

What steps can be taken to make the real loop deadtime step forward? Last week we found that a step made in the controller setpoint rather than its output for a controller gain less than one increases the deadtime because of the time it takes for the controller output to work through the valve deadband …

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Jun
01

Deadtime’s Secret Identity – Part 1

For a constant flow and set of process operating conditions, is the observed total loop deadtime relatively constant? We know from last week’s blog, the deadtime also depends upon the sensor time constant and hence it’s fouling. Less recognized is that it depends upon whether a step change is made in the controller output versus …

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