Sensible Sensor Speed–Part 1

Greg McMillan and Stan Weiner Talk Sensor Speeds

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May 2009 COmicGreg: The total of the sensor, transmitter and DCS filter time lags should be less than 1/5 of the process dead time and process time constant in the loop if you want to avoid a degradation of more than 10% in the integrated absolute error from a disturbance. This rule of thumb assumes that about one-half of the time lag effectively becomes loop dead time as a result of time constants in series, and that the controller was properly tuned for disturbance rejection. For compressor and header pressure control, the process dead times and time constants are in the 1-second ball park, which means the transmitter damping setting should be about 0.2 seconds or less.

Stan: It used to be the rule to set the damping setting at a minimum (e.g., zero) and use the signal filter in the DCS to set the smoothing just enough to keep the excursions of the controller output from measurement noise within the resolution setting of the control valve (e.g., 0.2%).

Greg: With wireless measurements we have a new ball game. The transmitter damping setting will be used to keep measurement noise from exceeding the resolution setting and triggering an unnecessary transmission to the DCS. Presently, the fastest update is once per second, which is good enough for most flow and all level, temperature, composition, percent solids and pH loops. For compressor, furnace and header pressure control without variable-speed drives, 0.1 second wired transmitter update time and 0.1 DCS scan\execution time are needed to keep the measurement and control faster than the response of a control valve with a diaphragm or piston actuator.

Stan: Now Randy Reiss offers his “Top 10 Ways to Get Noticed at Work.”

Top 10 Ways to Get Noticed at Work

10. inny dipping at a company picnic. Note: Getting noticed is not always a good thing.
 9. Always be at the coffee machine to help the plant manager with his espresso.
8. Dedication, working extra hours and sacrificing your personal life to better the project…Ha! Who am I kidding? That never works.
7. Seak up at meetings with new ideas. The boss will notice your initiative. Everyone else will hate you.
6. Every time new ideas are mentioned say, “We tried that and it didn’t work!” Then suggest the same idea an hour later, but claim it as yours.
5. Pretend English is your second language, and everyone will think you are real smart when you stutter and spew incomprehensible gibberish.
4. Rewrite history: Half-heartedly support a project until you see that, through other people’s hard work, it’s going to succeed. Then jump in and declare that it works just like you kept telling people.
3. Act dumb. This could be easy.
2. Act smart. Emphasis on “act.” Real smarts make management nervous.
1. Figure out what the plant manager thinks and repeat it to him. It won’t increase production, but you’ll get noticed and, possibly, a raise. 

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