If the measurement lag is the largest time constant in the loop, the plots will be smoother and the controller gain can be increased leading one to think a big measurement filter is the best idea ever. I have been at an ISA conference where the presenter said he almost did not get permission to give the paper because his company thought he was giving away a proprietary advantage. He had added a filter so large, everything was so smooth. He did not plot the actual process variable. When we have excessively slow sensors, transmitters, or filtered signals, we do not know what the actual process is doing. Making a sensor or transmitter faster may make a measurement look worse so if possible keep the old slow measurement to show the improvement from faster control. Low velocities, low heat transfer coefficients, low thermal conductivity protection tubes, and thermowell air gaps for temperature measurements, and low mass transfer coefficients, old glass, thick glass and low pH can make pH electrodes slow. Coatings can add a huge measurement lag especially for pH electrodes. The amplitude of the actual process variable can be estimated from the amplitude of the measurement by Equation 1-3 on page 56 of the ISA book Essentials of Modern Measurements and Final Elements. For more on the deception see So Smooth
If the measurement lag is the largest time constant in the loop, the plots will be smoother and the controller gain can be increased leading one to think a big measurement filter is the best idea ever. I have been at an ISA conference where the presenter said he almost did not get permission to give the paper because his company thought he was giving away a proprietary advantage. He had added a filter so large, everything was so smooth. He did not plot the actual process variable. When we have excessively slow sensors, transmitters, or filtered signals, we do not know what the actual process is doing. Making a sensor or transmitter faster may make a measurement look worse so if possible keep the old slow measurement to show the improvement from faster control. Low velocities, low heat transfer coefficients, low thermal conductivity protection tubes, and thermowell air gaps for temperature measurements, and low mass transfer coefficients, old glass, thick glass and low pH can make pH electrodes slow. Coatings can add a huge measurement lag especially for pH electrodes. The amplitude of the actual process variable can be estimated from the amplitude of the measurement by Equation 1-3 on page 56 of the ISA by CinemaxME" style="border: none !important; display: inline-block !important; text-indent: 0px !important; float: none !important; font-weight: bold !important; height: auto !important; margin: 0px !important; min-height: 0px !important; min-width: 0px !important; padding: 0px !important; text-transform: uppercase !important; text-decoration: underline !important; vertical-align: baseline !important; width: auto !important; background: transparent !important;">book Essentials of Modern Measurements and Final Elements. For more on the deception see So Smooth
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