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Apr
05

Common Control Myths – Part 5

To blog or not to blog, that is the question. When on vacation, I choose not to. Now I am back and ready to blog on. Here are some myths from my days (and nights) at Monsanto improving pH loops.

(16) More frequent buffer calibrations improve pH measurement accuracy – unless an electrode needs to be removed to be cleaned, it is better to leave it in the process and do an adjustment based on a statistical average of process samples. The buffer is not representative of the process in terms of liquid junction potentials and the glass electrode response especially when the process has salts or strong acids and bases. The reference electrode can take up to a day to reach equilibrium with the process when it is reinserted. Also, wiping the glass electrode or soaking it in cleaning solutions reduces its life expectancy. Finally, the effect of temperature on pH is not found via a buffer which leads to the next myth.

(17) The pH electrode temperature compensator corrects for changes in pH with temperature – the standard temperature compensation is the change in millivolts per pH unit per the Nernst equation. The change in the actual solution pH due to the change in dissociation constants with temperature requires another correction. Many smart pH transmitters now have this solution pH temperature compensation but it is up to the user to find the relationship. Information on how the dissociation constants change with temperature and how this would affect a complex solution is scarce to nonexistent so it requires a pH sample’s temperature to be varied in the lab. For the simple case of water and a strong base (e.g. caustic) where the effect is dominated by the change in the water dissociation constant with temperature, the error is about -0.03 pH per degree C.

(18) A second pH electrode always improves measurement accuracy and reliability – a second electrode offers more questions than answers since they never agree unless you are lucky enough to have a problem that a smart transmitter can detect. Every operator has a favorite electrode based on some interesting war stories but its anybodies guess as to which is right. Also, since electrodes can fail or develop errors in almost any direction, a decision to select the best electrode requires some extra intelligence such as changes in electrode resistance or a comparison of response times. At Monsanto we used middle signal selection of three electrodes to automatically and inherently ride out a failure of any type and reduce the noise and short term errors for concentration gradients.

(19) The rangeability of a control valve is per catalog specifications – the rangeability statements by valve suppliers usually does not take into account the stick-slip near the closed position. Many rotary valves carry impressive rangeability statements but don’t deliver the goods because the control at low flows is a big saw tooth from high seal friction and breakaway torque.

(20) A valve with a positioner is a good throttling valve – the response of a control valve depends upon the entire package (valve, seat, seal, actuator, packing, positioner, and feedback mechanism). Putting a positioner on an on-off valve (e.g. block or isolation valve) does not make it a throttling control valve. In fact the feedback mechanism may be lying to the positioner in which case the smart diagnostics mean nothing. For more info on this check out the article “Improve Control Loop Performance”

I have “Final Four” on my mind and I am off to see if my alma Mata Kansas can upset UNC but before I go, if you ever come to Austin check out the Oasis for sunsets and its new Starlight bar for star gazing over Lake Travis. I spent last night at the Oasis Starlight listening to the Eggmen (a great Beatles cover band) and having flashbacks to the 1960s and 1970s. Too bad I didn’t wear my plaid bellbottoms.