I became sensitized to the importance of measurement location when I found the easiest way to keep a pH electrode from fouling was to install it in a pipe with a flow velocity of 5 to 7 fps, preventing the usual 100X deterioration in the speed of response that resulted from just a few millimeters of coating. The higher velocity also made the electrode much faster-responding when clean. The conventional wisdom of putting an electrode into a vessel was proven wrong on several counts. The velocity in even the most highly agitated vessels is only 1 fps, resulting in a slow response and the need to remove the electrode more frequently. Furthermore, removing an electrode from a vessel in service is more problematic than removing it from a recirculation line that can be isolated.
I also found that pH electrodes installed too close to the outlet of a static mixer were too noisy. Moving the electrodes downstream 25 pipe diameters made a world of difference. The increase in loop deadtime was only 1.5 sec from the additional transportation delay (9 feet of 4-inch pipe at 6 fps). The decrease in noise allowed me to use a smaller filter time so that the actual total loop deadtime was less.
The same principle applies to thermowells, although the effect is less dramatic. Higher velocities decrease the fouling rate and decrease the measurement lag, the result of an increase in the heat transfer coefficient. (The annular clearance (air gap) between the sensor and the inside diameter of the thermowell has a bigger effect than velocity.) The thermowell should also be about 25 pipe diameters downstream of a heat exchanger to allow for mixing of the flows from the tubes.
Bubbles in liquid streams and droplets in gas streams cause noise when they hit a sensor. Bubbles from air, oxygen, and carbon dioxide spargers in bioreactors and chemical reactors can cause dissolved oxygen and pH signals to become noisy. Droplets of water at a desuperheater outlet cause a noisy temperature measurement. Ammonia bubbles at a static mixer outlet cause a noisy pH measurement.
The tip of an electrode or thermowell should be near the pipe centerline because the temperature and composition vary over the cross section of the pipe. For highly viscous fluids, the error is pronounced. I found that the temperature measurement in extruder outlets and the pH measurement in static mixer outlets with a sulfuric acid reagent are particularly sensitive to the depth of insertion of the sensor tip due to the effects of the high viscosity of polymers and of 98% sulfuric acid.
Differential head meters and vortex meters should be located where the velocity profile is uniform, the flow is turbulent, and there is a single phase – or wherever the piping designer tells you (just kidding).
Concept: The sensor location should provide sufficient residence time and mixing to ensure a single phase and a uniform mixture. The location should minimize the volume between the point of injection and the sensor to minimize delay. For differential head and vortex meters, a consistent velocity profile is required. Most importantly, the location must be sensitive to changes in both directions of the process.
Details: Maximize the detection of changes in the process from disturbances and setpoint changes. For composition, pH, and temperature choose the location that shows the largest change in both directions for a positive and negative change in the ratio of the manipulated flow to the feed flow, realizing that there are cross-sectional and longitudinal temperature and concentration profiles in pipes and equipment. For distillation columns, the best location for the thermowell is the tray with the largest change in temperature for an increase or decrease in the reflux to distillate ratio or steam to distillate ratio. A temperature or pH sensor and an analyzer sample tip should be near the center of a pipe and should extend well past equipment walls. A series of temperature sensors across a fluidized bed at several longitudinal distances is often necessary, with averaging and signal selection to get a representative measurement and prevent hot spots. The insertion length of a thermowell should be more than five times the diameter of the thermowell to minimize thermal conduction-induced errors from heat conduction along the thermowell wall between the tip and the process connection. To prevent vibration failure from wake frequencies in pipes, calculations should be run with the program supplied by the manufacturer on the allowable maximum length. A location with good mixing and a single phase will minimize fluctuations in measured temperature and concentration and the disruption caused by bubbles or solids in liquids and liquid droplets in gases hitting temperature or pH sensors or getting into sample lines for analyzers or into impulse lines for pressure and level measurements. Pressure probes in high-velocity gas streams and furnaces must be designed to minimize momentum and vacuum effects. Sensors and sample probe tips should not be installed on pump suctions and should be downstream of strainers. Minimize sensor deadtime and lag by reducing transportation delays and increasing velocities.
The transportation delay in a pipe or sample line is the volume divided by the flow rate or the distance divided by the velocity. The lag time of temperature and pH sensors decreases with velocity by an increase in the heat transfer and mass transfer coefficient, respectively. Fouling also decreases with velocity.
Watch-outs: Material volumes behind baffles or near the surface or bottom of an agitated vessel or at the outlet of inline equipment may not be well-mixed. Packed and fluidized bed equipment may have uneven composition and temperature distribution from flow channeling. Programs for vibration analysis may only be looking at thermowell failure and will not predict RTD failure. The use of calcium hydroxide (lime) or magnesium hydroxide as a reagent may seem cost-effective until you consider the cost of poor control and solids going downstream.
Exceptions: The best location may not be accessible or maintainable due to height or obstructions.
Insight: The best measurement location maximizes the sensor sensitivity and maximizes the signal-to-noise ratio and minimizes deadtime.
Rule of Thumb: Find a location that is sensitive to changes in the process, where the fluid has a uniform mixture and a single phase, and where sensor lag and transportation delay are minimized.