This blog is the only thing between me and dinner so here are some tidy tips on tips.
Sensor tips in a vessel located too close to a dip tube for pH or temperature and a sparger tube for dissolved oxygen result in a noisy measurement that may be responding more to the temperature or composition of the feed than the mixture of the volume. The probe should be on the opposite side of the vessel and at a different elevation than the feed entry, particularly for impellers that promote a radial circulation. The probes must always be immersed in the process fluid when the equipment is running but not in the bottom of the vessel where solids and wrenches hang out. For pH probes, it is desirable to keep them hydrated between runs and batches. When sensors are in an overflow line (not my favorite location), make sure the control loop is not trying to respond to an erroneous signal when the vessel level is not up to the overflow point. Other situations where sensors are not sensing the proper process temperature occur in condensers and kilns or dryers where the liquid and solids level, respectively, is not consistent.