Then the humidity at the point of use quickly spikes, and the ECU swings to the opposite extreme and begins dehumidifying its output. Temperature undergoes analogous swings. As charcoal filters absorb moisture, an exothermic reaction heats the airstream. In this case, the heat rise is detected at the process area, and the sensor tells the ECU to reduce the heat, until the system inevitably undershoots the air temperature set point. Positioning the ECU between the chemical filter and a stepper doesn't work well because this creates a negative pressure downstream of the filter.
Negative pressure makes it possible to suck contaminants into the system.
Customers told Semifab's engineers that they were experiencing a continuous problem with ECUs and chemical filters. For fab technicians, the short-term solution was to mount the ECUs sensor at the output of the air handler and before the filter. The technicians would enter the chamber and measure conditions with a handheld probe. Based on the delta between this reading in the chamber and the sensor's measurement at the ECU output, technicians would offset the air handler to achieve the desired set point. This allowed them to circumvent the filter-induced response lag and subsequent overshoot.