No amount of purging can guarantee the sensor tube will be fully purged of HCL. When the controller is removed from service and atmosphere hits the residual HCl inside the thin wall sensor tube, acid forms and starts etching the sensor tube wall and pin holes eventually form. This may not happen every time the HCl Mass Flow Controller is removed, but when it does happen, it can be very expensive and very dangerous.

We have seen Mass Flow Controller circuit boards, stainless steel valves and most importantly, edge card connector pins corroded by HCl which has leaked from small holes in the sensor. Replacing the circuit boards and valves is expensive.

A major problem is the often illogical symptoms resulting from corroded edge card connectors. These connectors are used to provide power and signal voltages to and from the Mass Flow Controller. Using (5) five volts as full scale means that millivolts can be very important o your process. We calibrate to within millivolts and a corroded connector pin can easily drop a hundred millivolts or more and can do this intermittently. One hundred millivolts is 2% of the MFC full scale output.

If you carefully observe the condition of the MFC edge card connector, on HCl Mass Flow Controllers, you will probably see an unusually high occurrence of corrosion and discoloration. This corrosion can be transferred to the system connector and thus to other MFCs, causing intermittent and difficult to solve problems that spread throughout your system.

We strongly recommend that any HCl mass flow controller should receive a new sensor during service.