Abstract: A fail-safe pneumatically operated by-pass sampling valve suitable for use n hazardous chemical processes utilizes a manually operated spool valve to initiate a pneumatically operated piston driven crank to rotate a sampling valve. The sampling valve withdraws a sample amount of fluid from a process line without disturbing normal process flow. A pneumatically connected spindle valve and a plurality of limiting valves are proximately positioned and operatively connected to the piston to automatically move the piston actuating member from a normally closed non-sampling position to a sampling position and then back to a normally closed non-sampling position.