This model is identical in design and characteristics to the STD 101, except in the output which consists of two sets of relay contacts with no potential. The STD 201a is designed to be used as an alert for filter failure by line breakage. There are also other applications in which flow/no flow indication is required for process control. In this application the STD 201a is the most adequate and economic solution. As with other instruments in the STD series, this alert has options to allow to work at high pressures.