Probably 1977 , is about right, from product numbers C26-P0606
2-77-2P Does whatever fills the tube change its character over the decades, expanding for any given temp?. Leaking I could understand causing problem, but that would be one-off. Does not seem to be any wear on the pivotted rocking plate inside , that transfers bellows motion to microswitch plunger. Other pivot point moves via cam action of the dial up knob, unlikely that is worn. Over time the setpoint on the dial has had to go up and up and now it is effectively off the top of thse scale and microswitch is hardly on. There is adjustment inside for the switching point of the microswitch, which at half the stem is probably correct. If the rocking plate bent wiht the pressures then I'd expect to see too low a set point developing. I'll take some temp / bellows deflection data just out of interest, but also confirm it has reasonable linearity. The original setting of the pivot seems to be by choice of the deformations in the pivot/rocker plate, no way of setting at initial assembly other than go/no-go testing of each one. "Sensor" tube is well buried and next to impossible to replace with new, entire or partial because impossible to make a join in the capilliary.- posted
7 years ago