As is happening more often as the years go on, I'm confused about what is being discussed.
Is this discussion about a
heater control valve, as used on Air-Flow (deluxe) heaters (47-54 Chevrolet & GMC trucks)? This has a capillary tube temperature sensor/controller. The tube has a screw tab near the end (often broken off - see picture at the link above); and, that tab is screwed to the firewall "dash mat", under the dash, on the upper right side of the hump. I will post a picture this evening, if I can find the installation manual that came with my Air-Flow (Deluxe) heater.
[a diagram can be found at the link given by Jim Meier, below] My understanding is that valve can be "trimmed" , by turning the dash-mounted control rod, so that water flows through the valve and thus through the heater at a variable temperature setting (a thermostatically controlled heater).
Or, is this discussion about a temperature gauge sensor/sendor, which has a capillary tube on Chevrolet trucks and was electronic on GMC trucks (at least for some, if not all, New Design years)?
Both parts seem to be brought into the discussion above.
Both parts (at least on on Chevrolet 47-55 trucks) use a capillary tube as a temperature sensor (gauge) and/or controller (heater).
