Filled System, or just filled thermometers are those
that work on pressure or volume change of a gas or changes in vapor pressure of
a liquid. The gas or gas and liquid are contained usually in a sealed metal tubing
and bulb system. If you have an electric or gas oven in the USA, chances are the
temperature sensor in the oven looks like a metal tube with a fat bump on the
end. That's a filled system thermometer.
The gas type were used
in many industrial applications and for establishing portions of the thermodynamic
temperature scale. They can be very simple, non-powered devices with great reliability
and repeatability. The vapor pressure types have a bulb, the sensing portion,
filled with a volitile liquid, instead of a gas. Since they are more sensitive
to temperature changes than a gas type, they can be physically smaller, however
their relative temperature measurement span is quite a bit smaller.