The Pirani gauge, invented in 1906, is used to measure vacuum. It has a wire (the sensor or filament), normally gold-plated tungsten or platinum, which behaves as a black body emitter. An electical, Wheatsone bridge, circuit is used to keep the temperature of the sensor constant. The whole is put inside a shell so that vacuum inside the shell can be measured down to about 0.001 Torr, up to 1 Torr. This range is dominated by conductive cooling. Below it, radiative cooling dominates. Above 1 Torr: conductive cooling tails off and convective cooling also comes into play. By conductive cooling, I mean the action of gas molecules, in the semi-vacuum, which cool the sensor by conductive loss. Each instrument also has 'end losses' but instruments are calibrated such that end-losses are ignored when measuring.
Consider the chart of showing typical Pirani gauge behavior. My annotations are written in a salmon (pink) colour. The green curve plots pressure (horizontal axis) against power used (vertical axis). Both axes have log scales. The horizontal axis shows pressure in Torr. 760 Torr ~ surface atmospheric pressure (labelled by the sp arrow). Below 0.1 Torr is normally called 'high vacuum'. The vertical axis shows the power used by the instrument to maintain a constant temperature at the Pirani sensor. We assume, that at any particular temperature the emission of infrared (heat) radiation is constant. Because physics shows that's the case: radiative emissions only depend on temperature for a 'black body'.
Over the Low vacuum range [ 1E-02 to 1E+03 (0.01 to 50 Torr), between points labelled by arrows rc to cc ) conductive cooling dominates. So most of the power is used to allay this conductive cooling to keep the Pirani sensor at a constant temperature.
The criticism of the Pirani gauge is that "it's not a good model for the earth". But the actual model of the earth used with backradiation of about 333 is a very, very bad model for the earth; and totally wrong. Climate alarmists are still using this very, very, wrong model today.
Nasif Nahle claims he measured some back-radiation (60 W/m² at night)
From: "Observations of Backradiation during Nighttime and Daytime" by Prof. Nasif Nahle, July 5, 2011
When we place a pyrometer on the ground under direct sunlight with its sensors facing up and measure the solar power flux during daytime, we record solar thermal radiation close to 1000 W/m² not 340 W/m²
If we place the same pyrometer on the ground, adjusted to longwave thermal radiation, during nighttime, the recorded power flux will be around 60 W/m². Nevertheless, the proponents of the greenhouse effect argue that backradiation is 340 W/m² day and night. Additionally, they assert they measured backradiation and it is around 300 W/m².
If we measure the power flux from the surface to the atmosphere during nighttime, we record around 310 W/m². This is five times higher than the power flux recorded from the atmosphere, which is around 60 W/m²
