Greenhouse Gas Confusion Magnified by Misuse of Infrared Thermometers
By Carl Brehmer
Instruments called radiometers are believed to measure both up-welling and down-welling longwave radiation, but what do radiometers actually sense and what do their readouts actually mean?
The core of a radiometer is a thermopile held within a vacuum under a dome that has one end attached to a case that holds a “reference temperature.” Depending upon the target a positive or negative electrical charge will be induced in the thermopile, which is transmitted to a circuit board. If the target is warmer than the reference temperature of the case (the ground for instance) radiative warming of the thermopile will occur and a positive current will be induced and if the target is cooler than the case (the open sky for instance) radiative cooling of the thermopile will occur and a negative current will be induced. Along with the electrical signal generated by the thermopile two other temperature signals are sent to the circuit board: 1) the case temperature and 2) the dome temperature. The voltages from these three signals are then mathematically converted into a readout in W/m2.
“The Eppley PIR has 3 output signals; the thermopile (mV), case temperature (V), and dome temperature (V). The 3 signals are combined in the Pyrgeometer Equation, which determines the thermal balance of the instrument and hence the contribution of down-welling longwave radiation (LW).” PIR – Precision Infrared Radiometer on Kilo Moana by Frank Bradley
“The mV output from the thermopiles is converted to W m-2, then corrected for the temperature effects on the PIR’s case.” Eppley PIR (Precision Infrared Radiometer) ® CAMPBELL SCIENTIFIC, INC. Copyright © 2001-2007 Campbell Scientific, Inc.
Here is an example of one such Eppley PIR calculation of DLWR examined in the paper PIR – Precision Infrared Radiometer on Kilo Moana by Frank Bradley. When pointed upwards towards the sky what the Eppley PIR (Precision Infrared Radiometer) actually sensed was radiative cooling of the thermopile. This radiative cooling induced a small negative voltage in its output wires and this was mathematically converted into a negative W/m2: in this example -66.6W/m2. The radiometer then, using S-B formulae, calculated the potential IR emission of the thermopile itself based on the reference temperature of the case and it’s presumed emissivity (the result of this calculation was 460.9 W/m2) These two numbers where then added together (-66.6 + 460.9 = 394.3). Finally the Eppley PIR estimated the affect of the dome temperature on this number, which was -10.9 W/m2. It then added this number to the previous sum:
(-10.9 + 394.3 = 383.4)
This is how a measured up-welling IR radiant energy flux of -66.6 W/m2 became a down-welling IR radiant energy flux of 383.4 W/m2. So, even though the thermopile was sensing a -66.6 W/m2 up-welling flux the Epply PIR readout said that there was a 383.4 W/m2 down-welling flux. In reality, the Epply PIR readout is a calculation of what the downward radiant energy flux from the atmosphere would be if it were not being cancelled out via destructive interference by up-welling IR radiation. As such, the existence of an atmospheric down-welling radiant energy flux is a mathematical confabulation; it is a hypothetical reality not a measured or sensed reality.
If you direct the Epply PIR towards the ground the thermopile experiences radiative warming and the same calculations are done to produce an up-welling IR radiation number. The readout is, again, a hypothetical rather than what is actually sensed. An honest radiometer would just tell you the actual radiant energy flux and in what direction it is flowing. In which case the “net” up-going IR radiation would be the total up-going IR radiant energy flux and the down-welling IR radiant energy flux would be nil.
I think that it would be valuable to review what the unit W/m2 actually means.
“A watt per square meter (W/m²) is a derived unit of heat flux density in the International System of Units SI. By definition, watt per square meter is the rate of heat energy of one watt transferred through the area of one square meter, which is normal to the direction of the heat flux.”
“Heat flux” is “The amount of heat transferred across a surface of unit area in a unit time. Also known as thermal flux.”
Flux (n) “the rate of flow of something, such as energy, particles, or fluid volume, across or onto a given area.” Encarta® World English Dictionary © 1999 Microsoft Corporation.
The unit W/m2 then is a measurement of the rate that thermal energy is actually moving from one place to another. It is not a measurement of potential energy flow. Actual thermal energy flow is always unidirectional down a temperature gradient. Think of the unit used to measure water flow in a river: gallons/min. This is a measurement of how much water is actually flowing past a particular point over a given time period. It is not a measurement of what the flow would be if the river bed where infinitely steep. Yet this is what the ULWR and DLWR numbers on the K-T Earth Energy Budget chart are; they are potential thermal energy flows and not actual energy flows. Radiometers, in reality, only sense the flow of radiant thermal energy in one direction, but the readout on radiometers, rather than being a readout of what is actually being sensed, is a calculation of what the heat flux would be if the radiating matter were in a vacuum radiating towards a perfect black body at zero °K.
Ergo, a radiometer takes something that is physically sensed (either the radiative cooling or the radiative warming that induces a small negative or positive current in the thermopile) and converts it into a hypothetical number. We can deduce from their design that engineers of IR radiometers are adherents of the two-way radiant energy exchange paradigm and design these instruments to manifest that paradigm. For example, if you took one of these instruments down into a wine cellar and allowed it to assume room temperature and then measured the IR radiation flux coming from room’s walls and ceilings it would say that ~300 W/m2 is coming from each surface. In reality, the actual “heat flux density” within a wine cellar in W/m2 would be 0.00 since the entire room is in thermal equilibrium and there is no heat flowing from anywhere to anywhere. From where then does the ~300 W/m2 number on the radiometer readout come from? It is the calculated amount of IR radiation that the thermopile would emit if it were in a vacuum opposite a blackbody at 0 °K. In reality a radiometer in a wine cellar whose temperature has equilibrated with that environment should read 0.00 W/m2, because the amount of heat that is actually flowing from one wall to the other within a wine cellar is nil.
Let’s move then to the outside world. Again, neither the DLWR number 333 W/m2 nor that the ULWR number 396 W/m2 seen on the K-T Earth Energy Budget chart’s are measurements of actual radiant energy fluxes. Rather they are instead mathematical estimates of what the upward or downward flux would be if the other were absent. This is like measuring the wind speed to be 10 mph from the west, but calculating that there is actually a 20 mph wind coming from the east that is being opposed by a 30 mph wind coming from the west to yield a net 10 mph wind from the west! Just as wind only flows in one direction so to does thermal energy. Again, the empirical evidence that DLWR is completely extinguished by ULWR is the very radiometer that presumes to measure its presence. These radiometers detect 0.00 W/m2 downward heat flux; what they do sense is an upward radiant energy flux and then calculate what the DLWR would have been had it not been extinguished.
Here is the kicker. The IPCC in its soon to be released AR5 report* will again affirms that they consider DLWR to be substantively identical to insolation in that they just add the hypothetical number seen on the readouts of IR radiometers to the actual measured short wave radiant energy flux coming from the sun.
“The instantaneous RF (radiative forcing) refers to an instantaneous change in net (down minus up) radiative flux (solar plus longwave; in W m–2) due to an imposed change.” AR5 draft chapter 8
This sets up the perspective that the atmosphere is actually the surface of the earth’s primary heat source since the readouts on radiometers assert that the down-welling longwave radiant energy flux coming from the atmosphere is twice that of direct sunlight! In reality it is nil.
*PSI has a fully-searchable copy of the recently leaked AR5 draft report. PSI members may enjoy use of this facility by entering the back end of the site (enter ‘LOGIN’ details on the right of this page).
Carl Brehmer
End Note: In one of my career paths I obtained a degree in Electronics Technology and it was from that knowledge base that I did the above analysis of the internal operation of the Epply PIR using Epply’s description of their own instrument found in the Owner’s Manual. I challenge anyone to find within the Owner’s Manual of any radiometer the claim that they actually sense down-welling IR radiation. What you will find on close inspection is that these Owner’s Manuals will reveal that DLWR readouts are always a calculated hypothetical based on the two-way radiant heat transfer paradigm, rather than an actual direct measurement of DLWR.
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