Tall bloke too?


thefordprefect says:Your comment is awaiting moderation.

Bryan says: February 10, 2013 at 12:38 pm

…This is because DLWIR cannot be measured directly. Instruments that claim to measure DLWIR are measuring an INFERRED value. The pyrgeometer is a typical device

From the nrel site linked in my post

PRECISION INFRARED RADIOMETER Model PIR The Precision Infrared Radiometer, Pyrgeometer, is intended for unidirectional operation in the measurement, separately, of incoming or outgoing terrestrial radiation as distinct from net long-wave flux. The PIR comprises a circular multi-junction wire-wound Eppley thermopile which has the ability to withstand severe mechanical vibration and shock. Its receiver is coated with Parson’s black lacquer (non-wavelength selective absorption). Temperature compensation of detector response is incorporated. Radiation emitted by the detector in its corresponding orientation is automatically compensated, eliminating that portion of the signal. A battery voltage, precisely controlled by a thermistor which senses detector temperature continuously, is introduced into the principle electrical circuit.

Isolation of long-wave radiation from solar short-wave radiation in daytime is accomplished by using a silicone dome. The inner surface of this hemisphere has a vacuum-deposited interference filter with a transmission range of approximately 3.5 to 50 µm.


Sensitivity: approx. 4 µV/Wm-2. Impedance: approx. 700 Ohms. Temperature Dependence: ±1% over ambient temperature range -20 to +40°C. Linearity: ±1% from 0 to 700 Wm-2. Response time: 2 seconds (1/e signal). Cosine: better than 5%. Mechanical Vibration: tested up to 20 g’s without damage. Calibration: blackbody reference. Size: 5.75 inch diameter, 3.5 inches high. Weight: 7 pounds. Orientation: Performance is not affected by orientation or tilt. ————————-

This looks to me as if it is measuring the heating effect (thermopile) of radiation hitting the dome of the sensor (transmission 3.5 to 50um. The thermopile of course generates a voltage dependant on the temperature difference between one side and the other The non-dome side is not exposed to external radiation so no effect there. However, the nondome side temperature must be measured and compensated. The instrument also compensates for its own generated IR. No assumption of BB radiation is assumed. It is the ACTUAL heating effect of IR radiation of narrow or wide bandwith hitting the sensor that is the cause.

This all sounds as accurate as a liquid in glasss thermometer to me.

If the radiative “temperature” is less than the receiver temperature then the thermopile still measures – see my series of posts about thermal imaging – the camera microbolometers sitting at 20+C shows temperatures down to -40C


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