Pyrgeometer (Precision Infrared Radiometer): Everything To Know

Any object that has a temperature greater than absolute zero emits electromagnetic radiation. The wavelengths and intensity of the emitted radiation are proportional to the temperature of the item.

The atmosphere and terrestrial surfaces (e.g., soil, plant canopies, water, snow) emit radiation in the mid-infrared region of the electromagnetic spectrum (about 4-50 µm).

The pyrgeometer is an instrument that measures the rate at which these emitting surfaces lose heat to space by measuring the downward longwave (infrared) radiation.

It is the standard instrument for measuring the longwave downward radiation flux from the atmosphere to the surface.

In this post, we will discuss the essentials of Pyrgeometers, how they work, and their importance.

What is a Pyrgeometer?

A pyrgeometer is an instrument that measures the spectrum of infrared radiation in the atmosphere, which ranges from 4.5 μm to 100 μm.

It provides us with the difference in net longwave radiation between the detector surface and the surface to which it is pointed (usually, the atmosphere or ground).

Pyrgeometer achieves this by measuring the changes in resistance/voltage of a material that is placed in the path of the infrared radiation. The most common type of pyrgeometer uses a thermopile as the resistance-changing element.

Using the voltage/resistance changes and the material's properties, a pyrgeometer can calculate the amount of energy that has been transferred between the two materials. This data can be used to calculate the infrared radiation flux, which is a measure of radiation power transferred per unit area.

When making measurements the instrument takes into account a view factor between the two surfaces (the ratio of the area of the detector's field of view occupied by the target surface to the total area of the detector's field of view) as well as atmospheric variables like water vapor and clouds that can absorb or reflect longwave radiation.

Using this information, pyrgeometers can produce reliable estimates of the downward longwave radiation.

How does a Pyrgeometer work?

Pyrgeometers measure downward longwave radiation with a resistance-changing element. When the element is placed in the path of infrared radiation, its resistance/voltage changes proportionally to the amount of energy transferred.

A typical pyrgeometer consists of:

Housing,
Thermopile sensor,
Aperture,
Solar Blind Filter Coating,
Readout Electronics,
Temperature Sensor,
Sun Shield

Housing

The housing protects the instrument from the environment and provides a mounting for the other components. The housing also has an aperture that allows the infrared radiation to reach the sensor.

Thermopile Sensor

The thermopile sensor is the heart of the pyrgeometer. It consists of an array of thin metal wires (usually made of copper or nickel) that are connected at both ends.

When infrared radiation hits the thermopile sensor, the wires absorb some of the energy and heat up. This difference in temperature between the wires and their surroundings creates a voltage difference that can be measured.

The thermopile sensor is encased in a black material (usually black paint) to absorb as much infrared radiation as possible.

Aperture

The aperture is an opening in the instrument that allows infrared radiation to enter and hit the thermopile sensor. The size of the aperture is important because it determines the field of view of the instrument.

A larger aperture results in a wider field of view, while a smaller aperture results in a narrower field of view.

Solar Blind Filter Coating

Solar blind filter coating is a special coating that is applied to the aperture. It is designed to reflect visible light (wavelengths shorter than 4.5 μm) while allowing infrared radiation to pass through.

This helps to reduce the amount of sunlight that enters the instrument and heats up the thermopile sensor, which can affect the accuracy of the measurements.

Readout Electronics

The readout electronics are used to amplify the signal from the thermopile sensor and convert it into a digital signal that can be read by a computer.

They also include temperature compensation to account for changes in the thermopile sensor's characteristics with temperature.

Temperature Sensor

Temperature sensor is used to measure the temperature of the instrument. This information is used to correct for any temperature effects on the thermopile sensor.

When the instrument is temperature-stabilized, the thermopile sensor's characteristics do not change with temperature and the measurements are more accurate.

The temperature sensor is usually located inside the instrument, but some pyrgeometers have them external to the instrument.

Sun Shield

A sun shield to prevent the instrument from overheating due to solar radiation. The sun shield is usually made of aluminum or another material that reflects sunlight.

It is important to note that sun shield doesn't block all of the solar radiation, so the instrument. So that it can work as it is intended to.

Pyrgeometers are usually mounted on a mast or tower so that they can be above the surrounding area and have a clear view of the sky.

The instrument must be calibrated in order to take into account the amount of solar radiation that is entering the instrument.

Ventilation Unit (Optional)

In order to keep the instrument from overheating, some pyrgeometers have a ventilation unit.

The ventilation unit consists of a fan that blows air over the instrument and helps to dissipate the heat. This is a very useful improvement that can help to keep the instrument working correctly in hot climates.

What is the maximum irradiance a pyrgeometer can measure?

When the sky is clear and there is little precipitable water vapor, the downwelling longwave irradiance is about 250 Watts per square meter.

At a ground temperature of 25°C, the upwelling longwave irradiance is 350 Watts per square meter. That means the maximum difference in longwave irradiance that a pyrgeometer can measure is about 600 Watts per square meter.

Where are Pyrgeometers commonly used?

Pyrgeometers are commonly used in atmospheric and energy balance studies to measure longwave radiation from the sky and the radiation emitted by terrestrial surfaces.

Frost Prediction

Frost prediction models rely heavily on pyrgeometer measurements of longwave radiation. It is also possible to use pyrgeometers in conjunction with net radiometers in order to measure net radiation at the land surface.

Measuring Climate Change

Pyrgeometer data is used to estimate the downward longwave radiation. This information can be used to study the Earth's climate, as well as to understand how different surfaces affect the Earth's radiation budget.

Agricultural Studies

Pyrgeometers are also used in agriculture to study how different crops affect the downward longwave radiation. This information can be used to optimize crop yields and to study the effects of different land use on the climate.

Solar Energy Studies

Pyrgeometers are also used in solar energy studies to measure the longwave radiation from the atmosphere.

This information can be used to improve the efficiency of solar panels and to understand how different atmospheric conditions affect the amount of solar radiation that reaches the Earth's surface.

Building Energy Studies

Pyrgeometers are also used in building energy studies to understand how the longwave radiation affects the heating and cooling of buildings. This information can be used to reduce the energy consumption of buildings.

Why are pyrgeometers mounted on automatic solar trackers?

Although the pyrgeometer dome is supposed to reject all shortwave light (wavelengths less than around 4 microns), the covering of this hemispherical interference filter may “leak” and enable some shortwave radiation to reach the detector.

In order to ensure that only longwave radiation is reaching the detector, pyrgeometers are often mounted on top of automatic solar trackers which keep the instrument working in the shade and away from direct sunlight.

What effects do ventilators have on pyrgeometer performance?

Ventilators can keep the pyrgeometer dome clean by circulating air at room temperature over it.

The ventilator has no effect on the pyrgeometer data because the irradiance measurement is calculated from the measured thermopile output and the case and dome temperatures.

Is it important how to set up the pyrgeometer?

Pyrgeometers should be installed so that the dome is pointing directly upwards and the instrument is level. The pyrgeometer should be mounted in a shady location away from direct sunlight.

It is also important to make sure that the pyrgeometer is mounted securely and that there is no risk of the instrument falling over.

What maintenance is required for a pyrgeometer?

Pyrgeometers should be regularly calibrated and their domes should be cleaned. The frequency of calibration and cleaning will depend on the specific instrument and the environment in which it is used.

Pyrgeometers can be calibrated using a longwave calibrator or by comparing measurements with those from another pyrgeometer.

The dome should be cleaned when it becomes dirty or dusty. The best way to clean the dome is to use a microfiber cloth and distilled water. Avoid using cleaners or solvents on the dome as these can damage the interference filter.

Pyrgeometer calibration

Pyrgeometers can be calibrated using a number of different methods. The most common method is to use a blackbody radiator. A blackbody radiator is a device that emits longwave radiation at a known temperature.

By measuring the output of the pyrgeometer when exposed to the blackbody radiator, the sensitivity of the instrument can be determined.

Another method of calibration is to use a source of longwave radiation that has a known spectral distribution. This method is often used when calibrating pyrgeometers that will be used for solar energy studies.

Pyrgeometers can also be calibrated by comparing measurements with those from another pyrgeometer. This method is most often used when calibrating pyrgeometers that are part of a network.

Pyrgeometer accuracy

Pyrgeometers are typically accurate to within +/-5%. The accuracy of the instrument can be affected by a number of factors, including the calibration procedure, the environment in which the instrument is used, and the age of the instrument.

Pyrgeometers should be regularly calibrated to ensure that they are providing accurate measurements. The frequency of calibration will depend on the specific instrument and the environment in which it is used.

Where should pyrgeometer be oriented?

Although the World Meteorological Organization (WMO) suggests that the signal lead be oriented towards the nearest pole to minimize heating of the electrical connections, no special orientation of the instrument is generally necessary.

How to level pyrgeometer?

Accurate measurement of global radiation necessitates correct leveling of the thermopile surface. This can be achieved by means of an optical level or a laser level.

The instrument should be leveled in two perpendicular directions. In order to avoid parallax errors, the leveling should be done with the observer's eye in line with the leveling vial.

When installing a pyrgeometer, the instrument should be mounted in a shady location away from direct sunlight.

How accurate are pyrgeometers?

The accuracy of a pyrgeometer reading can be affected by many factors. The most common cause of inaccuracy is incorrect calibration.

Other factors that can affect the accuracy of a pyrgeometer reading include the environment in which the instrument is used, the age of the instrument, and the level of maintenance that has been performed on the instrument.

Pyrgeometers are usually accurate to +/-5%. However, the instrument's accuracy can be influenced by a variety of factors, including the calibration technique, the environment in which the instrument is used, and the instrument's age.

It is critical to calibrate pyrgeometers on a regular basis to guarantee that they are producing accurate results. Calibration frequency is determined by the instrument and the environment in which it is used.

What specs should I look for when buying a pyrgeometer?

The important specs to consider when purchasing a pyrgeometer include:

Measurement range

It is important to know the minimum and maximum longwave radiation values that the pyrgeometer can measure.

Accuracy

The accuracy of a pyrgeometer reading can be affected by many factors. Make sure to check the specs of the pyrgeometer you are considering to see how accurate it is.

Non-linearity

Non-linearity is the deviation of the output signal from a linear function of the input. It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Drift

Drift is the change in output over time when there is no change in input. It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Response time

The response time is the time it takes for the pyrgeometer to reach 90% of its final output value when exposed to a step-change in longwave radiation.

It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Thermal drift

Thermal drift is the change in output over time due to changes in temperature. It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Sensitivity

The sensitivity is the change in output per unit change in longwave radiation. It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Calibration interval

It is important to know how often the pyrgeometer needs to be calibrated in order to maintain accurate measurements.

Operating Temperature Range

It is important to know the minimum and maximum temperatures at which the pyrgeometer can operate.

Window heating offset

Window heating offset refers to the heating of the pyrgeometer window by longwave radiation. It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Dome emissivity

The dome emissivity is the ratio of the longwave radiation emitted by the dome to the longwave radiation emitted by a blackbody at the same temperature.

It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Dome temperature

The dome temperature is the temperature of the pyrgeometer dome. It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Field of View FOV

The FOV is the angular extent of the scene that is visible to the pyrgeometer. It is important to consider when choosing a pyrgeometer because it can affect the accuracy of the measurements.

Cable Length

Cable length is the length of the cable that connects the pyrgeometer to the data logger.

It is important to consider when choosing a pyrgeometer. Because the cable between the pyrgeometer and the data logger should be sufficient enough to allow the pyrgeometer to be placed in the desired location.

Cables are typically 10 to 50 meters long make sure you get the appropriate length cable for your needs.

Data Logger Compatibility

It is important to make sure that the pyrgeometer you choose is compatible with the data logger you are using.

Pyrgeometer Weight

The weight of the pyrgeometer can be important when choosing a pyrgeometer.

Ventilation Unit:

The ventilation unit is a device that is used to cool the pyrgeometer. It allows for the heat generated by the pyrgeometer to be dissipated. The ventilation unit is typically a fan that is mounted on the back of the pyrgeometer.

The ventilation unit is an optional improvement for the pyrgeometer. However, it can be a great addition if the pyrgeometer is going to be used in a hot environment.

Albedo mounting kit

Albedo mounting kit is an accessory that can be used to improve the accuracy of the pyrgeometer measurements.

The albedo mounting kit allows for the pyrgeometer to be mounted in the sun. This allows for the pyrgeometer to measure the albedo of the surface.

The albedo mounting kit is an optional accessory. However, it can be a great addition if the accuracy of the pyrgeometer measurements is important.

Conclusion

Pyrgeometers are a type of instrument that is used to measure longwave radiation. They are typically used in meteorology and climatology. Pyrgeometers can be used to measure the albedo of a surface.

When choosing a pyrgeometer, it is important to consider the thermal drift, sensitivity, calibration interval, operating temperature range, window heating offset, dome emissivity, dome temperature, field of view (FOV), cable length, data logger compatibility, and pyrgeometer weight.

The ventilation unit and albedo mounting kit are optional improvements for the pyrgeometer.

However, they can be great additions if the pyrgeometer is going to be used in a hot environment or if the accuracy of the pyrgeometer measurements is important.

Finally, make sure to choose a pyrgeometer that is compatible with the data logger you are using. Use your pyrgeometer according to the manufacturer's instructions to get the most accurate measurements.