NL2014382B1

NL2014382B1 - Thermal sensor having two dome-shaped windows.

Info

Publication number: NL2014382B1
Application number: NL2014382A
Authority: NL
Inventors: Jan Van Den Bos Cornelis; Richard Hoeksema Eric
Original assignee: Hukseflux Holding B V
Priority date: 2015-03-03
Filing date: 2015-03-03
Publication date: 2016-10-14
Also published as: WO2016140565A1

Abstract

The invention relates to a detection device (1) with a housing (2), a sensor (5) in said housing, an inner window (6) and an outer window (7) both overlying the sensor (5). The invention is characterized in that the outer window (7) has a relatively high transmission coefficient below an upper wavelength value that is larger than the upper wavelength value of the inner window (6), wherein the outer window (7) has a thermal conductivity which is at least 2 times higher than the thermal conductivity of the inner window (6), preferably at least 4 times higher and most preferably at least 5 times higher,. The housing (2) includes a heating member (9) for transferring heat to the housing and to the inner and outer window for prevention of deposition of- and removal of moisture and/or ice.

Description

Thermal sensor having two dome-shaped windows Field of the invention

The invention relates to radiation detection devices with a housing, a sensor in said housing, an inner window and an outer window both overlying the sensor, the inner window having a relatively high transmission coefficient between a lower wavelength value of 100 nm and an upper wavelength value of 5000 nm and a thermal conductivity of between 1 and 5 W/(mK).

Background of the invention

It is commonly known in pyranometers (for instance model SRI 1, marketed by Hukseflux Thermal Sensors B.V., Delft, Netherlands) to utilize an inner and outer glass dome overlying the sensor. The two glass domes reduce wind related signal noise and thermal offset related error effects, resulting in improved measurement accuracy. Such a known device is described by the pre-characterizing part of claim 1.

Zero offsets and deposition of water on the instrument dome are important factors determining the reliability of measurements with pyranometers.

In traditional pyranometers, zero offsets, i.e. signals not related to the quantity to be measured, are a significant source of measurement uncertainty. Reduction of zero offsets is useful because this improves measurement accuracy.

The most significant offset is the sensitivity to far-infra-red radiation exchange, the “zero offset A” , as defined by the ISO 9060 standard which classifies pyranometers. Zero offset A is caused by the outer dome cooling down by radiation exchange with the sky, which is a relatively cold source of far-infra-red radiation. The balance of the far-infra-red radiation exchange from the outer dome to the sky is negative. The WMO manual and ISO 9060 define a reference condition of -200 W/m2, representing worst case conditions. The outer dome turns cold, and on its turn cools down the inner dome (3) by the same mechanism of radiation exchange. Finally the sensor produces a negative offset by its radiation exchange with the inner dome.

There are other significant offsets such as “zero offset B”, defined as the offset caused by heating or cooling the instrument with a fixed temperature rate of change of 5 K/hr. This temperature change produces internal temperature differences in the instrument. These differences not only cause far-infra-red radiation exchange but also generate energy flows to or from the sensor. Both mechanisms generate zero offsets adding up to zero offset B.

Heating a pyranometer, for example by using an electrical resistor or by heated ventilation air may independently produce zero offsets by the same mechanisms that cause zero offset B. Offsets caused by heating are not specifically mentioned or defined in the ISO 9060 standard. In practice they are an integral part of the measurement, and therefore part of the measured zero offset A and zero offset B. For one instrument model there may be offset A and B specifications with heating and without heating.

Deposited water on pyranometer domes leads to unpredictable and potentially very large but non-quantifiable errors. Deposition of rain and snow are quite common, but this usually goes together with cloudy conditions under which the measurement errors are small. Most pyranometers are located in moderate climate zones. Deposition of dew or frost on dome in the early morning regularly causes large errors. Water condenses on the dome because at night by far-infra-red radiation exchange with the sky these cool down to a temperature below dew point. A pyranometer with water deposited on the dome operates beyond its rated conditions. Prevention of deposition of water or fast removal of deposited water is useful because a dry dome is the rated condition for a reliable measurement. A dry dome also is unattractive for dust to stick to.

Lower zero offsets may be attained by improving thermal coupling between the thermal sensor, the instrument metal body and the inner dome. Better thermal coupling results in smaller temperature differences between these parts and thereby to reduced far-infrared radiation exchange.

For example, the model CMP22 pyranometer attains lower zero offsets than the otherwise equivalent model CMP11 by using two quartz domes with a higher thermal conductivity and larger thickness than the CMP11 domes. Zero offsets are reduced by a factor 2. A second example is that the inner dome acts as a radiation shield between the outer dome and the sensor, blocking radiation far-infra-red radiation exchange. By adding the inner dome to a pyranometer only employing a single outer dome, the far-infra-red radiation exchange from the dome to the sensor, and thus zero offset A, is reduced by a factor of approximately 1.5. This is illustrated by comparing zero offset A specifications of pyranometer models CMP11 and CMP3.

As a third example, high wind speed or artificial ventilation may reduce zero offset A by promoting thermal coupling between the pyranometer body and outer dome.

For zero offset B, the part of the zero offset caused by energy flows to or from the sensor may be reduced by symmetrically coupling a sensor to the instrument body, or by using a sensor with a low heat capacity. Some sensors employ a so-called compensation element.

Heating a pyranometer dome may help prevent dew and frost. A heated dome should have a temperature above dew point, so that moisture in the ambient air does not condense on it. In case water is deposited, heating accelerates evaporation of dew and rain, and promotes the process of sublimating or melting of snow and frost. To promote sublimation and melting, higher temperatures are beneficial. Melting requires a dome temperature above 0 °C.

The simplest option would be to directly heat a pyranometer, i.e. internally or with a heater connected to the instrument body, as opposed to externally via ventilation air. Using traditional pyranometers, already at low power levels, where heating is not yet effective to prevent humidity from condensing on the instrument dome, the added zero offsets caused by direct heating are beyond the specification limits of the ISO 9060 standard. The standards covering pyranometer use such as ISO TR 9901 therefore do not mention direct heating as a possibility. In some cases direct heating is nevertheless used, for instance in pyranometer model SR20, where it is typically switched on at night only when offsets do not matter because there is no sun. The zero offset caused by 1.5 W direct heating is - 8 W/m2 which is beyond the specification limits of ISO 9060 for the accuracy class. Applying direct heating at higher power, for example to promote evaporation and sublimation or to melt snow or ice is possible, but creates still larger errors and therefore is not mentioned in any standard.

The present invention aims at improving performance under moist and/or icy conditions while at the same time not compromising performance under normal fair weather. The invention furthermore aims at providing a detector with low maintenance and low power requirements and large data availability.

Summary of the invention A thermal sensor according to the invention is thereto characterized in that the outer window has a relatively high transmission coefficient below an upper wavelength value that is larger than the upper wavelength value of the inner window, wherein the outer window has a thermal conductivity which is at least 2 times higher than the thermal conductivity of the inner window, preferably at least 4 times higher and most preferably at least 5 times higher, the housing including a heating member for transferring heat to the housing and to the inner and outer window for prevention of water deposition and removal of moisture and/or ice.

By utilizing an outer window that has optical properties different from the optical properties of the inner window, it is possible to utilize a high thermal conductivity material. This improves the thermal coupling between outer window and the sensor and reduces the thermal offset. The potential error due to higher transmission of far infrared radiation from the sky and from the sensor through the outer window is corrected by using the inner window to filter out this far infrared radiation. The effective reduction of zero-offset by the high conductivity outer dome surprisingly allows active heating of the housing and window for removal of moisture and /or ice while maintaining a low zero offset and remaining within the performance limits of the thermal sensor class.

It is noted that in Journal of Geophysical Research vol. 110, D06107, 2005, Michalsky et al: diffuse irradiance working standard, on page 2 a sapphire outer dome is mentioned for the Kipp & Zonen detector model CM 22. However, no internal heating member in the housing is mentioned for evaporation of moisture from the windows or removal of ice. The known instrument is employed with conventional external ventilation.

In an embodiment of a thermal sensor according to the invention, the lower wavelength value of the outer window is between 200 nm and 500 nm and the upper wavelength value is between 4500 nm and 6000 nm.

In a pyranometer, a typical choice for an outer dome is sapphire, (transmitting from 0.2 to 6 x 10'6 m approximately), the typical choices for the inner dome are glass (transmitting from 0.3 to 3 x 10'6 m approximately) or quartz (transmitting from 0.2 to 4 x 10'6 m approximately). Sapphire has a typical nominal thermal conductivity of the order of 30 W/(m K), glass of 1.1 W/(m K), quartz of 1.4 W/(m K).

Using a film heater, which is attached to the body beneficially forces the temperature of all components to the body temperature.

To be effective preventing dew or frost deposition at night, the power level of heating must at least compensate for the energy loss of the pyranometer to the sky by far-infrared radiation exchange. As an example, with an instrument surface area of 0.008 m2 facing the sky and an estimated far-infra- red radiation exchange of - 100 W/m2, energy is lost by the pyranometer at a rate of 0.8 W.

The heating power of the heating member generally lies between 0.5 and 5 W. At a heating level of lower than 0.5 W and a typical pyranometer surface area, the heating power is not sufficient to keep the entire instrument above dew point. At a heating level of higher than 5 W, at a typical pyranometer surface area and construction, the offsets created by the heating become significant sources of measurement error.

As an example illustrating capability to work effectively against dew and frost at low power: for pyranometer model SR20 experiments show that in case the normal glass outer dome is replaced by a sapphire dome, the zero offset generated by 1.5 W direct heating is reduced by a factor 5, from 8 to 1.5 W/m2.

As an example illustrating capability to work effectively against dew and frost at low power:

During field tests in The Netherlands the 1.5 W heating power proves effective against dew deposition.

The 1.5 W power consumption and the offset in the order of 1.5 W/m2 of an SR20 with a sapphire dome compare favourably to the typical heated ventilator. To have similar effectivity pyranometer model SR20 and ventilation unit model VU01 require 11 W generating a 2 W/m2 offset.

Typically the heating will be limited to a specified maximum level of permissible heating, in W, at which the instrument still performs within certain target zero offset limits, in W/m2, for example the maximum limits as specified by the user or in the classification system.

The invention may be combined with traditional features of pyranometers such as indirect heating and external ventilation. More windows or domes may be added. It may be combined with model-based zero offset corrections for example from analysis of temperature measurements in the instrument or estimates of the far-infra-red radiation exchange between the instrument and the sky using a pyrgeometer. It may be combined with other measures to prevent zero offsets that are not included in the classification system. For example measures to reduce offsets induced by thermal shocks, such as increasing body weight or insulating the instrument body from contact with the ambient air.

Brief description of the drawing

An embodiment of a thermal sensor according to the invention will by way of example be described with reference to the accompanying drawing which is a partially cut-away perspective view of a pyranometer having a double dome window and a heating element according to the invention.

Detailed description of the invention

In the sole figure, a pyranometer 1 is shown with a housing 2 encompassing a detector body 3 and a thermal sensor 5. The sensor 5 is covered by an inner dome 6 and an outer dome 7 that are in thermal conducting contact with the detector body 3. The inner dome 6 is for instance made of glass, the outer dome 7 being made of sapphire. A film heater 9 is placed on the detector body 3. The heater 9 is powered via a signal and power lead 11, while signals are transported along the same signal and power lead.

Claims

A radiation detection device (1) with a housing (2), a sensor (5) in said housing, an inner window (6) and an outer window (7) both covering the sensor (5), the inner window (6) has a comparatively high transmission coefficient between a lower wavelength value of 100 nm and an upper wavelength value of 5000 nm and a thermal conductivity between 1 and 5 W / (mK), characterized in that the outer window (7) has a comparatively high transmission coefficient below an upper wavelength value which is greater than the upper wavelength value of the inner window (6), the outer window (7) having a thermal conductivity that is at least two times greater than the thermal conductivity of the inner window (6), preferably at least four times greater and more in in particular at least five times larger, the housing (2) comprising a heating element (9) for transferring heat to the housing and to the inner and outer to prevent the deposition and removal of moisture and / or ice.

The radiation detection device (1) according to claim 1, wherein the lower wavelength value of the outer window (7) is between 200 nm and 500 nm and the upper wavelength value is between 4500 nm and 6500 nm.

The radiation detection device (1) according to claim 1 or 2, wherein the outer window (7) comprises sapphire and the inner window (6) comprises glass or quartz.

The radiation detection device (1) according to claims 1, 2 and 3, wherein the inner and outer window (6, 7) are dome-shaped.

The radiation detection device (1) according to claim 1, wherein the heat generated by the heating means (9) substantially corresponds to the heat loss from the inoculation parts directed outwards, such as windows (6, 7) by infrared radiation.

A radiation detection device (1) according to any one of the preceding claims, wherein a power generated by the heater is between 0.5 and 5 W.