This section includes results of preselection according to Criteria 1 and 2, selection according to Criteria from 3 to 5, and post-selection, applied if needed. Experiments for selection purposes were conducted in Krakow, and pre-selection was carried out on the basis of meteorological data from this city and the specifications of particular sensors. The final results of the selection process are summarized in the last subsection.
5.1. Pre-Selection
The sensor selection, based on experimentation, follows the sensor pre-selection based on the cost of the sensors and the technical literature review. The aim of this sensor pre-selection is to choose a set of low-cost sensors that are able to take weather measurements to the extent known from site-dependent historical data. During the pre-selection, the maxima and minima of weather factors measured over the whole of the 20th century were taken as the basis for the determination of the minimal boundaries of the ranges of weather sensors.
In Krakow, atmospheric pressure has been accurately measured since 1792, including throughout the whole of the 20th century, at the same measuring point inside the Śniadecki College building of the Jagiellonian University, 220 m above sea level and not reduced to sea level [
30]. The minimum atmospheric pressure recorded in Krakow in the 20th century was 978.2 hPa (1915), and the maximum one was 1004.5 hPa (1959) [
30]. These values are inside of the range of both atmospheric pressure sensors listed in
Table 4, so they both passed the pre-selection. Low-cost atmospheric pressure sensors that pass the pre-selection were able to perform measurements from 300 hPa to 1100 hPa.
Temperature measurements also were carried out in Krakow from 1792, and in the 20th century the minimum registered air temperature was −32.7 (1929), and the maximum one was 37.4 (1921) [
31]. Thus, the minimum value measured by temperature sensors that pass the pre-selection was −40
C, and the maximum one was at least +80
C. This condition was met by 9 out of 10 sensors listed in
Table 5. The popular and often used DHT11 low-cost temperature and humidity sensor was not able to pass pre-selection for the weather station operating in Krakow, because the temperature sensor does not offer a suitable operating range. The old series of DHT11 were able to measure ambient temperatures from 0
C to +50
C (instead of at least from −33
C). Although DHT11 specifications indicate the operating range of new series is from −20
C to +60
C, it is still insufficient, because temperatures below −20
C are not uncommon in winters in Krakow.
Humidity measurements have been performed in Krakow since 1830 [
32]. The lowest recorded relative humidity was 23% (1906), and the maximum one was 100% (the state of saturation, which was achieved from time to time) [
32]. Humidity sensors that pass pre-selection were able to measure relative humidity over the entire range, from 0 to 100% (
Table 6). This condition was met by almost all sensors except DHT11, i.e., the same sensor that was excluded from the further temperature measurements. The operating range of the old series of DHT11 is 20% to 90%, and also in this case the specification of the new series of this sensor gives a wider operating range (5% to 95%) when compared to the old ones. However, it is still not enough to measure humidity in Krakow, which is in a valley, on the banks of the greatest Polish river, where relative humidity sometimes reaches 100%.
Only two decades have passed since the World Health Organization (WHO) introduced the global solar ultraviolet (UV) index [
33]. Thus, long-term measurements in the 20th century were, for obvious reasons, impossible. As a result, we decided not to apply Criterion 2 for pre-selection of UV index sensors with the use of other historical data. We limit our considerations to Criterion 1 only, which is met by both considered sensors. Nevertheless, ranges of considered UV index sensors are high. Although the SI1145 documentation lacks precise data on the scope of work, the graph presented here shows the range of the UV index from 0 to about 10. For VEML6070, the documentation only gives a maximum 5
W/cm
(UVA) power of 328 mW/cm
, which when converted gives a UV index of approximately 11. It is therefore safe to assume that both sensors are able to measure the UV index in the range of 0 to at least 9.
5.2. Selection
Sensors that passed the pre-selection were the subject of an actual selection, based on Criterion 4 and Criterion 5. This section presents three examples of selections, which differ in the number of sensors participating in the selection and the results obtained. They are: selection carried out on a small set of atmospheric pressure sensors (selection criteria were met by all sensors, there is a clear result of selection), selection carried out on a larger set of air temperature and relative humidity sensors (selection criteria were not met by some sensors, there is a clear result of selection), and selection that was carried out on a small set of UV index sensors (selection criteria were met by all sensors, no decisive results of selection).
If the sensor selection experiments do not bring decisive results, two decisions are possible. The first one is that all sensors that pass the selection will be used in the weather station. The second one is that post-selection will be performed. The post-selection may be based on the analysis of features of the selected sensors to find the most useful one (or ones). But just as well, the determination of the sensor can be done randomly or it can be an arbitrary decision which sensor will be used in the weather station.
5.2.1. Atmospheric Pressure Sensors
As was presented in previous section, both considered atmospheric pressure sensors, namely the Bosch Sensortec BMP280 and the Bosch Sensortec BME280, passed the pre-selection. These sensors were then laboratory-tested in the flying weather station in terms of response time (the electrical one).
The results collected in
Table 7 show that the response times achieved by the BMP280 and the BME280 being a part of the fully operational flying weather station were usually slightly slower (maximum 0.6 ms slower, on average 0.3 to 0.4 millisecond slower) than the nominal ones included in the specification of each sensor. During laboratory tests, the Bosch Sensortec BME280 was not able to achieve its nominal response time, and its mean response time was 0.1 ms slower than the one determined for the Bosch Sensortec BMP280. Nonetheless, such a small difference in response time, as described above, cannot be considered as decisive for weather monitoring.
Because during field pre-tests both analyzed sensors met the minor criteria (
Table 8), the final decision was taken on the basis of the additional capabilities of the atmospheric pressure sensors. The Bosch Sensortec BMP280 offers a limited capability relative to the Bosch Sensortec BME280 (pressure and temperature measurements vs. pressure, temperature, and humidity). As a result, the BME280 was selected to be one of the input devices of the flying weather station.
5.2.2. Temperature and Humidity Sensors
The classic problem-solving principle, formulated by the 17th-Century philosopher Johannes Clauberg states “entities should not be multiplied beyond necessity” (“entia non sunt multiplicanda praeter necessitatem”). Thus, the typical sensor’s selection presented in the literature assumes that the use of the two-in-one and the three-in-one weather sensor (usually, temperature and humidity in one, and atmospheric pressure, temperature, and humidity in the other) entails the necessity of the use of this sensor for all possible measurements. According to this common practice, the natural candidate for the temperature and humidity sensor of the weather station was the Bosch Sensortec BME280, chosen in the previous subsection for atmospheric pressure measurements.
The ability to fulfill the major criterion by particular temperature and humidity sensors is summarized in
Table 9. It was checked on the basis of the results of experiments presented in the article [
17], which include tests of the UAV-based and WebRTC-based framework co-operating with the same sets of popular low-cost sensors. However, when compared to other fast response sensors, with the mean response time of 12–13 ms the BME280 turned out to be one of the three slowest temperature sensors (besides the Bosch Sensortec BMP280 and Measurement HTU21D), and the slowest humidity sensor. Nonetheless, it still met the major criterion for sensor selection (response time was small enough to enable proper work of this sensor in the target monitoring system), and its response time was two orders of magnitude smaller than the response times of the popular Aosong DHT22, or Aosong AM2320 sensors, which are used, among other things, in weather stations, and which do not fulfill the major criterion (
Table 9).
In the second stage of selection, the Bosch Sensortec BME280 achieved the worst quality of temperature information of all tested temperature sensors. It often happened that it was the worst of all tested temperature sensors (
Table 10). It frequently happened that the error bound of the temperature measurements of the reference station (±1
C) was exceeded, which made the measurement non-satisfactory in terms of believability. This meant that the Bosch Sensortec BME280 did not pass the selection in terms of temperature, so it cannot be used as the primary sensor and should not be used as the secondary one for temperature measurements for weather station purposes.
The Bosch Sensortec BME280 was one of the three sensors that met the minor criteria during humidity measurements and, as a result, passed the selection in terms of the humidity measurement. The other ones were the Sensirion SHT30, which achieved the worst accuracy of these three sensors, and the Measurement HTU21D, which achieved the best accuracy of all tested humidity sensors (
Table 10). Although both SHT30 and HTU21D have passed minor criteria in terms of temperature (
Table 10), the most accurate temperature sensor turned out to be the Silicon Laboratories Si7021. It, in turn, failed its selection as a humidity sensor. Therefore, if the principle of “entities should not be multiplied beyond necessity“ was disregarded, or if, in the light of the results summarized in
Table 9, it is necessary to use two different sensors, the Si7021 and the HTU21D would be selected as a result of the selection. Otherwise, a choice must be made between Sensirion SHT30 and Measurement HTU21D.
The SHT30 better reflects the temperature measurements made by the reference station than the HTU21D. In the case of the humidity measurement, the opposite is true. In other words, the SHT30 achieved a greater LoA in measuring temperature, and the HTU21D in measuring humidity. Because the LoA of the SHT30 in humidity is close to the LoS, and the LoA of the HTU21D in temperature is in the middle between the perfect fit and the LoS, in the post-selection the HTU21D was designated as the temperature and humidity sensor.
5.2.3. UV Index Sensors
The selection of the UV index sensors was carried out by using the Silicon Laboratories SI1145 and the Vishay Semiconductors VEML6070. Generally, the VEML6070 is less convenient to use as a UVI sensor. As is shown in
Table 11, the VEML6070 has a relatively slow response time—an order of magnitude slower than was measured for the SI1145. Bearing in mind the fact that this slow response time is mainly influenced by the long integration time (the standard integration time is 125 ms), and a longer integration time usually goes hand in hand with higher accuracy, this cannot be considered a disadvantage. Especially considering the longer integration time is in the order of tenths of a second, not seconds. Consequently, despite the relatively long response time of the VEML6070, both sensors fulfill the major criterion.
During field experiments, both sensors achieved comparable worst-case results (
Table 12). At this point, it is worth mentioning that the manual of the SBS-WS-400 weather station [
34], used as the reference weather station, does not offer precise accuracy of the UV index measurement. Due to the lack of precise data, we have assumed that for selection purposes the measurement accuracy is half a gradation, i.e., 0.5. On this basis, we concluded that both sensors fulfilled the minor criteria.
The analyses shown in
Table 12 are necessarily very coarse and did not produce a conclusive result. The decision should therefore be made at the post-selection stage. However, both the Silicon Laboratories SI1145 and the Vishay Semiconductors VEML6070 have good ambient temperature compensation (from −40
C to +85
C). They are both equipped with internal integration circuits and low-pass filters. Because the major criterion, and the two minor criteria showed that there is no clear advantage of either sensor, nor any clear contraindication, both of these sensors were found to be equally useful for a weather station. As a result, one should now choose one of them in any way, for example by drawing lots, or by installing both sensors in the target weather station and continuing their tests.
5.3. Discussion
The low-cost atmospheric pressure sensors market is currently dominated by a very successful design by one of the manufacturers. As a result, the solutions listed in
Table 1 use only three types of low-cost air pressure sensors. They are the Bosch BMP180 [
19,
20,
22,
24,
28], the Bosch BMP280 [
23] and the Bosch BME280 [
2,
7,
9,
22]. The BMP180 is the older version of the BMP280. The use of the old version, which currently has an end of life (EOL) status, instead of the new one, was usually explained by a search for easily available and cheap components.
Almost the same set of sensors (with the exception of the BMP180, which has lost its value as a budget solution in Europe: the currently available BMP180s are more expensive than the newer BMP280s) were considered as atmospheric pressure sensors for weather station purposes. The pre-selection was passed by both BMP280 and BME280. The BMP280 and the BME280 also passed the selection. The three-in-one (air pressure, temperature, and humidity) BME280 sensor was chosen in post-selection, as potentially more useful than the two-in-one (air pressure and temperature) BMP280. However, in light of the selection criteria, the decision to use the BMP280 was equally legitimate.
The same situation concerning the lack of a variety of solutions, albeit for a different reason, occurred in the case of the UV Index sensors. Most of the weather stations known from the literature do not yet implement the UV index measurement. In the case of the solutions listed in
Table 1 only two such sensors were used. They are the UV index sensor being a part of the multi-sensor Xiaomi Plant Monitor device [
27] and the Silicon Laboratories SI1145 [
7]. The PlantMonitor does not directly measure the UV index but is said to have a wide range and is accurate. The indirect method of measurements was also used in the case of the Vishay Semiconductors VEML6070 that was the one of two UVI sensors that was selected for the use in the flying weather station. The second one was the SI1145, also selected in [
7].
Overall, the above comparison of the selection results shows that although both the measurement range (pre-selection criterion) and the IQ dimensions used in the selection criteria do not contraindicate the use of some sensors, the results of our selection are consistent with the mean observed in the literature.
The group of temperature and humidity sensors is very diverse, and there is no sensor that is used the most. In papers [
19,
21] the Adafruit DHT11 was used, as it was considered very cheap and popular. This sensor does not meet the Criterion 2 of our pre-selection, carried out for use in Krakow, due to the too-high minimum measured temperature (
Table 5). However, according to the same Criterion 2 (as a reminder, this is a site-dependent sufficient range of measurements), authors from Khulna (Bangladesh) [
19] and Phagwra (Punjab, India) [
21], can use this sensor because there is no frost in these locations.
A relatively large group of papers [
20,
22,
24,
28] considered another sensor produced by Adafruit, namely the DHT22. The paper [
8], in which the SparkFun RHT03 is used, should also be included in this group. The RHT03 is a DHT22 clone with its pros and cons. The criteria that led to the selection of the RHT03 as the most suitable for temperature and humidity measurements in a small UAV were weight, size, range, resolution, and cost [
8]. The authors of the paper [
24] chose DHT22 because they were very keen on the low cost and availability of parts. In the paper [
22], the DHT22 was tested as one of three possible sensors but, ultimately, it was not the best. The DHT22 was also used in [
20] for measurements of temperature. In our selection, this sensor was too slow to pass the major criterion, but in the above work it was either not considered or, as in the case of [
20], it was negligible. The paper [
28] presents another disadvantage of the DHT22, namely it less closely approximates the distribution of a typical mercury thermometer than other tested sensors, including the BMP180. In our selection, because this sensor did not pass the major criterion, it was not checked against the minor criteria, i.e., the accuracy of the measurements.
As mentioned in
Section 5.2.2, solutions using the three-in-one sensors typically use them for all three possible measurements. As an example, in the works [
2,
7,
9,
22] the Bosch BME280 was used. In [
7,
9], the BME280 was installed on a UAV as a combined ambient sensor. The BME280 was used in stationary weather station [
2] because it is integrated, has low power consumption and small size. The paper [
22] compares three sensors (the BME280, DHT22, and BMP180) in terms of the use in stationary weather stations, and the BME280 was indicated as ideal for this application due to its interfaces and the integration of all three measurements. In our selection, the BME280 was rejected as the temperature sensor (it does not meet the minor criteria—
Table 10) and passed the selection as the humidity one.
The same rule, mentioned in
Section 5.2.2, was used for the Bosch BMP280 and its predecessor, the Bosch BMP180, intended for the atmospheric pressure and temperature measurements. In [
23], the BMP280 and the Texas Instruments HDC1080 were used as temperature sensors, and the HDC1080 was the humidity one. Both these sensor passed our selection as temperature sensors only. In [
20], both the Bosch BMP180 and the Adafruit DHT22 (which do not pass the major criterion of our selection in temperature) were used as temperature sensors, and the DHT22 was used as the humidity one. The same set of sensors, enriched with the Sensirion SHT21, was used in [
28], where the BMP180, the DHT22, and the SHT21 were used as temperature sensors, and the DHT22 and the SHT21 were used as humidity sensors. In our selection, the BMP180 has not been tested, but according to the technical literature, it is to be slightly worse than the BMP280, which took part in the proposed selection.
The SHT10, mentioned above as used in [
20], and other products of Sensirion, such as the SHT75 used in [
25], and the SHT21 used in [
26] had either the EOL status (SHT10, SHT75) or were not recommended for new designs (SHT21). In the case of the SHT10, the results accuracy was satisfactory to the authors [
20]. The SHT75 was chosen for its resolution and stability over different atmospheric conditions [
25]. The SHT21 was used for the needs of a low-cost system for use on farms with minimal power requirements (so that it can be powered by solar panels) [
26]. In this paper, the newer Sensirion products, SHT30 and SHT35, with similar technical parameters to the SHT75 and the SHT21, were a subject of the proposed selection. Both passed the selection as temperature sensors, and the SHT30 also as a humidity sensor.
The sensors selected with the use of a preliminary and less-formalized version of our selection criteria were installed on the target weather station [
18], built with the use of the UAV and WebRTC-based universal framework [
17]. Results of long-term tests of this flying weather station, which lasted from early June to late November [
18] confirm the good accuracy of the selected set of sensors.