1、 1 中文 2386 字 英文 : The right design for a relative humidity sensor system Optimizing the response characteristics and accuracy of a humidity sensor system 1 Overview To make the right choice when selecting a relative humidity sensor for an application, it is important to know and to be able to judge
2、the deciding factors. In addition to long-term stability, which is a measure on how much a sensor changes its properties over time, these factors also include the measurement accuracy and the response characteristics of the sensor. Capacitive humidity sensors are based on the principle that a humidi
3、ty-sensitive polymer absorbs or releases moisture as a function of the relative ambient humidity. Because this method is only a spot measurement at the sensor location, and usually the humidity of the surroundings is the desired quantity, the sensor must be brought into moisture equilibrium with the
4、 surroundings to obtain a precise measurement value. This process is realized by various transport phenomena (cf. the section titled The housing effect on the response time), which exhibit a time constant. Accuracy and response time are thus closely dependent on each other, and the design of a humid
5、ity measurement system becomes a challenge. 2 Measurement accuracy The term measurement accuracy of a humidity sensor is understood primarily to refer to the deviation of the value measured by the sensor from the actual humidity. To determine the measurement accuracy, references, such as chilled mir
6、ror hygrometers, whose own tolerance must be taken into account, are used. In addition to this trivial component, humidity sensors require a given time for reaching stable humidity and temperature equilibrium (the humidity is a function of temperature and decreases with increasing temperature； a dif
7、ference between sensor and ambient temperature leads to measurement errors). This response time thus has a significant effect on the value measured by the sensor and thus on the determined accuracy.This time-dependent characteristic is explained in more detail in the following. 3 Response characteri
8、stics and response time The response characteristics are defined by various parameters. These are: The actual response characteristics of the humidity sensor at constant temperature. (1) How quickly the sensitive polymer absorbs or releases moisture until equilibrium is reached (intrinsic response t
9、ime) 2 (2) How fast the entire system reaches humidity equilibrium (housing effect) The thermal response characteristics of the humidity sensor at a non-constant temperature (3) The thermal mass of the sensor (4) The systems thermal mass, which is thermally coupled to the sensor (e.g. printed circui
10、t board) (5) Heat sources in the direct surroundings of the sensor (electronic components) (1) and (3) are determined entirely by the sensor itself, (1) primarily by the characteristics of the sensitive polymer. (2) and (4) are primarily determined by the construction of the entire system (shape and
11、 size of housing andreadout circuitry). (5) is determined by heat-emitting electronic components. These points will be discussed in more detail in the following. The intrinsic response time (1) Qualitatively, the response characteristics of capacitive humidity sensors look like the following (Fig. 1
12、). Fig. 1: Typical and idealized response characteristics of capacitive humidity sensors (schematic) Because these response characteristics are especially pronounced at high humidity values, an isothermal humidity jump from 40% to 100% was selected here for illustration. The desired ideal behavior o
13、f the sensor is indicated in blue. In practice, however, the sensor behaves according to the red line, approximately according to: RH -t（ t ） =(E-S)*(1-e )+S 3 Here, the time span 1 is usually very short (typ. 1 30 min.), in contrast, the time span 2 is very long (typ. Many hours to days). Here the
14、connection of measurement accuracy and response characteristics becomes clear (t until RH=100% is reached). The value at t4 (Fig. 1) is considered to be an exact measured value. However, this assumes that both the humidity and also the temperature remain stable during this entire time, and that the
15、testing waits until this very long measurement time is completed. These conditions are both very hard to achieve and unusual in practice. For the calibration, there are the following two approaches, which both find use in practice (cf. Fig. 2): 1.The measured value at t2 (Fig. 1) is used as a calibr
16、ation reference. Advantage: The required measurement time for reaching the end value (in the example 100%) is clearly shortened,corresponds to practice, and achieves an apparent short response time of the sensor (cf. Fig. 2). Disadvantage: If the conditions are similar for a long time (e.g., wet per
17、iods in outdoor operation), the sensors exceed the correct end value (in the example 100%) undesirably by up to 10% (cf. Fig. 2). 2. The measured value at t4 (Fig. 1) is used as a calibration reference. Advantage: Even for similar conditions over a long time (e.g., wet periods in outdoor operation),
18、 an exact measurement result is obtained (cf. Fig. 2). Disadvantage: For a humidity jump like in Fig. 1, the sensors very quickly deliver the measured value at t2, but reaching a stable end value (about 3-6% higher) takes a long time (apparent longer response time)(cf. Fig. 2). In order to take into account both approaches optimally, the measured values at t3 (cf. Fig. 1) are used as the calibration reference by Sensirion AG.
