Humidity is a term for the amount of water vapor in the air. Relative humidity (RH) is defined as the ratio of the partial pressure of water vapor (in a gaseous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. So, simply put, RH is the amount of water vapor in the air at a specific temperature compared to the maximum water vapor that the air is able to hold without it condensing, at a given temperature. It is an important metric in weather forecasts since it is an indicator of the likelihood of precipitation, dew, or fog.
Types of humidity sensors
Humidity sensors¹ are usually of the capacitive or resistive type. Capacitive sensors have a more linear response than the resistive sensors (these have nearly a log response but are high sensitivity at low humidity). Capacitive sensors are also usable over the entire range of 0 to 100 percent relative humidity, where the resistive element is typically limited to about 20 to 90 percent relative humidity.
Capacitive relative humidity sensors typically use an industrial-proven thermoset polymer, three-layer capacitance construction, platinum electrodes, and except for high- temperature versions, some have on-chip silicon integrated voltage output signal conditioning. These sensors work on a delta C or change in capacitance with humidity level. AC excitation, instead of DC, is used to prevent polarization.
Capacitive sensor signal conditioning
Multiple circuit architectures may be used with a capacitive humidity sensor. Extra care must be taken during the layout of the pc board in these designs. Any stray capacitance must be minimized in the layout since any added capacitance will act as a parallel capacitance with the sensor and create a measurement error.
A careful conformal coating of the pc board and components is advised to prevent unexpected deviations in FOUT of the 555 timers, especially in high humidity conditions.
The first and simplest is the capacitance-to-frequency conversion circuit. Figure 1 shows the circuit diagram using the Honeywell HCH-1000 series, a polymer-based sensor with a delta C form factor.
Figure 1: Capacitance to frequency conversion circuit using a simple 555 timer. (Courtesy of Honeywell.)
The HCH-1000 is a variable capacitor connected to the THRES and TRIG pins. RV is a variable resistor that compensates the output frequency about the basic capacitance of the HCH-1000. The external capacitor charges through R1+(Rv+R2) and discharges through (Rv+R2). Thus, the duty cycle may be precisely set by the ratio of these two resistors if the duty cycle is close to 50 percent. Therefore, the frequency is independent of the supply voltage. This method gives an accuracy of less than ±3.9 percent RH.
Next, we have a voltage output circuit², generated by two 555 timers, the first timer in Figure 2 provides a continuous timing pulse to trigger the second timer which operates in pulse width modulation (PWM) mode. The second timer frequency output goes to a low-pass filter (LPF) and amplifier made up of the dual LM2904 op amp. This circuit method is known as a one-point calibration voltage output. This gives a voltage output with an accuracy of less than ± 2.7 percent RH.
Figure 2: Circuit for a one-point correction in a capacitance to voltage conversion architecture. (Courtesy of Honeywell.)
Then, in order to get the best accuracy, a two-point calibration method (shown in Figure 3) is used giving an accuracy of less than +/- 0.5 percent RH.
Figure 3: Circuit for a two-point correction in a capacitance to voltage conversion architecture. (Courtesy of Honeywell.)
Finally, there are capacitance-to-digital converters available such as Analog Devices’ AD7147, which provide a highly integrated way to convert capacitance to a digital serial output using a capacitive humidity sensor directly connected to its input.
As a final capacitive humidity sensor solution for a specific, but popular, product area, TDK has a series of ultrasonic nebulizer units with their NB family of devices. These are fully integrated with electronics and ready to install into a nebulizer product design.
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