超声雾化加湿系统英文文献和中文翻译(3)

菜单

Relative humidity measurement system

The Monolithic IC (HIH3605A, Honeywell)-type hu- midity sensor measures the RH of the incubator environment. If the humidity level of the incubator environment changes, the capacity of the IC sensor also changes. These changes are then converted to a voltage by internal signal conditioning circuits.

The sensor has a linear voltage output in the range

0.8 – 3.9 V versus RH% of the environment in the range 0 – 100% RH. That is; 0.8 V represents 0% and 3.9 V 100% humidity. Linearity of the sensor is + 0.5% RH. It has fast response time of 15 s even in slowly-moving air at 258C, low-drift performance and is stable with respect to time. Response time of the sensor is reasonable with respect to the others. The accuracy of the sensor is

+ 2% RH in the range 0 – 100% RH in non-condensing ambient air at 258 C [6]. The nebulizer does not produce saturated air. However, the water particles produced by the nebulizer are circulated into the incubator chamber. They are heated by the tempera- ture of the incubator chamber. Therefore, water particles are converted into water vapour. If the condensation happens in the incubator chamber, this affects the accuracy of the sensor. However, an extended exposure 490% RH causes only a reversible shift of 3% RH of the sensor output. In other words, the measured RH by the sensor is 5 3% RH than the ambient RH level.

The operating temperature of the sensor is in the range – 40 to 858 C. The accuracy of the sensor changes with temperature. The effect of the temperature is only

+ 0.007% RH/8C at 0% RH, and – 0.22%RH/8C at

100% RH. In addition to these, the sensor is sensitive to electrostatic discharge (ESD) [6]. Although the sensor has been protected by internal protective diodes against the ESD 516 kV, some precautions have been taken. Two fast static protective diodes were connected to the input and output pins of the sensor. In addition, the sensor is placed in a small aluminum can, which has

one open end. Since the sensor is light sensitive, for the best result, it should be shielded from bright light, such as sunlight, phototherapy, etc.

The block diagram of the IC-type humidity measure- ment circuit is shown in figure 2. The power of the measurement system is obtained by mains on the AC/ DC converter. A reference supply provides the power for the sensor. Since the reference supply has a low drift performance and its output is stable with respect to time, measurements can be obtained more accurately. The output voltage of the reference supply was 2.5 V. As the drift of the reference supply was around 50 ppm/ 8C, the temperature dependency of the measurement circuit was very low. The buffer circuit consisted of an op-amp that provided high-input and low-output impedance for the sensor. Since the inherent design of the RH sensor caused it to be sensitive to ESD, the ESD protection circuit had been used both at the sensor input and output, as explained above.

The zero and span circuit was used at the output of the RH measurement circuit. This circuit shifted the minimum and maximum level of the input signal that came from RH sensor. Since the RH sensor produced

0.8 V versus 0% RH, and 3.9 V versus 100% RH, these voltage levels are shifted 0 – 4 V respectively by the zero and span circuit. As a result, 0 V represents 0% RH and 4 V 100% RH. In this way, 0 – 4 V output voltages versus humidity changing in the range 0 – 100% RH were obtained in the incubator environment.