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.