This document describes the development of a digital hygrometer system using a low-cost capacitive moisture sensor to measure trace moisture in the range of 3.7-100 ppm. The sensor consists of a nonporous thin film of alumina coated between two parallel gold electrodes. An AVR microcontroller is used to convert the analog sensor data to digital and calibrate it to a moisture measurement in ppm. Testing showed the system achieves an accuracy of ±1 ppm when compared to a commercial dew point meter.

1. A Digital Hygrometer for Trace Moisture
Measurement
ABSTRACT
A microcontroller based digital hygrometer system using a low cost moisture
sensor in the range of 3.7-100 ppm has been developed. The sensor is of capacitive
type and consists of nonporous thin film of alumina (γ-Al2O3) dip-coated in
between two parallel gold electrodes onto an alumina substrate. Behaviour of the
sensor will be observed by using AVR microcontroller which convert the analog
data to digital using A/D converter .
The converted digital value is calibrated in terms of ppm. The accuracy of the
digital hygrometer when compared with commercial dew point meter is ± 1PPM.
INTRODUCTION
Humidity sensors play an important role in process industries and environment
control. In domestic applications, it is used for humidity control in microwave
ovens, in buildings, and in laundry etc.
Humidity sensors are also used for green-house air-conditioning, plantation
protection (dew prevention), soil moisture monitoring, and cereal storage in
agriculture. In industry, humidity sensors are used for chemical gas purification.
The type of humidity sensor required depends on the measurement range and
application. The humidity sensors may be of capacitive type, resistive type, optical
and gravimetric but large number of commercial humidity sensors are of capacitive
type because of its high sensitivity
In capacitive type, when a thin porous dielectric layer between the parallel
electrodes absorbs moisture, its apparent dielectric constant increases and thus
increasing capacitance. Parallel plate structure may be suitable for developing low
cost trace moisture meter.

2. A microcontroller based digital hygrometer has been developed to measure
moisture by converting analog data to digital using A/D converter.
The capacitance of the sensor changes nonlinearly with ppm but almost
independent with change in temperature and one can neglect its change for
accurate measurement
Methodology
The methodology we are using in making digital hygrometer is AVR
microcontroller. The analog data from the sensor is converted to digital using
analog to digital converter.
AVR microcontroller has 10 bit resolution with different sampling frequency. The
converted digital value is calibrated in terms of PPM. This PPM value will be
displayed on LCD.
Block Diagram

3. Circuit Diagram:-
Hardware used
1) AVR Microcontroller
2) Humidity Sensor
3) Liquid crystal display
4) Power supply
5) AVR programmer
6) PCB board
Software Used:-
1) Proteus (Simulator)
2) AVR Studio 5(Compiler)
3) Prog ISP (Compiler)
4) Matlab Simulink(Simulator)