TABLE OF CONTENTS There are many sensors, other than level, pressure, flow, and temperature sensors, which may not be encountered on a daily basis, but still play an equally important part in process control in high technology industries and in operator protection. This chapter discusses several sensors that are very important in modern processing. They are:
Humidity;
Density, specific weight, and specific gravity;
Viscosity;
Sound;
pH;
Chemical.
It is necessary to control the amount of water vapor present in many industrial processes. Textile, wood, and chemical processing is very sensitive to humidity.
Humidity is a measure of the relative amount of water vapor present in the air or a gas.
Relative humidity ( ?) is given by:
where P is the pressure and T is the temperature.
An alternative definition using vapor pressures is:
The term saturated means the maximum amount of water vapor that can be dissolved or absorbed by a gas or air at a given pressure and temperature.
Specific humidity, humidity ratio,or absolute humidity can be defined as the mass of water vapor in a mixture in grains (where 7,000 gr = 1 lb), divided by the mass of dry air or gas in the mixture. The measurement units could also be defined as a ratio (pounds of water vapor per pound of dry air), or be defined in SI units (grams of vapor per kilogram of dry air).
where P (water vapor) is...
