Sensors which are mainly in use for general purpose pressure measurement in compressed air systems are piezo resistive type. Piezo resistor convert stress into resistance and then resistivity changes into output signals mainly 4... 20 mA. Such a sensor is then integrated into a compact housing with process connection and a cover/housing for the electronics.

If we want to measure flow in the pipeline we can choose between several measuring technologies but in compressed air or gas system pitot tube and thermal mass flow measurement are the most common. In this case we are going to talk about thermal mass flow measurement, which is based on heating a wire that is exposed to air stream. The wire is kept at a constant temperature difference, compared to the gas temperature, which is measured at the same time. This constant temperature difference is achieved with a constant electrical current. If now a gas, or compressed air, is passing this heated wire, the temperature of the wire will get lower, since the gas is taking some of the heat energy. Now the current is increased to keep the temperature difference constant again. This current needed is direct proportional to the mass flow of the gas/compressed air. By knowing the diameter of the pipe the volume flow can be calculated. Advantage of thermal mass flow meters is that the measurement is independent of system pressure. This means that measured value can be expressed as mass in kg or as volume in m3/h or l/s without knowing actual pressure of the system.

DEW POINT

Beside solid particles and oil, compressed air also contains large amounts of water. In order to provide stable product quality contaminants including water must be removed or reduced to the acceptable level based on specific application requirements. Typical applications where water content is limited are outdoor installations where the risk of freezing occurs and applications with high quality requirements in terms of air dryness such as the process air in process industry (food and beverage, pharmaceutical, electronic, chemical…). Humidity (water vapour content) is expressed in the terms of Pressure Dew Point (PDP) where dew point is the temperature at which air is 100 % saturated with moisture. Dew point sensor gives us reliable information about actual PDP of compressed air and makes it possible to react quick in case PDP is out of limit.
 

POLYMER SENSOR TYPE OS 215

OSCILLATING QUARZ CRYSTAL SENSOR TYPE OS 220

PORTABLE OS 505

How it works? Dew point sensors which are common in compressed air application mainly use capacitance measurement as main operating principle. We have polymer sensors (OS 215, OS 212) and quartz crystal oscillating sensors OS 220. Quartz crystal is oscillating at a certain frequency, if now a water molecule sits on this oscillating quartz, the frequency will be affected. This change can be measured which is in proportion of humidity. By measuring the temperature and the humidity, the dew point can be calculated.