Liquid and Gas Sensors

The following pages are application notes for various types of Liquid and Gas sensors

Index

• Pressure

• Flow

• Anemometry

• Humidity

• Chemical

Pressure Sensors

Principals of operation

Pressure measurements are normally made with a strain gauge or silicon sensor diaphragm. They are also normally come with signal conditioning to provide either a voltage or current loop output.

Interfacing

Generally, the full bridge strain gauge input module is required for non-signal conditioned sensors. for signal conditioned sensors, the voltage or current input modules should be used.
NPAI16SG-3 NPAI24SG-3 NPAI16C NPAI24C NPAI16V-2 NPAI24V-2

Sources of Supply

• Druck
  Large supplier of pressure sensing devices

Flow Sensors (liquid)

Principals of operation

There are three basic ways of measuring flow:

• Impeller counting
  A simple turbine is placed in the flow. A proximity sensor produces a pulse for every revolution of the turbine, and the output frequency is proportional to the flow.

• Centrifuge flow metering
  
• Magnetohydrodynamic flow metering
  Generally used for high flows in oil. An electric field is generated in the liquid when a magnetic field is produced perpendicular to the flow. This electric field is sensed by electrodes and is used to calculated the flow.

Interfacing

Contact Keynes Controls for details of interfacing.

Sources

Anemometry

Principals of operation

Anemometry is flow sensing for gasses. Some of the techniques used for flow can also be used for anemometry, however there are a number of additional techniques than can be used:

• Thermal resistance anemometry.
  A heater is placed in the gas flow with a constant power. The air flowing past the heater cools the device, and the temperature difference between this heater, and ambient temperature is used to determine (non-linearly) the flow.
  
• Pieto Tube
  A pieto tube relies on dynamic pressure principal. The open end of a tube is placed in the flow and the pressure is monitored. This is a very common method for measuring velocity for aircraft.

Interfacing

Contact Keynes Controls for details of interfacing.

Sources

• Keynes Controls Ltd. (www.keynes-controls.com)
  Manufactures the FlowCheck thermal resistance anemometer system for air conditioning ducts.
  Features high accuracy, and no moving parts in the air flow reduce maintenance costs.

Humidity

Principals of operation

The most common method of humidity sensing is by capacitance measurement. It is important therefore that signal conditioning is placed close to the sensor, since otherwise cable capacitance would have a considerable and unknown effect.

Interfacing

Contact Keynes Controls for details of interfacing.

Sources

Chemical Sensors

Principals of operation

The chemical to be monitored reacts with other chemicals in the sensor to produce a change of resistance, capacitance or some other electrically measurable parameter. Some emit light which can be sensed with a photodetector.

Interfacing

Contact Keynes Controls for details of interfacing.

Sources