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Monday, October 8, 2007

Pressure Transmitters.

In pressure transmitters, the full signal conditioning circuitry
is integrated in the housing. The sensor signal is conditioned
into standard output signals of 0...100mV, 0...10V, 0.5...4.5V,
and 4-20mA.

Piezoresistive OEM Pressure Transducer.

The silicon sensor on the glass feed-through is mounted in
a stainless steel housing, isolated by a thin stainless steel
diaphragm and filled with silcone oil. The pressure acts on
the diaphragm and is transfered through the oil onto the
sensor. These transducers are fully tested for temperature
and linearity and the compensation resistor values given on
the individual test sheets.

Monday, September 10, 2007

Leadouts from the bridge.

1). Gold or aluminium wires are welded to the aluminium contacts
on the chip and to the glass feed-through, pins of the header.

2). TAB (Tape Automated Bonding). The contacts on the chip
have a gold dot.

A pretinned felxible printed circuit is directly soldered to these
gold dots and the other end to a PC-board, or the header.

In the first method, the sensor must be fixed on the header. The
TAB printed circuit, however, holds the sensor in place itself.

Thursday, September 6, 2007

The piezoresistive pressure sensor, or silicon cell.

This type of pressure sensor consists of a micro-machined
silicon diaphragm with piezoresistive strain gauges diffused
into it, fused to a silicon or glass backplate.
The resistors have a value of approx. 3.5 kOhm. Pressure
induced strain increases the value of the radial resistors (r),
and decreases the value of the resistors (t) transverse to
the radius. This resistance change can be high as 30%.

The resistors are connected as a Wheatstone Bridge, the
output of which is directly proportional to the pressure.

Wednesday, September 5, 2007

Pressure Sensor

Pressure transducers are pressure measuring instruments,
ready to use. It is an OEM transducer with pressure port,
integrated compensation resistors and a cable or connector.

Transducers give an unamplified signal into a separate
instrumentation amplifier or indicator. They can be
considered as passive bridges, being interchangeable
between different manufacturers.

Low Cost Sensor

Low cost sensors are devices where the they are exposed to
the media without protection.

The glass feed-through and the silicon cell is mounted in a
plastic housing with pressure ports for positive and negative
pressure. The silicon sensor with the TAB print
is fixed between two plastic mouldings with pressure ports.

The silicon sensor is bonded to a brass pressure port. The
contacts are made either by gold wires to soldering pins, or
by TAB flexible printed circuit.

Standard Pressure Units and Conventions

To communicate the amount of pressure requires the use of an agreed on scale and convention. Just as the conventions and units for temperature are referred to as Fahrenheit, Centigrade or Kelvin; industry uses PSID, PSIA, PSISG and PSIVG notation for pressure. PSI (Pounds per Square Inch) is the pressure unit while the last letter(s) refers to the convention used to measure the pressure. You would use PA( Pascels) units instead of PSI in the metric system.

By nature, pressure is the difference between two points or ports. Pressure between two ports (or pipe connections) is differential pressure. A designation of 10 PSID means 10 Pound per Square Inch Differential.

Tuesday, September 4, 2007

The insides of a modern pressure transducer

Further on in our voyage, we come to small circuit board.

The two voltage out points from the bridge circuit are fed to an amplifier that changes the very small voltage into a 0-5V signal or most commonly to a 4-20 mA signal. This signal is fed out the cable (sometimes along with a vent tube) which finishes our voyage.

The output of a Strain gauge is sometimes referred to as ratiometric to the supply voltage. The term ratiometric means that the output varies as a ratio of the the supply voltage. An example helps make all things things clear.

· Let us assume that With a 1/2 full scale pressure input the out put is x volts, if we double the supply voltage, the voltage output will now be 2X.

By varying the supply voltage we have a handy way to scale the output to what ever units we want. Changing the value of the reference element resistor gives us a way to offset the zero point. Between the two adjustments we can scale and offset the output to what ever we want. In real word practice the output voltage and currents are kept low to minimize thermal error so the output is usually followed with an amplifier stage.

Pressure Transducer Basic

Pressure transducers used in industry today take on a rather typical appearance. A round, tubular stainless steel body with a pipe fitting on one end and a cable coming out of the other end. Stainless is used because of its high strength and resistance to corrosion.

Monday, September 3, 2007

The Pressure Sensor

With the steam age came the demand for pressure measuring
instruments. Bourdon tubes or bellows, where mechanical
displacements were transferred to an indicating pointer were
the first pressure instruments, and are still inuse today.

Pressure metrology is the technology of transducing pressure
into an electrical quantity. Normally, a diaphragm construction
is used with strain gauges either bonded to , or diffused into it,
acting as resistive elements. Under the pressure-induced strain,
the resistive values change.

In capacitive technology, the pressure diaphragm is one plate
of a capacitor that changes its value under pressure-induced

Pressure sensing using diaphragm technology measures the
difference in pressure of the two sides of the diaphragm.
Depending upon the relevant pressure, we use the terms
ABSOLUTE, where the reference is vacuum (1st picture),
GAUGE, where the reference is atmospheric pressure
(2nd picture), or DIFFERENTIAL, where the sensor has two
ports for the measure of two different pressure.

Types of sensor

  • Thermal Sensor
  • Electromagnetic Sensor
  • Mechanical Sensor
  • Chemical Sensor
  • Optical radiation Sensor
  • Motion Sensor
  • Distance Sensor
  • Pressure Sensor
  • Oxygen Sensor
  • Parking Sensor
  • Humanity Sensor
  • Temperature Sensor
  • Voltage Sensor
  • Backup Sensor
  • Speed Sensor
  • Level Sensor
  • Flow Sensor
  • Wireless Sensor
  • Optical Sensor
  • Gas Sensor
  • Color Sensor
  • Photo Sensor
  • Heat Sensor
  • Force Sensor
  • Water Sensor
  • ph Sensor
  • Laser Sensor
  • Touch Sensor

What is sensor?

A sensor is a type of transducer . Direct-indicating sensors, for example, a mercury thermometer , are human-readable. Other sensors must be paired with an indicator or display, for instance a thermocouple. Most sensors are electrical or electronic, although other types exist.

Sensors are used in everyday life. Applications include automobiles, machines, aerospace, medicine, industry and robotics.

Technological progress allows more and more sensors to be manufactured on the microscopic scale as microsensors using MEMS technology. In most cases a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. See also MEMS sensor generations.