Sensors

Pressure and level

• Most Common industrial sensors:– Pressure– Flow– Temperature– Level

Pressure

1. Absolute pressure zero referenced against a perfect vacuum.it is equal to gauge pressure plus atmospheric

pressureAbsolute Pressure (psia) = Gauge Pressure +

Atmospheric PressureAbsolute pressure = gauge pressure + 14.7 psi

2. Gauge pressure zero referenced against ambient air pressure. It is equal to absolute pressure minus atmospheric

pressure Gauge Pressure (psig) = Absolute Pressure -

Atmospheric PressureGauge pressure = absolute pressure - 14.7 psi

0 psig = 14.7 psia

3. Differential pressureThe difference in pressure between two points

Engineering units = Pa, psi, bar, kg/cm2, mmH2O, inH2O

1. what is the reading of the pressure in figure 1?– in psig– in psia

2. What is the absolute pressure in the vacuum?3. What is the gauge pressure in the vacuum?

Figure 1

vacuum atmospheric

0 psi 14.7 psi

0 psia 0 psig

Pressure sensors

Pressure sensor

Pressure gauge: a self-contained indicator that converts the detected process pressure into the mechanical motion of pointer.

Bourdon tube

– most common pressure sensor

• A piece of tubing in the form of horseshoe

• One end sealed and the other connected to the pressure source

• The cross section of the tube is elliptical or flat, so the tube tend to straighten when pressure is applied.

• Returns to its original form when pressure is released.

• Amount of the tube straighten proportional to the applied pressure.

• If the open end of the tubing is fixed, the close end is connected to the pointer to indicate pressure / transmitter to generate signal

Appendix C page C-1

• Pressure range measured by the Bourdon– Depends on

– Wall thickness– Material of tubing

Bellows-Looks like a corrugated capsule -Made up by elastic material –

stainless steel or brass-When pressure increase, the bellows expands--when pressure decrease, the bellows contracts.-Amount of expansion and contraction is proportional to the applied pressure.

• Stainless steel bellows

Diaphragm sensor

• Diaphragm sensor– Similar to the bellows– Pressure increase, the center of diaphragm moves

away from the pressure.– The amount of motion is proportional to the

applied pressure

Level sensor-transmitter

Level Sensor

• -3 most important level sensors– Differential pressure– Float– Air bubbler sensors

• Differential pressure method-Sensing the difference in pressure between the pressure at the bottom of a liquid and above the liquid level.-The differential pressure is caused by the hydrostatic head developed by the liquid level.-The side that senses the pressure at the bottom –high pressure side-The side that senses the pressure above is the low pressure side.-From The differential pressure and density of the liquid we can obtain the level

-If vapor above the liquid level are noncondensable – wet leg can be empty-If vapor tend to condense – wet leg must be filled with suitable seal liquid

Float sensor-Detect the change in buoyant force on a body immersed in the liquid.-The force required to keep the float in place, which is proportional to the liquid level, then converted to a signal by transmitter.-Major disadvantage

-Unable to change their zero and span

Bubbler sensor

-One type of hydrostatic pressure sensor-Consist of air or inert gas pipe immersed in the liquid.-The air / inert gas flow through the pipe is regulated to produce continuous stream of bubbles.- The pressure required to produce this continuous stream is a measure of the liquid level.