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Pressure Sensor

Motivation

why do we measure pressure?

  • To calculate the depth at which the sensor is placed (CTD).
  • These sensors can also measure the tidal cycles accurately.
  • Data also used to study Decompression sickness in Divers.

Measurement Principle

Measured with a precision quartz crystal resonator whose frequency of oscillation varies with pressure-induced stress.

Pressure normally measured in decibars (db). Pressure in decibars is almost equal to depth in meters(Freshwater applications).

Hydrostatic Pressure Equation :

  • Pressure(p) is the weight per unit area of the water column above the sensor.
  • Pressure is dependent on the depth(z).

For Seawater applications, an empirical formula(in the UNESCO Technical Papers in Marine Science No. 44) that takes density into account is used.

The gravity variation with latitude and pressures is computed as,

g (m/sec2) = 9.780318 * [ 1.0 + ( 5.2788x10-3 + 2.36x10 -5 * x) * x ] + 1.092x10 -6 * p

where
x = [sin (latitude / 57.29578) ] 2
p = pressure (decibars)

Then, depth is calculated from pressure:

depth (meters) = [(((-1.82x10-15 * p + 2.279x10 -10 ) * p - 2.2512x10 -5 ) * p + 9.72659) * p] / g

where
p = pressure (decibars)
g = gravity (m/sec2)

Typical Use instruments

  • High precision quartz crystal resonator mounted on the CTD.
  • Digital sensors combined with microprocessor-based systems are accurate and perform better in extreme environmental conditions.
  • They also have high resolution and low noise levels.