Th2 Pressure Measurement and Calibration

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Pressure Measurement and Calibration
1. Objective:
To convert an arbitrary scale of pressure sensor output into engineering units. To calibrate a semiconductor pressure sensor.

2. Equipment:

The equipment comprises a Dead-weight Pressure Calibrator (DPC), Bourdon gauge and diaphragm-type pressure sensor.
The DPC consists of a precision ground piston and cylinder with a set of weights. In normal use, the required combination of weights is applied to the piston to produce the required pressure, and the piston is then spun to reduce vertical friction while the readings from the sensors are taken. The Bourdon gauge is a traditional instrument with a rotary scale and mechanical indicator. The clear dial face allows observation of the mechanism. The dial is calibrated both in arbitrary units (degrees of rotation) and engineering units of pressure (kN/m² or kPa).

The electronic pressure sensor incorporates a semi-conductor diaphragm that deflects when pressure is applied by the working fluid. The deflection generates a voltage output that is proportional to the deflection, and hence to the applied pressure.

The two pressure sensors are mounted on a manifold block with a reservoir to contain the hydraulic fluid. An isolating valve allows the water flow to be restricted, to demonstrate the effect of damping. The voltage from the electronic pressure sensor is displayed on a digital meter on the electrical console. An additional conditioning circuit incorporates zero and span adjustments, and allows the voltage output from the pressure sensor to be converted and displayed as a direct pressure reading. The zero and span controls are mounted on the front of the console, and a selector switch allows the voltage output or the direct reading to be displayed as required.

3. Theory:
Pressure sensor calibration
Variation in a pressure sensor reading may be calibrated, using known pressures, to give a gauge reading in engineering units. As has been seen in exercise A, the dead-weight calibrator used in the TH2 produces a known reference pressure by applying a mass to a column of fluid. The pressure produced is


Fa = gMa, and


Fa is the force applied to the liquid in the calibrator cylinder. Ma is the total mass (including that of the piston)
g is the acceleration due to gravity, and
A is the area of piston.
The area of the piston can be expressed in terms of its diameter, d, as:

The pressure in the fluid may then be calculated in the relevant engineering units. These known pressures may then be compared to the pressure sensor outputs over a range of pressures. The relationship between sensor output and pressure may be turned into a direct scale, as on the Bourdon gauge scale. Alternatively, a reference graph may be produced. Where the relationship is linear and the sensor output is electrical, the sensor may be calibrated using simple electronics.

When using SI units, the units of pressure are Newtons per square meter (N/m², also known as Pascals). To calculate the pressure in N/m², M must be in kg, d in m, and g in m / s². For the pressure range covered in this exercise, it will be more convenient to use units of kN/m², where 1 kN/m² = 1000 N/m² (1 N/m² = 0.001 kN/m²). Barometric pressure: Pressure units and scale conversion

Barometric pressures are frequently measured in bar. One bar is equal to a force of 105 N applied over an area of 1m². While bar and N/m² have the same scale interval, pressures in bar frequently have more convenient values when measuring barometric pressure.

Pressure may also be measured in millimetres of mercury (mmHg). The pressure is given in terms of the height of a column of mercury that would be required to exert an equivalent pressure to that being measured. Another possible unit of...
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