Calibration of Pressure gauges with dead weight tester

Field Instrumentation
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Pressure gauge

Pressure gauges are Instruments that measure and display pressure. Pressure ,applied force by a fluid (liquid or gas) on a surface, is measured as force per unit of surface area

Classification of Pressure Gauge

Pressure gauge can be classified into three types based on their working principle.

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1. Hydrostatic:

  • They compare pressure to the hydrostatic force per unit area at the base of a column of fluid.
  • The measurements are independent of the type of measurant.
  • Have a very linear calibration.
  • They have poor dynamic response.

2. Mechanical (Aneroid)

  • Aneroid (meaning without fluid) gauges have a metallic pressure-sensing element that deforms elastically due to pressure difference across the element.
  • Used to measure the pressure of a liquid and gas.
  • Aneroid gauges are not dependent on the type of measurant,
  • Do not contaminate the system .
  • The pressure sensing element may be a Bourdon tube, a diaphragm, a capsule, or a set of bellows, that change shape when pressure is applied.
  • These most defenetly need a secondary transducer to read the deflection in sensing element
  • Capacitance is the most used secondary transducer due to displacement of the sensing element

3. Spinning-rotor gauge

  • The spinning-rotor gauge contains a steel ball ( that is rotated)that is magnetically levitated inside a steel tube closed at one end and exposed to the gas to be measured at the other.
  • The speed of rotation of the ball is the measure of viscosity of the gas being measured.
  • These utilize electromagnetic transducers as secondary transducers
  • Best used as secondary standards.

Standards and Calibration of Pressure Gauges

  • Pressure is a quantity derived from force and area which by itself is a derivation of fundamental units mass, length and time. Hence having a standard of measurement is not straight forward.
  • Accurate instruments for Pressure measurement are used as standards to calibrate less accurate measuring devices for Pressure.
  • Though these standards depend on the fundamental standard for accuracy.
  • The standards are in terms of mercury column length and unit of pressure is in Pascal.

The basic standards of Pressure

Pressure Range Standard Instrument for calibration
10-1 mmHg to several hundred MPa Precision Mercury columns, Dead weight Piston gages
10-1 to 10-3 mmHg McLeod gauge
Below 10-3 mmHg Pressure division through successive orifices till the upward higher pressure is proportional to the low pressure. The Higher pressure is measured by McLeod Gauge

Dead weigh tester

  • These are hydrostatic dependent piston gauges where the pressure of the fluid is countered by a known weight or spring for calibration
  • Weights of known quantity are used to apply a force on the fluid level and the accuracy of the pressure gauges are determined.

Dead weight Tester image

These are primary standards for pressure as the pressure measured by this is defined in fundamental quantities; length, mass and time.
Dead weight Tester schematic diagram image

1 - Handpump 2 - Testing Pump 3 - Pressure Gauge to be calibrated 4 - Calibration Weight 5 - Weight Support 6 - Piston 7 - Cylinder 8 - Filling Connection

Working of Dead weight Tester (DWT)

  • The testing pump (2) is connected to the instrument to be calibrated (3).

  • A special hydraulic oil or gas such as compressed air or nitrogen is used as the pressure transfer medium.

  • The pressure in the testing chamber is adjusted by a bleeding valve or pump (1). It is also connected to a chamber with piston with standard weights.

  • The measuring piston is then loaded with calibrated weights (4) .

  • The pressure is applied via an integrated pump (1) or, if an external pressure supply is available, via control valves in order to generate a pressure until the pressure builds.

  • The loaded measuring piston with weights (6) rises and ‘floats’ on the fluid.

  • At this point there is a balance between gauge pressure and the mass load(weights).

  • The piston is rotated to reduce friction as far as possible.

  • Since the piston is spinning, it exerts a pressure that can be calculated by application of a derivative of the formula P = F/A. where :

    P : reference pressure [Pa] F : force applied on piston [N] A : effective area PCU [m2]

Design constraints for DWT

  • Small clearance between the piston and the cylinder.
  • The effective area is taken as the average of the piston and cylinder area.
  • Corrections needed for Temperature that effects the area of piston, cylinder, air or fluid buoyancy, local gravity conditions,height difference between the lower end of the piston and reference point of the calibrated gauge.

Tare Pressure = Piston Weight / Area.

  • DWT can not measure pressure lower than the tare pressure.
  • Tilting the cylinder - piston setup from vertical to a calculated degree will expand the pressure range measured from 0 Pa to Tare pressure.
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    The accuracy of a pressure balance is characterized by the deviation span, which is the sum of the systematic error and the uncertainties of measurement.

See Also

For Mcleod gauge click here For measurement of low pressure click here

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