Pressure Measurement

Errors Involved in Pressure Gauge Measurement

PRESSURE GAUGE

What is an error in the pressure gauge?

  • Inaccuracy in pressure gauge occurs when an output signal diverges from the desired value.
  • When the pointer on the pressure gauge scale indicates a higher or lower value instead of actual.
PRESSURE GAUGE
PRESSURE GAUGE

What are the factors affecting accuracy and performance in Pressure Gauges?

  • The majority of pressure gauges used are the bourdon tube type or diaphragm type having a pointer and dial arrangement for indicating the pressure
  • During the selection and installation of the pressure gauge process temperature and ambient temperature is important to be considered.

The three factors that affect pressure gauge accuracy and performance:

1. Temperature:

  • For the calibrated gauge the user may experience a 2% error in reading for every 30 °C temperature shift.
  • This is due to variations in elasticity or spring rate of bourdon tube elements with temperature.
  • This is caused by change in the elasticity or spring rate of the bourdon tube.
  • But it is quite hard to overcome the influence of ambient temperature.
  • To avoid gauge damage from the saturated heat source of steam service common practices such as
    1. Installing coil siphons or pigtail siphons to vanish the process heat.
    2. Installing a diaphragm seal with capillary to avoid high heat.

2. Vibration:

  • Vibration due to pumps, motors, and other rotating equipment causes excess wear and unseasonable damage to the pressure gauge’s internal parts such as the bourdon tube and gear mechanism.
  • Pointer oscillation caused by continuous exposure to vibration makes it difficult to accurately gauge reading.
  • This difficulty is solved by using a liquid-filled pressure gauge. The liquid used is either glycerine or silicone.
  • Liquid-filled gauge protects and lubricates the movement of the internal gear mechanism.

3. Pulsation:

  • The pulsation effect occurs around pump discharge and quick operating valves. Liquid (Glycerine or Silicone) filled gauge reduces the effect of pulsation
  • Pulsation reducer such as pistontype snubber or sintered metal snubber are installed at the upstream side of the gauge socket.
  • To address pulsation in the gauge a needle valve is slightly opened or pinched down at the upstream side of the gauge.
  • But for addressing the pulsation effect the needle valve is not recommended because the user may accidentally open the valve, thereby negating restriction in fluid flow.
  • To address pulsation
    1. A clean or low viscousfluid like gases or liquids, a threaded orifice flow restrictor, or a sintered metal snubber is usually installed.
    2. In dirtier and higher viscosity fluids like massecuite or oil, a piston snubber is usually installed.

What are the common reasons for Pressure Gauge Failure?

1. Mechanical vibration:

  • Vibration is an irregular oscillation of mechanical parts.
  • In most cases, pressure gauge failure occurs during mechanical vibration in the system. It has a negative impact on gauge accuracy.
  • Vibrating gauge makes difficult to read the pointer location on a dial.
  • Incremental damage to the pointer mechanism produces inaccurate readings.

Indications

  • Detached pointer if the vibration is severe
  • Metal erosion or dust inside the gauge window from worn pinion and segments gears.

Risks

  • Wear and tear of internal components
  • Loss of accuracy and functionality
  • Pressure system failure

2. Pulsation Effect:

  • It is the regular occurrence of rapid increase or decrease in pressure of the flowing media.

Indications

  • Pointer vibration
  • Loose or broken pointer

Risks

  • Difficult to obtain an accurate reading.
  • Wear and tear of internal components.
  • Loss of accuracy and functionality
  • Pressure system failure.

3. Extreme Temperature:

  • Generally, the gauges we are using have various tolerances for extreme temperatures. We observe both ambient temperatures and the temperature of the process media.

Indications

  • Liquid or Dial is discoloured, usually yellow, orange, brown, or black.
  • Dial, the Case is melted due to high-temperature media.

Risks

  • Difficulty in obtaining an accurate reading
  • Loss of accuracy and functionality
  • Pressure system failure

4. Pressure Spikes:

  • These spikes occur when the pressure rises sharply and drops suddenly.
  • Spikes cause some sort of problems for gauges that are not designed for a particular application or condition.

Indications

  • Pointer damage
  • Bending of a pointer like a fishtail or fish hook from hitting the stop pin too hard
  • Stop pin broken.

Risks

  • Increased wear on movement and components
  • Loss of accuracy/functionality
  • Split Bourdon tube, leading to released media
  • Pressure system failure.

5. High or over Pressure:

  • This condition is somewhat similar to pressure spikes.
  • This occurs when the gauge measures a pressure of maximum range.
  • Generally, this overpressure is observed in water or wastewater treatment and gas lines.
  • The effect of overpressure causes the Bourdon tube to twist.

Indications

  • Pointer buried against stop pin
  • Pointer dislodges stop pin

Risks

  • Loss of accuracy and functionality
  • Damages Bourdon tube
  • Pressure system failure.

6. Corrosion Effect:

Most process industries work with harsh chemicals such as 

  • Hydro fluoric acid in refineries
  • Flocculants andChlorine in wastewater treatment.
  • Chlorinated gases in fibre optic products.

Indications

Stain in the gauge case, pointer, connection, and dial.

Risks

  • Loss of accuracy/functionality
  • Pressure system failure

7. Clogging or Jamming:

  • Jamming is a major issue because a slurry, viscous, and dust media may block the system.
  • This occurs in major industries such as paper plants, wastewater treatment plants, pharmaceuticals, and other industries

Indication

Indication at zero or near zero position during system operation hour.

Risks

  • Loss of accuracy
  • Possibility of overpressure

Which devices are used to calibrate the pressure gauge?

The pressure gauge can be calibrated by using tools like

  1. Dead Weight tester
  2. Standard Pneumatic calibrator
  3. Calibrator with an accurate reading.

How do you calculate pressure gauge error?

Error in pressure gauge for each pressure can be calculated by using the relation

pressure gauge 1

What are the four 4 types of pressure measurement?

  1. Gauge Pressure.
  2. Sealed Pressure.
  3. Absolute Pressure and
  4. Differential Pressure.


What is the error of the pressure gauge?

Pressure measurement errors:

 Pressure Gauge 2

What are the types of errors in the Pressure Gauge?

The specific types of errors

  • Zero Point Error: An Initial point of measurement range is too high or too low, thus shifting the entire scale up or down by zero offset value.
  • Span Errors : It is actual distances between the individual divisions from the zero point to the full scale are even but have a wrong value with magnifying error effect at the upper end of the scale.
  • Nonlinearity: It is the actual distances between the individual divisions from the zero point to the full scale that are non-linear or not even thus making the ideal straight line into a curve.

How to correct zero error in pressure gauge?

  • Zero error is corrected by releasing the pressure by opening the sealed vent.
  • Note that re-sealing the vent is done to avoid oil leakage and gauge damage.
  • Due to variations in temperature and elevation, the pressure inside the gauge will build up and affect the needle settings to zero.

Why is it important to calibrate a pressure gauge?

  • To ensure accurate measurement of pressure the gauges are required to be calibrated at regular time intervals.
  • Using inaccurate or un-calibrated gauges causes safety issues, production downtime, or faulty products.

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