Height Difference in Pressure Calibration: Calculating Head Correction Correctly

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During pressure calibration, the reference and the device under test should ideally be located at the same hydraulic or pneumatic reference height. In practice, however, this is often not possible: the pressure controller is positioned on the laboratory bench while the device under test is mounted above it on a test stand, or a pressure transmitter is located below the reference instrument.

A vertical column of the pressure medium then exists between the two measuring points. Due to gravity, a hydrostatic pressure gradient develops within this column.

The result: although the reference and the device under test are connected by the same pressure line, the pressure at the two devices is not exactly identical.

The resulting head correction is calculated using the height difference, the density of the pressure medium and gravitational acceleration. With liquids, a height difference of only a few centimeters can already cause several millibars of pressure difference. With gases, the effect is much smaller, but it can still become relevant for low pressure ranges, high gas pressures or very precise reference instruments.

For highly accurate stationary pressure calibration, for example, the Druck PACE5000E high-precision pressure controller is suitable. Depending on the control module used, measuring ranges from 25 mbar to 210 bar and very low measurement uncertainties are possible.

For mobile calibration tasks, the Druck DPI610E and the modular DPI620G calibration system are available, among others.

Further solutions can be found under calibration technology at ICS Schneider.

Why does a pressure difference arise between two heights?

A stationary fluid has a vertical pressure gradient in the Earth’s gravitational field.

The farther downward you move within a continuous fluid column, the greater the pressure becomes.

The reason is the weight of the liquid or gas column above it.

This also applies within a calibration line

For example, if:

• a pressure controller is positioned on the laboratory bench

and:

• a pressure transmitter is located 50 cm below it,

the weight of the 50 cm-high fluid column acts between the two devices.

The pressure at the lower device is therefore higher.

Conversely

If the device under test is located above the reference, the pressure at the device under test is lower than at the reference.

This difference exists even if:

• there is no flow,
• the pressure is completely stabilized,
• the reference and device under test are perfectly connected.

This is not a measuring-instrument error, but a real physical pressure difference.

Formula for hydrostatic head correction

For a liquid, or approximately for a gas when the height difference is sufficiently small, the following applies:

Δp = ρ · g · Δh

where:

• Δp = hydrostatic pressure difference in Pa,
• ρ = density of the pressure medium in kg/m³,
• g = gravitational acceleration in m/s²,
• Δh = vertical height difference in m.

For standard calculations, the following approximate value can be used:

g = 9.80665 m/s²

Conversion to mbar

Since:

1 mbar = 100 Pa

the following applies:

Δp [mbar] = ρ · g · Δh / 100

Important

Only the vertical height difference is relevant.

The length of the connecting hose is not decisive for the hydrostatic correction.

A five-meter hose running almost horizontally can therefore cause a smaller head correction than a 50 cm hose running completely vertically.

What sign should the correction have?

The most common practical source of error is not calculating the magnitude, but applying the wrong sign.

A clear definition prevents confusion.

We define

Δh = hReference − hDUT

with a height coordinate increasing upward.

Then:

pDUT = pReference + ρ · g · Δh

Device under test is below the reference

Then:

Δh > 0

and:

pDUT > pReference

The pressure at the device under test is therefore higher.

Device under test is above the reference

Then:

Δh < 0

and:

pDUT < pReference

The pressure at the device under test is lower.

Arrangement	Head correction at the DUT
DUT below the reference	Pressure at the DUT is higher
DUT at the same height	No height correction
DUT above the reference	Pressure at the DUT is lower

For instruments with an integrated head-correction function, the sign convention used by the manufacturer must always be observed. The software definition may differ from your own calculation convention.

Which reference levels must be compared?

The outer housing heights of the reference and the device under test are not automatically the correct reference points.

What matters is the metrological pressure reference level of each device.

Depending on the device, this may be, for example

• the center of a pressure connection,
• the plane of a diaphragm seal,
• the position of the internal sensor diaphragm,
• a reference line marked by the manufacturer,
• the reference level of a deadweight tester.

For the device under test

the pressure reference level defined by the manufacturer should be used whenever possible.

If no specific level is specified, the geometric plane of the process connection or measuring diaphragm is a sensible approximation for many industrial pressure instruments.

For a precision reference

the operating manual should always be consulted.

High-precision pressure controllers may have a defined reference level or a specified measuring connection.

Measuring the height between the lower edges of two housings, even though the actual pressure reference levels are located elsewhere, can itself introduce an error into the head correction.

Head correction with liquids

Height correction is particularly important for hydraulic calibrations.

Liquids have a much higher density than gases.

Typical orders of magnitude

Medium	Typical density	Pressure difference per 1 m height
Water at approximately 20 °C	approx. 998 kg/m³	approx. 97.9 mbar/m
Hydraulic oil	approx. 850 kg/m³	approx. 83.4 mbar/m
Air at approximately 1 bar(a)	approx. 1.2 kg/m³	approx. 0.12 mbar/m

This already demonstrates why height correction should practically never be ignored as a general rule in hydraulic calibrations.

10 cm height difference with oil

At a density of:

850 kg/m³

a height difference of:

0.10 m

already produces approximately:

8.34 mbar

of pressure difference.

For low pressure ranges, this can be considerably larger than the permissible measurement deviation of the device under test.

Head correction with gases

For pneumatic calibrations, head correction is considerably smaller because the density of air or nitrogen is much lower.

Nevertheless, it should not automatically be neglected in highly accurate measurements.

It becomes particularly relevant for

• low pressure ranges,
• large height differences,
• high-precision reference instruments,
• high absolute gas pressures.

Unlike an almost incompressible liquid, gas density changes significantly with pressure.

The simple equation:

Δp = ρ · g · Δh

can still be used for small height differences if a suitable mean gas density is applied.

For larger height differences or the highest accuracy requirements

the change in gas density along the vertical line must be taken into account.

In principle:

dp/dz = −ρ · g

where the density:

ρ = ρ(p,T)

depends on absolute pressure and gas temperature.

Why is gas pressure crucial for density?

For gases, the air density at atmospheric pressure must not simply be used for high-pressure calibration.

Density increases significantly with increasing absolute pressure.

For an approximately ideal gas

ρ = p · M / (Z · R · T)

where:

• p = absolute pressure,
• M = molar mass of the gas,
• Z = compressibility factor,
• R = universal gas constant,
• T = absolute gas temperature.

In simplified terms

At approximately the same temperature, density increases with absolute pressure.

Air at approximately atmospheric pressure and room temperature has a density of around:

1.2 kg/m³

At approximately:

10 bar(a)

the density is already approximately in the range of:

12 kg/m³

.

Head correction therefore increases as well

At approximately 1 bar(a), a 1 m column of air produces around:

0.12 mbar

.

At approximately 10 bar(a), it can already be around:

1.2 mbar

per meter.

Absolute pressure is therefore relevant when calculating pneumatic head correction, even if a gauge-pressure instrument is being calibrated.

Influence of temperature on medium density

The density of the pressure medium depends on temperature.

For liquids

density changes comparatively moderately with temperature.

Nevertheless, for precise hydraulic calibrations, the density of the actual oil or liquid being used at the current temperature should be applied.

For gases

the influence of temperature is considerably more significant.

At the same pressure, warmer gas has a lower density.

This also reduces the head correction.

For high accuracy, the following should therefore be known

• pressure medium,
• temperature of the medium,
• density or equation of state,
• absolute pressure.

For longer pressure lines, it may be necessary to use the mean gas temperature along the relevant vertical section.

What role does local gravitational acceleration play?

For normal industrial calibrations, standard gravitational acceleration is frequently used:

g0 = 9.80665 m/s²

However, the actual local gravitational acceleration depends, among other factors, on:

• geographical latitude,
• height above sea level

.

For the highest accuracy requirements

a local g value can therefore be used.

This is particularly familiar from deadweight testing and primary pressure standards.

Some precision pressure controllers also take local gravitational acceleration into account in their head-correction function, in addition to height and gas density.

Practical example: hydraulic calibration with oil

A precision pressure gauge is calibrated using a hydraulic reference instrument.

The device under test is located:

0.50 m

below the reference level.

The oil used has a density at the current temperature of:

850 kg/m³

Calculation

Δp = 850 · 9.80665 · 0.50

This gives:

Δp ≈ 4,168 Pa

or:

Δp ≈ 41.7 mbar

The device under test is lower

The pressure at the device under test is therefore approximately:

41.7 mbar

higher than the pressure at the reference level.

Example reference indication

The reference indicates:

100.000 bar

.

The pressure at the lower-mounted device under test is therefore approximately:

100.0417 bar

.

If the height difference were ignored, this real pressure difference would incorrectly be interpreted as a measurement deviation of the device under test.

Practical example: water as the pressure medium

In a calibration using water, the height difference is only:

100 mm

.

For water with a density of approximately:

998 kg/m³

the result is:

Δp ≈ 998 · 9.80665 · 0.10

and therefore:

Δp ≈ 979 Pa

or:

Δp ≈ 9.8 mbar

A height difference of only 10 cm with water therefore already produces almost 10 mbar of pressure difference.

Practical example: pneumatic calibration with air

A pressure transmitter is calibrated pneumatically.

The pressure reference is located:

0.50 m

below the device under test.

At approximately atmospheric pressure and room temperature, we assume an approximate air density of:

ρ = 1.2 kg/m³

.

Calculation

Δp = 1.2 · 9.80665 · 0.50

This gives approximately:

5.9 Pa

or:

0.059 mbar

Is this relevant?

For a 10 bar pressure gauge, this value is usually insignificant.

For a highly accurate measuring range of, for example:

0 … 100 mbar

however:

0.059 mbar

may already be relevant.

The effect increases at higher absolute pressure

If the same measurement is performed at a significantly higher gas pressure, gas density increases and therefore so does the height correction.

The statement:

Head correction can always be ignored with gases

is therefore incorrect.

Why are low pressure ranges particularly critical?

What matters is not only the absolute magnitude of the head correction, but also its relationship to the measuring range and the permissible measurement uncertainty.

Example

A pneumatic height correction of:

0.10 mbar

represents practically no relevant proportion of a measuring range of:

0 … 100 bar

.

For a measuring range of:

0 … 10 mbar

the same:

0.10 mbar

already represents:

1% FS

.

Whether head correction must be taken into account therefore cannot be determined from the height difference alone. Measuring range, accuracy and medium must be considered together.

Handling gauge and absolute pressure correctly

The hydrostatic pressure difference occurs regardless of whether the connected device under test measures gauge or absolute pressure.

For liquids

the basic calculation remains unchanged.

For gases, however, it is important

that the density is determined from the actual absolute gas pressure.

A gauge pressure of:

10 bar(g)

corresponds, at approximately 1 bar atmospheric pressure, to a gas pressure of approximately:

11 bar(a)

.

For the gas density, therefore:

10 bar

must not simply be used.

Special considerations for differential pressure measurements

Differential pressure measurements have two pressure sides:

• High Pressure,
• Low Pressure.

Each side can have its own hydrostatic correction.

In simplified form

ΔpDUT = pHP,DUT − pLP,DUT

and both pressures can differ from their respective reference pressures by their individual head corrections.

Symmetrical setup

If:

• both pressure lines are filled identically,
• both media are identical,
• both reference levels are at the same height

and the HP and LP sides are routed identically, the height effects can largely cancel each other out.

Asymmetrical setup

Different:

• heights,
• liquid columns,
• gas densities,
• temperatures

on the other hand, directly result in a differential pressure error.

For very low differential pressure ranges, this influence can be particularly critical.

Distinguishing hose routing from height difference

A common misconception is to use the entire hose length in the hydrostatic formula.

This is incorrect.

Only the vertical component is relevant

A hose may be:

3 m

long, while the reference and device under test are only:

0.2 m

apart vertically.

The height relevant for the head correction is therefore:

0.2 m

and not:

3 m

Hose loops do not change the static head correction

provided that:

• the medium is continuous throughout,
• no second phase is trapped,
• the system is in static equilibrium.

With liquids, air bubbles can nevertheless be problematic

An air bubble locally changes:

• the effective fluid column,
• the compressibility of the system,
• the dynamic behavior.

Hydraulic calibration lines should therefore be carefully vented.

Measuring the height difference correctly

The height difference should be determined vertically between the two actual reference levels whenever possible.

Suitable tools include, for example

• steel ruler,
• measuring tape,
• laser distance meter,
• leveling instrument for larger setups.

For high accuracy

the uncertainty of the height measurement should also be taken into account.

Example with oil

At:

ρ = 850 kg/m³

a height-measurement uncertainty of only:

±1 mm

causes a pressure uncertainty of approximately:

±0.083 mbar

.

For very low pressure ranges, even the accuracy of the height measurement itself can therefore become relevant.

Can head correction simply be zeroed out?

In some calibration setups, the reference and device under test are zeroed in the unpressurized condition.

However, this does not generally eliminate head correction.

With a liquid column

the hydrostatic pressure difference caused by the existing liquid remains present.

Depending on the setup, a zero adjustment may appear to compensate for this effect at one point, but it does not replace a correct definition of the reference level.

With gases

the situation is additionally pressure-dependent.

Since gas density increases with absolute pressure, the head correction also changes over the pressure range.

A zero adjustment at atmospheric pressure therefore cannot fully compensate for a height error at, for example, 10 bar.

A physically calculated head correction is therefore preferable to simply zeroing the system.

Head correction in the measurement uncertainty

When height correction is applied, not only its value is relevant.

Its uncertainty also belongs in the measurement uncertainty budget.

Influencing quantities include, for example

• uncertainty of the height difference,
• uncertainty of the medium density,
• temperature dependence of the density,
• gas pressure during pneumatic tests,
• local gravitational acceleration,
• non-uniform temperature along a gas line.

For:

Δp = ρ · g · Δh

the relative uncertainty can, in simplified terms, be derived from the uncertainties of:

ρ

,

g

and:

Δh

.

In many industrial cases

the dominant uncertainty components for liquids are:

• height determination,
• actual liquid density.

For pneumatic high-pressure measurements, the thermodynamic state properties of the gas may additionally be relevant.

When may the height correction be neglected?

Not every calculated head correction necessarily has to be applied as a separate correction.

It may be neglected if it can be demonstrated that its influence is sufficiently small compared with the required measurement uncertainty.

A sensible procedure

First, the maximum possible height-related error is calculated.

It is then compared with:

• the permissible DUT tolerance,
• the reference uncertainty,
• the overall measurement uncertainty budget.

Example

Calculated head correction:

0.02 mbar

Permissible process deviation:

±20 mbar

In this case, the influence is probably practically insignificant.

Another example

Calculated head correction:

0.02 mbar

Target measurement uncertainty:

0.01 mbar

In that case, the same height correction can no longer be ignored.

It is not the absolute magnitude that matters, but its relationship to the required accuracy.

Practical example: calibration station with PACE5000E

A high-accuracy pressure transmitter is calibrated at a stationary test station using a PACE5000E.

The controller is installed in a benchtop rack.

The device under test is mounted in a fixture above it.

1. Determine the reference levels

The relevant pressure reference level of the controller and the process-connection level of the device under test are identified.

2. Measure the height difference

The device under test is:

320 mm

above the reference.

3. Define the medium

Dry nitrogen is used as the pressure medium.

4. Determine the gas state

The calculation takes into account:

• absolute pressure,
• mean gas temperature,
• nitrogen density.

5. Calculate the height correction

Because the device under test is located higher, the pressure acting on it is slightly lower than the pressure measured at the controller.

6. Correct the test points

The pressure generated by the controller is converted to the pressure level of the device under test.

7. Document the correction

The calibration report records:

• height difference,
• medium,
• density or calculation method,
• temperature,
• applied correction.

This does not change the controller indication; instead, it clearly defines the pressure actually present at the reference level of the device under test.

Special considerations for on-site calibration

During mobile calibration, maintaining a common reference height is often more difficult than in the laboratory.

Typical situations include

• transmitters mounted on piping above the calibrator,
• pressure gauges on machines close to floor level,
• devices under test on vessels or platforms,
• long vertical impulse lines.

Especially during mobile hydraulic calibrations, this can result in a significant head correction.

A simple improvement

is to position the mobile reference calibrator as close as possible to the height of the device under test.

This:

• reduces the correction,
• reduces its uncertainty,
• simplifies documentation.

With a portable pressure calibrator such as the DPI610E or a DPI620G system, the test setup can therefore often be arranged so that the reference and the device under test are closer to the same height level.

Typical fault patterns

Observation	Possible cause	Recommended check
All test points show nearly the same offset	Head correction with liquid not taken into account	Calculate height difference and medium density
Error changes when the mounting height changes	Hydrostatic height influence	Compare reference and DUT levels
Hydraulic calibration shows several mbar of unexplained deviation	Reference and DUT are not at the same height	Measure vertical distance
Pneumatic error increases with pressure	Gas density and therefore head correction increase with absolute pressure	Calculate gas head correction as a function of pressure
Calculation gives the wrong sign	Height convention reversed	Check which device is positioned higher
Head correction remains despite zero adjustment	Physical fluid column still present	Apply correction to the reference level
Two technicians calculate different values	Different density or height reference used	Standardize calculation parameters
Gas correction significantly too small	Density at 1 bar used instead of density at operating pressure	Take absolute pressure into account
Oil correction no longer matches exactly after a temperature change	Change in oil density	Check medium temperature and density
Differential pressure measurement shows a constant zero offset	Different hydrostatic columns on HP and LP sides	Consider both sides separately
Calculation is based on total hose length	Hose length confused with vertical height	Use only the height difference
Hydraulic system responds sluggishly or spongily	Air bubbles in the line	Carefully vent the system
Correction is of the same order as the target uncertainty	Head correction not included in uncertainty budget	Include correction and associated uncertainty

Recommended calibration procedure

1. Define the measuring task: Specify pressure type, range and required measurement uncertainty.
2. Select the reference: Use a suitable pressure standard or calibration system.
3. Define the pressure medium: Clearly document gas or liquid.
4. Determine the reference level of the reference instrument: Use the manufacturer’s specification.
5. Determine the reference level of the device under test: Define the process connection or specified measuring level.
6. Measure the height difference: Record only the vertical distance.
7. Define the sign: Clearly establish whether the device under test is above or below the reference.
8. Determine the medium density: Use manufacturer data or suitable material-property data.
9. Measure the medium temperature: Take it into account when determining density.
10. Use absolute pressure for gases: Do not derive gas density from gauge pressure.
11. For high accuracy, check the local g value: If required for the uncertainty budget.
12. Calculate the head correction: Apply Δp = ρ · g · Δh.
13. For gases, consider pressure dependence: Use a suitable mean or point-specific density.
14. Reference the test pressure to DUT level: Apply the correction with the correct sign.
15. Stabilize the system: Allow temperature and pressure to stabilize sufficiently.
16. Record test points: Measure an increasing and, if required, decreasing pressure sequence.
17. Apply head correction consistently: Do not use different conventions between individual test points.
18. Determine the uncertainty of the height correction: Include height, density and other influencing quantities.
19. Evaluate the results: Compare corrected DUT values with the tolerance.
20. Create the calibration report: Document the reference level and head correction.
21. Reproduce the setup for repeated calibrations: Use the same height conditions or determine them again.

Suitable calibration technology from ICS Schneider

Druck PACE5000E – high-precision stationary pressure controller

The PACE5000E is a modular high-precision pressure controller for calibration laboratories, test benches and production.

Key features include:

• measuring ranges from 25 mbar to 210 bar,
• interchangeable PACE-CM control modules,
• measurement accuracy, depending on the module, down to the range of 0.001% FS,
• high control stability,
• fast pressure control,
• automatable test sequences,
• integration into calibration and test systems.

At these accuracy levels in particular, influences such as:

• head correction,
• temperature,
• stabilization,
• reference level

must be systematically taken into account.

Druck DPI610E – portable pressure calibrator

The DPI610E combines pressure measurement and pressure generation in one portable system.

Depending on the version, available functions include:

• pneumatic pressure and vacuum generation,
• hydraulic pressure generation up to 1,000 bar,
• HART communication,
• integrated calibration routines,
• data logging,
• optional external PM700E pressure sensors.

For on-site calibrations, the portable design is particularly useful because the reference instrument can often be positioned closer to the height of the device under test.

Druck DPI620G – modular calibration system

The DPI620G combines electrical, temperature, frequency and pressure calibration functions.

Using:

• PM620 pressure modules,
• MC620G module carriers,
• PV62xG pressure-generation stations

a modular pressure calibration station can be configured for a wide range of measuring ranges.

The system is particularly suitable for complex on-site calibrations where, in addition to pressure, the following may need to be measured or tested simultaneously:

• 4–20 mA,
• HART,
• temperature.

Further systems can be found under calibration technology at ICS Schneider.

Conclusion

A height difference between the pressure reference and the device under test creates a real hydrostatic pressure difference and cannot generally be ignored in precise calibrations.

The basic equation is simple

Δp = ρ · g · Δh

However, the correct input values and the correct sign are decisive.

The lower point has the higher pressure

If the device under test is below the reference, its pressure is higher. If it is above the reference, its pressure is lower.

The effect is particularly large with liquids

With water, a height difference of only 10 cm already produces approximately 9.8 mbar.

Gases can also be relevant

For low pressure ranges, large height differences or high gas pressures, pneumatic head correction can significantly influence the target measurement uncertainty.

Absolute pressure must be taken into account for gases

Gas density increases with absolute pressure. Air density at atmospheric pressure must therefore not be used indiscriminately for high-pressure calibration.

The correct reference level is decisive

The measurement is not simply made between device housings, but between the metrological pressure reference levels of the reference and the device under test.

Only the vertical height matters

The length of a pressure line is not decisive for hydrostatic head correction.

The correction belongs in the measurement uncertainty

Uncertainties in height, density, temperature and, where applicable, local gravity must also be taken into account for high accuracy requirements.

For practical applications

Connect reference and DUT → determine pressure reference levels → measure vertical height difference → define medium and density → for gases, take absolute pressure and temperature into account → calculate head correction → check the sign according to the height arrangement → determine the corrected pressure at DUT level → evaluate the uncertainty of the correction → record test points → document results and reference level in the calibration report.

FAQ: Head Correction in Pressure Calibration

What does head correction mean in pressure calibration?

Head correction refers to the correction of the hydrostatic pressure difference caused by a vertical height difference between the pressure reference and the device under test.

Why do two connected pressure measuring instruments have different pressures?

If they are located at different heights, the weight of the liquid or gas column between them creates a pressure gradient.

What is the formula for head correction?

For a liquid, or approximately for small gas height differences, Δp = ρ × g × Δh applies.

What unit is used for the height difference?

When using the SI equation, height is entered in meters.

What unit is used for density?

Density is entered in kg/m³.

Which gravitational acceleration can be used?

For many calculations, the standard gravitational acceleration of 9.80665 m/s² is used.

Which point has the higher pressure?

Within the same continuous fluid column, the lower point has the higher pressure.

What happens if the device under test is below the reference?

The pressure at the device under test is higher than at the higher reference by the hydrostatic head correction.

What happens if the device under test is above the reference?

The pressure at the device under test is correspondingly lower.

Is hose length important for head correction?

No. The decisive factor is the vertical height difference between the reference levels, not the total hose length.

What is the hydrostatic pressure of a 1 m water column?

At room temperature, it is approximately 98 mbar per meter.

How large is the head correction for hydraulic oil?

For an oil with a density of approximately 850 kg/m³, around 83 mbar per meter of height difference is produced.

Can head correction be neglected for gases?

Not generally. It can become relevant for low pressure ranges, large height differences, high absolute pressure or very low target measurement uncertainties.

Why does the head correction of a gas increase with pressure?

Because gas density increases with absolute pressure. A denser gas column produces a larger hydrostatic pressure gradient.

For a gauge-pressure sensor, must absolute pressure still be used to calculate gas density?

Yes. Physical gas density depends on absolute pressure, not on the displayed gauge pressure.

Which temperature should be used for the density?

Preferably the actual temperature of the pressure medium or a suitable mean temperature of the relevant fluid column.

Is the density of a liquid pressure-dependent?

Liquids are comparatively incompressible. For many industrial calibrations, their density can be treated as approximately constant over the pressure range. For the highest accuracy requirements, however, temperature and, where applicable, pressure dependence must also be considered.

Why is head correction particularly important in hydraulic calibrations?

Liquids have a high density. As a result, only a few centimeters of height difference can already produce several millibars of pressure difference.

Which reference height should be used for the pressure controller?

The pressure reference level defined by the manufacturer should be used, for example the center of a measuring connection or an explicitly specified reference line.

Which reference height applies to the device under test?

Ideally, the measuring level defined by the manufacturer, or the plane of the pressure-sensitive diaphragm or process connection.

Can I simply compare the lower edges of the two devices?

No. Housing dimensions do not automatically correspond to the actual pressure reference levels.

Can head correction be eliminated by zero adjustment?

Not reliably. Particularly with gases, head correction changes with absolute pressure. A mathematical correction to the correct reference level is therefore preferable.

Does head correction have to be documented in the calibration certificate?

If it is relevant to the result, the reference level used or the applied correction should be documented in a traceable manner.

Does head correction belong in the measurement uncertainty?

Yes. If the correction is relevant, the uncertainties of its influencing quantities, such as height and density, should also be included.

How accurately must the height be measured?

This depends on the medium and the target measurement uncertainty. With liquids, an uncertainty of only a few millimeters can already be metrologically relevant.

Does local gravity matter?

For normal industrial calibrations, standard gravitational acceleration is often sufficient. For very high accuracy requirements, a local g value can be taken into account.

How does head correction work for differential pressure?

The hydrostatic corrections of the High-Pressure and Low-Pressure sides must be considered separately and then included in the resulting pressure difference.

Can the head corrections cancel each other out in differential pressure measurement?

Yes. With a fully symmetrical setup using identical media, heights and line conditions, the effects can largely cancel each other out.

What happens if there are air bubbles in a hydraulic calibration line?

They change the dynamic behavior and can influence the effective fluid column. Hydraulic systems should therefore be carefully vented.

Which instrument is suitable for high-precision stationary pressure calibration?

The Druck PACE5000E is designed for high-precision laboratory, test-bench and production applications.

Which pressure calibrator is suitable for on-site calibration?

The Druck DPI610E combines portable pressure measurement and pressure generation and is therefore particularly suitable for mobile test applications.

Which modular system is suitable for pressure and electrical signals?

The Druck DPI620G combines pressure calibration with electrical measurement and simulation functions as well as optional HART communication.

Where can I find further pressure calibration technology?

Further solutions can be found under calibration technology at ICS Schneider.