How Do Bourdon Tube Gauges And Dp Cells Differ? Choose The Right Pilot Plant Instrument

Bourdon gauges provide simple local pressure readings, while DP cells measure pressure differences for monitoring, control, and inferred process variables. A Bourdon gauge converts the deformation of a pressurized metal tube into pointer movement on a dial. A DP cell detects the force caused by pressure on two sides of a diaphragm and can transmit the resulting differential measurement to a control or data-acquisition system.

The practical distinction is direct local indication versus comparative process measurement. Bourdon gauges are rugged and independent of electrical power; DP cells are more versatile because the pressure difference between two points can reveal flow, liquid level, filter condition, or line pressure.

How the Measurement Mechanisms Differ

Bourdon gauge: pressure becomes mechanical movement

A Bourdon gauge contains a hollow, curved metal tube fixed at one end and sealed at the other. When the internal process pressure rises above the reference pressure, usually atmosphere, the tube tends to straighten slightly.

A linkage and gear mechanism magnify this small motion and rotate a pointer across a calibrated dial. The operator reads the gauge pressure directly at the instrument.

DP cell: pressure difference becomes diaphragm force

A DP cell has two isolated pressure compartments connected to the process through separate high-pressure and low-pressure impulse lines. The pressures act on opposite sides of a flexible diaphragm.

The diaphragm displacement is proportional to the applied pressure difference within the calibrated operating range. An electronic, pneumatic, or mechanical force-balance system converts that displacement into a measurement signal.

The reference pressure is different

A Bourdon gauge normally compares process pressure with atmospheric pressure. Its reading is therefore gauge pressure:

[ P_\text{gauge}=P_\text{process}-P_\text{atmospheric} ]

A DP cell measures:

[ \Delta P=P_\text{high}-P_\text{low} ]

If the low-pressure side is vented to atmosphere, a DP cell can approximate a gauge-pressure measurement. Its more important capability, however, is measuring pressure between two process locations.

Why Their Pilot Plant Uses Differ

Bourdon gauges support immediate field checks

Bourdon gauges are commonly installed near pumps, vessels, reactors, compressors, and process lines. They allow operators to confirm pressure locally without electrical power, signal wiring, or a control system.

This makes them useful for routine checks, startup activities, troubleshooting, and confirming that equipment is operating within a safe pressure range.

DP cells support remote monitoring and control

A DP cell can provide a standardized output to an indicator, data logger, distributed control system, or feedback controller. This is valuable when pilot plant experiments require continuous recording or automated response.

The instrument can also be installed away from the operator station, provided the impulse-line arrangement and process connections are properly designed.

DP across a restriction can indicate flow

When a DP cell measures the pressure drop across an orifice plate or another flow restriction, the differential pressure can be related to flow rate through the applicable flow equation and calibration.

This is useful in pilot plants for studying flow behavior, comparing operating conditions, or controlling feed rates. The relationship is not generally linear: for many restriction-based measurements, flow is approximately proportional to the square root of differential pressure.

DP across a vessel can indicate liquid level

A DP cell can measure the hydrostatic pressure difference between the lower and upper connections of a tank or column. That differential corresponds to liquid head and can therefore be calibrated as liquid level.

For a vessel operating under pressure, using both connections allows the common vessel pressure to cancel while retaining the hydrostatic component. This is one reason DP cells are more useful than a single local pressure gauge for level measurement.

DP can measure pressure drop through equipment

Pressure loss across filters, packed beds, heat exchangers, valves, piping sections, and distillation-column internals can indicate hydraulic resistance or equipment condition.

In a pilot plant, this supports unit-operation studies and helps identify fouling, plugging, flooding, or changes in flow regime.

What Each Instrument Tells the Operator

A Bourdon gauge answers “What is the local pressure?”

The gauge reports the pressure at its connection relative to atmosphere. It is well suited to questions such as whether a pump discharge is pressurized, whether a vessel is approaching its operating limit, or whether a line has depressurized.

It does not inherently compare two process points or calculate flow and level.

A DP cell answers “How different are these two pressures?”

The DP cell reports the pressure relationship between its high and low sides. The same basic instrument can therefore serve several functions depending on where the impulse lines connect.

Its reading has meaning only when the connection locations, reference side, density, elevation, and calibration are understood.

Understanding the Trade-offs

Bourdon gauges favor simplicity and ruggedness

Their principal advantages are low complexity, low cost, immediate visual access, and operation without external power. They are often effective as independent local indicators or as a second indication of a critical pressure.

Their limitations include manual observation, limited suitability for automated data collection, mechanical hysteresis, vibration sensitivity, and possible reading errors caused by parallax or a damaged mechanism.

DP cells favor versatility and signal integration

A DP cell can support remote indication, control, trending, alarms, and inferred measurements such as flow and level. It is also better suited to small pressure differences when a properly selected and calibrated transmitter is used.

However, it requires more installation effort. Impulse lines can plug, leak, condense, freeze, or develop unequal liquid columns, and those effects can create measurement errors.

Process compatibility matters

Bourdon tubes and DP-cell diaphragms must be compatible with the process fluid, pressure, temperature, corrosion environment, and cleaning requirements. Chemical service may require appropriate alloys, diaphragm seals, flushing arrangements, or chemical isolation.

A mechanically robust instrument is not automatically suitable for corrosive, viscous, reactive, toxic, or high-temperature service.

Do not confuse DP cells with differential manometers

A differential manometer measures pressure difference using the height of a known fluid column, commonly in a U-tube. It can be highly useful for small pressure drops in pilot experiments, but it is a different instrument from a diaphragm-based DP cell.

The manometer is a direct fluid-static measurement and is often simple and accurate for low-pressure laboratory work. A DP cell is generally more convenient for remote transmission, control, and integration with plant instrumentation.

Measurement range affects the choice

A Bourdon gauge is usually selected for the expected direct pressure range and required local accuracy. A DP cell must be selected for both the expected differential-pressure range and the static pressure that may be present on either side.

Choosing a DP transmitter with an excessive range can reduce resolution for a small pressure drop. Choosing one with insufficient static-pressure capability can damage the sensor even when the differential pressure itself is small.

Making the Right Choice for Your Goal

Select the instrument according to whether the pilot plant needs a local pressure check, a transmitted process variable, or a pressure difference between two defined points.

If your primary focus is local operator verification: Use a Bourdon gauge for a rugged, readily visible indication of gauge pressure at the measurement point.
If your primary focus is automated monitoring or control: Use a DP cell when the measurement must be transmitted to a control system, recorder, alarm, or data-acquisition platform.
If your primary focus is flow measurement: Use a DP cell across a calibrated restriction such as an orifice plate, with the appropriate flow relationship and installation correction.
If your primary focus is vessel or column level: Use a DP cell across suitable high- and low-side connections to measure hydrostatic head while accounting for fluid density and vessel pressure.
If your primary focus is small experimental pressure drops: Consider a differential manometer or a suitably ranged DP cell, depending on whether direct visual measurement or remote data collection is more important.

Bourdon gauges show local pressure directly, whereas DP cells turn pressure differences into versatile process measurements for pilot plant analysis and control.

Summary Table:

Feature	Bourdon Tube Gauge	DP Cell
Measurement	Local gauge pressure	Pressure difference between two points
Mechanism	Pressurized tube moves a pointer	Diaphragm responds to differential pressure
Output	Direct visual dial reading	Transmitted electronic, pneumatic, or mechanical signal
Common uses	Local checks at pumps, vessels, reactors, and lines	Flow, level, pressure drop, remote monitoring, and control
Main strengths	Rugged, simple, low cost, no external power	Versatile, remote-capable, suitable for automation and data logging
Key considerations	Vibration, hysteresis, parallax, and manual reading	Impulse-line plugging, leaks, condensation, calibration, and range selection

Need reliable instrumentation for your pilot plant? Contact LABPARK to discuss the right Educational and Vocational Unit Operations Pilot Plant for your application. LABPARK provides integrated solutions for chemical engineering, bioprocess and biotech, and environmental and water treatment training, research, and process development. Our team can help universities, research institutes, and enterprises select suitable Bourdon gauges, DP cells, flow and level measurement systems, process compatibility options, and data-acquisition features for safe, accurate, and practical pilot-scale experiments.