Gas and LPG Pressure Gauges: Low Pressure Capsule and Bourdon Types

For low pressure gas work, the key is a gauge with a very low range and enough sensitivity to read a few kilopascals, such as a capsule gauge reading 0 to 60 kPa. Higher pressure gas, such as cylinder or regulator inlet pressure, uses a conventional Bourdon tube gauge with the right range, materials and gas-specific cleaning. This guide explains how gas pressure gauges work, the difference between low pressure and high pressure gas gauges, how to choose a range and material, how to install them, and the rules around gas work in Australia, which must be done by licensed people.

Important: gas work is regulated

Installing, testing or altering gas fittings in Australia is controlled work. In most states and territories, only a licensed gasfitter may work on gas installations, and testing a gas system for tightness is part of that work. This guide explains the instruments, not how to carry out regulated gas work yourself. If you are not licensed for the task, engage someone who is.

Low pressure vs high pressure gas

Gas pressures vary enormously, and the gauge has to suit the pressure.

• Very low pressure (a few kPa): found at appliance inlets and in low pressure reticulation. Typical appliance supply pressures are in the order of one to three kilopascals, depending on the gas, so a standard Bourdon gauge reading 0 to 1,000 kPa would barely move.
• Medium pressure (tens to hundreds of kPa): found in commercial and industrial supply lines before the final regulator.
• High pressure (cylinders and bulk supply): cylinder contents can be hundreds of bar, and the gauge sits on a regulator.

Each of these needs a different gauge, so start by establishing what pressure range you are working with.

Capsule gauges for low pressure gas

Where pressures are only a few kilopascals, a capsule gauge is the right tool. Instead of a Bourdon tube, a capsule gauge uses a thin, flexible capsule made of two diaphragms joined at the rim. A tiny pressure creates a visible movement. Our KPCh low pressure gauge is a capsule design with a 63 mm dial and a 304 stainless steel case. Its key features are:

• Ranges from 0 to 2.5 kPa up to 0 to 60 kPa, with compound ranges starting at -60/0 kPa
• Accuracy of ±1.6% of full scale
• A 1/4″ BSPT lower mount (LM) or centre back mount (CBM) connection
• Brass, copper/beryllium and Buna-N wetted parts, with an acrylic window
• A scale in kPa only, supplied dry
• Designed for air and gas only. It is not suitable for liquids

The datasheet describes the KPCh as suited to air and gas, and the capsule and seals are made from specific materials. If your gas is a particular type, such as LPG or a corrosive gas, ask us to confirm that the wetted parts are compatible before you buy.

Bourdon gauges for higher gas pressures

For medium and high pressures, a standard Bourdon tube gauge works well. Choose a range with sensible headroom, as covered in our range selection guide, and the wetted material to suit the gas. Brass is common for many non-corrosive gases, and stainless steel suits corrosive gases or clean applications. How a Bourdon tube works is explained in Bourdon tube pressure gauges explained.

Gas gauges used on compressed gas cylinders usually have two gauges on the regulator. One shows the cylinder contents (high pressure) and the other the delivery pressure (low pressure). Both should be rated and marked for the specific gas.

Dry or liquid filled for gas?

Gas gauges are normally supplied dry. Liquid fill can contaminate certain gases and is not suitable for oxygen. For gas service, consider the gas type first. Our guide to liquid filled vs dry pressure gauges explains why oxygen service needs a cleaned, oil-free gauge, and why liquid filled models are not suitable.

Gas type and materials

Different gases demand different materials and cleaning.

• Air, nitrogen, argon, carbon dioxide: usually suit standard brass or stainless steel gauges. Check the datasheet.
• Oxygen: requires a gauge that is cleaned for oxygen service and is oil and grease free. Never use a standard gauge, and never use a liquid filled one.
• Acetylene: must not contact copper-rich alloys, so it needs gauges made for it. Use only gauges marked for acetylene.
• LPG and natural gas: need gauges compatible with the gas and rated for the pressure. Confirm the wetted materials and seals with the supplier.
• Corrosive or toxic gases: usually require stainless steel or special alloys and specialist advice.

When in doubt, ask. A wrong gauge on a reactive gas is a safety hazard.

How to read a low pressure gauge

1. Check that the pointer rests at zero with the gauge open to atmosphere.
2. Read the dial from straight on to avoid parallax.
3. Note the units. A kPa-only scale such as the KPCh is not interchangeable with a psi scale. One kPa is about 0.145 psi, and 100 kPa equals one bar.
4. Allow the reading to settle before recording, because a capsule can take a moment to stabilise.
5. Record the value, the time and the conditions.

Digital gauges and manometers for low pressure gas

A digital manometer or low range digital gauge reads small pressures to a fine resolution and can store minimum and maximum values. These are widely used by technicians for appliance checks and for logging the pressure over time. Our article on pressure gauge vs pressure transmitter vs manometer explains the differences. For higher gas pressures, digital gauges in the digital pressure gauge range suit testing and calibration.

Installation notes

• Use the correct thread and sealant. Tapered BSPT threads use a sealant approved for the gas. See pressure gauge connection types and thread sizes.
• Fit an isolation valve so the gauge can be removed or tested.
• Do not exceed the gauge range. The KPCh has overpressure protection of 130% of full scale, and exceeding it will damage the capsule.
• Mount in the correct orientation, and keep the gauge away from heat and vibration.
• Leak test every connection with a method approved for the gas, such as an approved leak detection fluid, never with a naked flame.
• Vent the gas away from ignition sources and in a ventilated area.

Accuracy and calibration

At low pressure, a small absolute error is a big percentage of the reading. A ±1.6% gauge on a 0 to 6 kPa range may be wrong by about 0.1 kPa, which could matter for appliance setting. Choose the range close to the pressure of interest, and have the gauge checked against a reference at regular intervals. See pressure gauge calibration.

Choosing between a gauge and a manometer for gas

For appliance and low pressure checks, many technicians prefer a digital manometer because it displays a fine reading and can hold the value. A capsule gauge, such as the KPCh, is simple, needs no battery and gives a continuous visual indication, which suits a permanent installation or a workshop test rig. Choose according to the job: a permanent indicator, a portable tester or a data record.

Cylinder regulators and gauge faces

The gauges on a gas regulator are part of a pressure control assembly, and they are matched to the regulator and the gas. If a gauge on a regulator fails, replace it with the same type and range specified for that regulator and gas, rather than swapping in a general-purpose gauge. Check the markings on the dial for the gas and the maximum pressure, and do not use a gauge whose markings are missing or unclear.

Storage and handling

Keep spare gauges clean, dry and capped, and store them in a protective case. Dust and moisture can enter an open port and affect a gas system. Label gauges that have been used on a particular gas so they are not mixed up. Check for damage before use, especially after any knock, as a bent pointer or a cracked window means the gauge should not be fitted.

Leak testing and record keeping

After any work on a gas line, a tightness test is normally required. The method, the test pressure and the permitted pressure drop depend on the installation type and the regulations that apply, and they are carried out by a licensed person. A gauge of suitable range and resolution is part of that test, and the readings should be recorded with the date, the time and the instrument used. Keep the calibration record of the instrument as well, as it shows the reading can be trusted.

Frequently asked questions

Can I use a water gauge for gas?

Not unless the range, materials and rating suit the gas. Many water gauges are too high in range to read low gas pressures.

What is the difference between kPa and mbar?

One kPa equals 10 mbar. A gauge reading 2.75 kPa is also reading 27.5 mbar.

Is the KPCh suitable for liquids?

No. The datasheet states that it is not suitable for liquids and is intended for air and gas.

Can I use any gauge on an oxygen cylinder?

No. Oxygen requires a gauge cleaned and marked for oxygen. Oil or grease and oxygen can ignite violently.

Do I need a licence to test a gas system?

For regulated gas installations, generally yes. Check with your state regulator or use a licensed gasfitter.

Find the right gas gauge

Tell us your gas, pressure range and connection through our contact page, or browse the pressure gauges category.