Choosing a pressure gauge: an overview of types, measuring ranges and accuracy classes
The pressure in your plant is correct, but is the pressure gauge accurate? An incorrectly selected pressure gauge may well display a reading, but how close that reading is to the actual value determines process reliability, product quality and, in extreme cases, plant safety. Three factors make all the difference: design, measuring range and accuracy class.
Why the right pressure gauge is crucial to your process quality
Imagine this: a Class 2.5 Bourdon tube pressure gauge displays a reading of 6 bar when the full-scale range is 10 bar. It sounds accurate, but the permissible deviation is ±0.25 bar. The actual pressure could therefore be anywhere between 5.75 and 6.25 bar. In a food processing application with tight pressure tolerances, this would be a problem.
Choosing the right pressure gauge is therefore no trivial matter. It directly influences how reliably you control your processes, how safely your plant operates, and whether you meet the requirements of standards such as DIN EN 837-1. There are three key factors to consider: the design of the measuring mechanism, the measuring range and the accuracy class.
Pressure gauge types: Which measuring principle is right for your application?
Not all pressure gauges work in the same way, and that is precisely the key to making the right choice. The design determines which pressure ranges can be measured, how robust the device is and which media it is suitable for.
Bourdon tube pressure gauges
The classic among pressure gauges, and still the most commonly used design in industry. A curved metal tube (the Bourdon tube) deforms under pressure and transmits the movement to the pointer. Simple, robust and tried and tested.
- Measuring range: -1 bar to approx. 2,500 bar
- Typical media: compressed air, water, hydraulic oil, industrial gases
- Accuracy: Class 1.0 to 2.5 (depending on nominal size)
- Advantages: wide pressure range, cost-effective, versatile
Diaphragm pressure gauges (membrane pressure gauges)
Where Bourdon tube pressure gauges reach their limits, diaphragm pressure gauges take over. A flexible diaphragm separates the measuring mechanism from the medium, making it ideal for aggressive, viscous or crystallising media. This design is standard in the chemical and food industries.
- Measuring range: -1 to approx. 40 bar
- Typical media: Aggressive chemicals, slurries, pasty substances
- Accuracy: Class 1.6 to 2.5
- Features: Resistant to media, hygienic connections possible, up to 5-fold overload protection
Capsule-spring manometers
For low-pressure measurements below 0.6 bar, neither tubular springs nor diaphragm springs are sufficient; this is where the capsule spring comes into play. Two diaphragms welded together form a capsule that deforms even at the slightest pressure changes. Typically used in ventilation technology and for filter monitoring.
- Measuring range: -25 mbar to 1000 mbar (low pressure)
- Typical media: air, non-corrosive gases
- Accuracy: Class 1.6 to 2.5
- Strengths: accurate measurement at low pressures, very good overload protection
High-precision pressure gauges
When standard accuracy is not sufficient, high-precision pressure gauges are used. They are also based on the Bourdon tube principle, but utilise higher-quality measuring mechanisms and larger casings (from NG160 upwards). They are indispensable in laboratories, on test benches and in quality assurance.
- Measuring range: Same as bourdon tube manometers
- Accuracy: Class 0.1 to 0.6 %
- Features: Highest precision, suitable for use as a reference and for calibrations
A direct comparison of pressure gauge types
The following table shows you at a glance which model is suitable for which application, from compressed air treatment to pharmaceutical production.
| Criterion | Tubular spring | Leaf spring | Capsule spring | Precision measuremen |
|---|---|---|---|---|
| Measuring range | -1 – +2’500 bar | -1 – +40 bar | -25 mbar – +1000 mbar | -1 – +1’000 bar |
| Accuracy class | 1.0–2.5 | 1.6–2.5 | 1.6–2.5 | 0.1–0.6 |
| Nominal sizes (NG) | 25 – 250 mm | 63–160 mm | 63–160 mm | 160 + 250 mm |
| Aggressive media | Subject to conditions (stainless steel version) | Yes | Subject to conditions (stainless steel version) | Subject to conditions (stainless steel version) |
| Typical sectors | Mechanical engineering, pneumatics, hydraulics | Chemicals, Pharmaceuticals, Food | Ventilation, HVAC, filter monitoring, medical gases, low-pressure applications | Laboratory, Quality Assurance, Calibration |
| Standard | DIN EN 837-1 | DIN EN 837-3 | DIN EN 837-3 | DIN EN 837-1 |
Choosing the correct measuring range: the 2/3 rule
The most common source of error when selecting a pressure gauge is choosing the wrong measuring range. A pressure gauge with a 0–100 bar scale that operates continuously at 5 bar will display a reading, but the accuracy of that reading will be poor. Conversely, continuous operation close to the full-scale value damages the measuring mechanism.
The rule of thumb for static pressures
The operating pressure should be around two-thirds of the full-scale value, i.e. between 60 per cent and 75 per cent of the display range. Within this range, accuracy is at its highest and the mechanical stress remains within safe limits.
Practical example
Your system operates at a working pressure of 6 bar. The optimal full-scale range:
- 6 bar ÷ 0,75 = 8 bar (minimum)
- 6 bar ÷ 0,60 = 10 bar (maximum)
- Recommendation: Pressure gauge with a 0–10 bar scale
Special case: Pressure peaks and dynamic loads
If regular pressure peaks occur in your system – for example, due to pumps, valve switching or hydraulic surges – the 2/3 rule is not sufficient. In such cases, the peak pressure should not exceed 90 per cent of the full-scale value. In addition, pressure gauges with ‘DAMP’ shaft damping or glycerine filling are recommended, as these dampen vibrations and stabilise the pointer. For severe pressure spikes, an adjustable restrictor for pressure gauges is also suitable.
Practical tip: In systems with severe pressure surges, an upstream restrictor (DIN 16272) protects the pressure gauge from sudden pressure spikes. This significantly extends its service life.
Understanding accuracy classes: what the number on the dial really means
The accuracy class is the most important parameter when it comes to measurement quality. It specifies the maximum permissible deviation as a percentage of the full-scale value, as defined by DIN EN 837-1 or DIN EN 837-3.
In practical terms, this means that a Class 1.6 pressure gauge with a full-scale range of 10 bar may deviate by a maximum of ±0.16 bar from the true value at any point on the scale. The lower the class number, the more accurate the instrument.
An overview of the accuracy classes according to DIN EN 837
| Class | Max. deviation | Typical nominal size | Area of application |
|---|---|---|---|
| 0.1 | ±0,1 % | From NG160 | Calibration laboratory, reference measurement |
| 0.6 | ±0,6 % | From NG160 | Precision measurement, quality assurance |
| 1.0 | ±1,0 % | NG100/NG160 | Process engineering, high-end industrial applications |
| 1.6 | ±1,6 % | NG63/NG80 | Industry standard, operational monitoring |
| 2.5 | ±2,5 % | NG40 | Compressed air networks, heating, basic monitoring |
Calculation example: What does Class 2.5 mean in practice?
A Class 2.5 pressure gauge with a full-scale value of 10 bar:
- Maximum deviation: 10 bar × 2.5% = ±0.25 bar
- The displayed value of 6.0 bar may actually be between 5.75 and 6.25 bar
- With a displayed value of 2.0 bar, the deviation is still ±0.25 bar, which is 12.5% in relative terms
Important: The deviation always refers to the full-scale value, not to the value currently displayed. This is precisely why selecting the correct measurement range is so crucial; if you take a reading at the lower end of the scale, the measurement will be significantly less accurate in relative terms.
Practical guide: How to choose the right pressure gauge in five steps
Now that you’ve got the theory sorted, it’s time to make a specific choice. These five questions will guide you systematically to the right pressure gauge.
What medium flows through your system? For compressed air, water or hydraulic oil, a Bourdon tube pressure gauge is sufficient. Corrosive, viscous or crystallising media require a diaphragm pressure gauge, or a Bourdon tube pressure gauge with an upstream pressure transmitter.
Determine the operating pressure and any possible pressure peaks. Apply the 2/3 rule: operating pressure = approx. 66 per cent of the full-scale value. For dynamic loads, keep the peak pressure at a maximum of 90 per cent.
Ask yourself: How critical is measurement accuracy for your process? For compressed air monitoring in a workshop, Class 1.6–2.5 is sufficient. For pharmaceutical processes or test benches, you will need Class 1.0 or better.
The nominal size determines the readability and limits the available accuracy class. For Class 1.0, you need at least NG100; for Class 0.6, at least NG160. Common process connections are G¼" and G½" in accordance with DIN EN 837.
The position of the process connection is also important. Should the connection face downwards (radial) or towards the rear (axial)?
Vibrations, extreme temperatures or humidity? If so, choose a pressure gauge with “DAMP” shaft damping or one with a glycerine filling and a stainless steel case. For use in potentially explosive atmospheres, additional ATEX requirements apply.
When the standard pressure gauge isn’t enough: special versions and accessories
In practice, a standard pressure gauge alone is rarely sufficient to meet all requirements. Often, the right accessories are key to the gauge’s service life and the quality of the measurements.
- Pressure transmitters: Protect the measuring mechanism from aggressive, hot or particulate-laden media. Indispensable in the chemical and food industries
- Throttle valves: Dampen pressure peaks and protect against hydraulic shocks (in accordance with DIN 16272)
- Shut-off valves / test valves: Enable the pressure gauge to be replaced whilst under pressure; important for systems requiring frequent maintenance
- Electrical contacts: Add switching functions to the pressure gauge, for example for limit value monitoring via a pressure switch
TRI-MATIC offers a carefully selected range of pressure gauge accessories, such as restrictor nozzles, shut-off valves and connectors, to suit all standard pressure gauges in its range.
Pressure gauges as part of an assembly, all from a single source
In many systems, the pressure gauge is not a stand-alone component but part of a complex measurement and control assembly. Pressure gauges, pressure transmitters, shut-off valves, pipework and documentation – when all these elements come from different sources, the coordination effort increases exponentially.
TRI-MATIC supplies such measurement assemblies fully assembled, tested and documented. From assembly to the fully tested unit, you receive a ready-to-install unit rather than individual components. This saves installation time, reduces sources of error and simplifies stock management.
Conclusion: Three choices, one suitable pressure gauge
Choosing a pressure gauge is not rocket science, but it does require a systematic approach. Three decisions determine whether your pressure gauge will provide reliable readings:
- Design: Bourdon tube for the vast majority of applications. Diaphragm for aggressive media. Capsule spring for low pressure. Precision gauge for the highest accuracy.
- Measuring range: Operating pressure at two-thirds of the full-scale value. Select a higher full-scale value for pressure peaks.
- Accuracy class: From Class 2.5 for non-critical monitoring to Class 0.1 for calibration laboratories; the requirement defines the class, not the price.
Are you unsure which pressure gauge your application requires? TRI-MATIC’s measurement technology specialists will advise you personally, from the selection of individual instruments to a fully assembled measurement unit.
Contact measurement technology experts now View our range of pressure gauges
Frequently Asked Questions
That depends on the criticality of your process. Class 2.5 is sufficient for compressed air monitoring in a workshop. In process engineering – for example, in the food, pharmaceutical or chemical industries – Class 1.6 or 1.0 is standard. For test benches and calibrations, you should opt for Class 0.6 or better. The key point is this: the accuracy class always refers to the full-scale value, not the measured value. That is why selecting the appropriate measuring range is at least as important.
Whenever the medium could corrode the measuring mechanism, for example in the case of aggressive chemicals, crystallising liquids or slurries containing solids. The pressure transmitter separates the measuring mechanism from the process medium via a diaphragm and transmits the pressure via a filling fluid. This protects the pressure gauge and ensures a reliable measurement. For severe pressure spikes, an adjustable restrictor for pressure gauges is also recommended.
A glycerine-filled pressure gauge contains a viscous liquid in its housing which fulfils two functions: it dampens vibrations, ensuring the pointer remains steady even under pulsating pressures; and it protects the measuring mechanism from condensation and corrosion. A glycerine filling is particularly recommended for pumps, compressors and hydraulic systems. You can recognise the filling by its oil-like appearance behind the sight glass.
There is no legally prescribed timeframe; the calibration interval depends on the application. In practice, an interval of 12 to 24 months has proven effective. In safety-critical or quality-relevant processes (e.g. under ISO 9001 or GMP), shorter intervals may be required. Documentation is key: keep a calibration log and define thresholds beyond which replacement is necessary.
The basic rule: Bourdon tube pressure gauges are the first choice for clean, non-corrosive media; they cover the widest pressure range and are the most cost-effective. Use diaphragm pressure gauges if the medium could corrode or clog the Bourdon tube. In the food and pharmaceutical industries, diaphragm pressure gauges are often used with hygienic flush-mounted flange connections. A third option is a Bourdon tube pressure gauge with an integrated pressure transmitter, which combines the advantages of both types.
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