The accuracy of dial pressure gauges is crucial in industrial processes and decision-making!
Inaccurate pressure gauges can lead to errors in industrial processes or any decisions involving pressure range limitations, potentially resulting in catastrophic outcomes. No matter how exact the measurement process is, or how careful the operators are, if the accuracy of the pressure measuring instrument is compromised, errors will result.
This is where a professional instrumentation supplier comes in to perform pressure calibration for you.
Interested in understanding how to calibrate dial pressure instruments?
In the following article, SJ Gauge will introduce you to the accuracy grade and tolerance error of dial pressure gauges, explaining why pressure gauges need calibration. We will accompany this with simple and understandable examples to provide you with a thorough understanding.
If you're interested, you can continue reading this article, or reach out to us directly. SJ Gauge's professional sales team can also provide the most suitable pressure measurement solutions for your application!
Contents
1. What does "Accuracy" mean in dial pressure gauges? Understanding the definitions of terms related to pressure calibration.
a. Accuracy
b. Accuracy Grade
c. Tolerance (Permissible Error)
2. What is pressure calibration? Why do instruments need calibration?
3. Introduction to SJ Gauge's comprehensive instrument calibration process before and after delivery.
1. What does "Accuracy" mean in dial pressure gauges? Understanding the definitions of terms related to pressure calibration.
What does accuracy mean in a dial pressure gauge?
Accuracy is the combination of "precision" and "repeatability," indicating the degree of closeness or deviation between the measured pressure value (observed or read value) and its true value.
The accuracy indication of a pressure gauge is typically expressed as a percentage of full-scale (F.S.) accuracy (± accuracy %). High accuracy implies a high level of precision and accuracy.
What does Accuracy Grade mean in a dial pressure gauge?
Accuracy Grade is classified based on the percentage of tolerance allowed to the measurement instrument's range. The British Standards Institution (BSI), generally divides it into seven percentage grades: 0.5, 1, 1.5 (European standard 1.6), 2, 2.5, 3, 4, with smaller values indicating higher accuracy. According to the American Society of Mechanical Engineers (ASME), there are seven additional grades: 4A, 3A, 2A, A, B, C, D.
The accuracy percentage is usually followed by R (reading) or F.S.. If not specifically marked, it is considered ± accuracy % F.S. The use of R is reserved for instruments requiring precise measurements.
European Standard:
BS EN 837-1 specifies the relationship between the diameter of pressure gauges and the achievable accuracy grades (as detailed in the table below).
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American Standard:
Reference the commonly used accuracy grade table listed in ASME B40.100:
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Note: Grade 4A gauges must remain within specified tolerance before and after lightly tapped.
Referring to the above table, let's explore the optimal accuracy/accuracy grades that can be achieved by American and European standards for pressure gauge dial sizes related to pressure ranges. Let's illustrate with a simple example:
- Dial Size: 100mm
For a dial pressure gauge with a 100mm dial size, the optimal accuracy achievable is 0.25% per the European standard and 2A per the American standard.
- Dial Size: 250mm
For a pressure gauge with a 250mm dial size, the optimal accuracy achievable is 0.1% per the European standard and 4A per the American standard.
From these two examples, it is evident that the actual value and the magnitude of the tolerance error are not only related to accuracy but also to the pressure gauge's range and dial size.
When the range and dial size are the same, the higher the accuracy, the smaller the allowable error for the pressure gauge.
Tolerance represents the "maximum permissible error of measured data from the standard value" and can be denoted in the pressure unit of measurement.
For instance, a dial pressure gauge with a range of 0 ... 100psi and an accuracy of ±1% F.S. exhibits a tolerance of ±1.0 psi at any point on the measurement scale.
Referring to BS EN 837-1, specifies the maximum allowable tolerance for each accuracy grade.
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In summary, we can illustrate the understanding of these terms with an example:
- Scale range: 0 ... 10psi
- Accuracy: ±1.5% F.S.
The calculation of tolerance is as follows:
10psi (Range) x ±1.5% F.S. (Accuracy) = ±0.15psi (Tolerance).
This means that when the actual pressure is 6psi, the allowable tolerance range is between 5.85psi and 6.15psi.
2. What is pressure calibration? Why do instruments need calibration?
What is Pressure Calibration?
Calibration is the process of adjusting and testing to ensure that the pressure displayed by the instrument is within the specified accuracy range.
During calibration, we put this gauge and another one (a calibration standard gauge) in a separate system where they both experience the same pressure. Then, we compare the readings they show. Any differences between them are noted and expressed as a percentage of the gauge's full range.
The ASME recommends that the calibration standard gauge used be at least four times more accurate than the gauge being tested. We often call this a 4:1 ratio. It's like ensuring our measurement tool is super precise to guarantee the accuracy of the instrument we're checking.
Example:
Instrument Under Test
- Scale range: 0 ... 10psi
- Accuracy: ±1.0% F.S.
- Tolerance：10psi (Scale range) x ±1.0% F.S. (Accuracy) = ±0.1psi (Tolerance)
- 1/4 of Tolerance: ±0.1 psi (Tolerance) x 1/4 = ±0.025psi
Calibration Standard Gauge A
- Scale range: 0 ... 30psi
- Accuracy: ±0.1% F.S.
- Tolerance：30psi (Scale range) x ±0.1% F.S. (Accuracy) = ±0.03psi (Tolerance)
Calibration Standard Gauge B
- Scale range: 0 ... 20psi
- Accuracy: ±0.1% F.S.
- Tolerance：20psi (Scale range) x ±0.1% F.S. (Accuracy) = ±0.02psi (Tolerance)
In this example, Calibration Standard Gauge A's tolerance is not less than 1/4 of the allowable tolerance value (±0.025psi), but Calibration Standard Gauge B complies.
Therefore, Calibration Standard Gauge B should be used to meet calibration specifications.
Why do pressure instruments need calibration?
Reliable measurements and accurate pressure data contribute to enhanced production efficiency, reduce risks of unexpected shutdowns, and establish a foundation that meets environmental requirements. This, in turn, optimizes profits through more efficient production.
Even the highest-quality pressure gauges can lose some accuracy over time. To maintain the optimal reliability of your instruments, it's recommended to verify and calibrate them regularly.
When should pressure instruments undergo calibration?
Several common situations warrant calibration:
- New instruments: Fresh from the factory, they need calibration.
- Aged instruments: Calibrate regularly if used for over a year.
- Critical measurements: Calibrate before and after important data collection.
- Environmental impact: Calibrate instruments after exposure to events like strong vibrations or impacts.
- Display doubts: If you suspect inaccuracies, it's recommended to calibrate.
- Specified requirements: Calibration may be needed per customer or manufacturer specifications.
In all these situations, there's a potential for inaccurate readings. It's advisable to promptly send your instruments to a reliable service provider, such as SJ Gauge, for pressure instrument calibration and maintenance.
3. Introduction to SJ Gauge's comprehensive instrument calibration process before and after delivery.
Calibration Process at SJ Gauge:
We equip ourselves with the following devices for the calibration process:
- Test environment meeting the reference temperature of 20℃.
- Pressure manual pump with different pressure ranges.
- Calibration standard gauge.
- Process connection (threads) adapters.
- Pressure gauge to be calibrated.
Once all components are connected, we slowly apply pressure using the pressure pump. If the gauge to be calibrated is precise, both the calibration standard gauge and the gauge to be calibrated will display identical values.
The recommended number of test points for pressure gauge calibration, based on the BS EN 837-1 standard by the British Standards Institution, is outlined below:
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Example:
If the specifications of the pressure gauge to be calibrated are as follows:
- Scale range: 0 … 100 bar
- Accuracy: ±1.0% F.S.
- Accuracy grade: A
Then, testing and comparison should be conducted at five different test points:
- 0 bar (*Zero point must be tested when the needle is unblocked)
- 20 bar
- 40 bar
- 60 bar
- 80 bar
- 100 bar
If the accuracy of the pressure gauge after calibration is still lower than the originally guaranteed accuracy by the gauge manufacturer, it is recommended to communicate with the manufacturer or consider a direct replacement.
Benefits of choosing SJ Gauge as your pressure gauge manufacturer:
- High and consistent product quality
- Compliance with industry standards and regulations
- Process optimization and increased productivity
- Prevention of unexpected production downtime
- High-cost efficiency
- Quality to cost efficiency
With nearly three decades of professional measurement experience, SJ Gauge's Technical Research Department is dedicated to providing exclusive custom measurement solutions. Whether it's the selection of pressure gauge specifications, component assembly, calibration, or regular maintenance, we are committed to offering excellent products and support.
Click to contact us and let SJ Gauge provide the best custom measurement solutions for your industry application.
