API 6A - Calibration Interval Extension of Pressure Gauge


Hi, I would like to have suggestions on increasing the calibration interval from 3 months. Currently we do 2 calibrations at 3 months/6 months / 9 months and if all results are within the acceptable range and no adjustments were done we increase the frequency which is documented on our internal procedures.

As per API 6A - Calibration intervals shall be established based on repeatability and degree of usage how we can show the compliance to this statement as to calculate the usage of Pressure gauge seems difficult and not having idea regarding the repetability with respect to pressure gauges.

Would love to hear from the experts.



Quite Involved in Discussions
Do you have any customer or industry standards that require you to conduct calibration of the gauge at pre-defined (maximum) intervals? What does the gauge manufacturer recommend? If you are not going against any of these then what you propose appears eminently sensible - but do you specify a maximum interval yourselves (e.g. 12 months)? Are measurements on this gauge critical to a process in any way? (How accurate is the gauge - to within 0.1%, or 10% or ??)


Trusted Information Resource
In practice, manufacturers mostly rely on the second paragraph in
Calibration intervals shall be a maximum of 3 months until recorded calibration history can be established by the manufacturer. Extension of intervals shall be limited to 3-month increases, with a maximum calibration interval to not exceed 1 year.
So usually the initial calibration interval is 3 months. Then if the gauge passes as-found calibration after at least two 3-month intervals, you could increase it to 6 months (you might also want to check that any deviation away from the desired value during a 3-month interval is less than half of the allowable deviation). In general, you probably need at least two passing as-found calibration results at an interval length before increasing it.

Many manufacturers also just leave it at 3 months. The cost of pressure gauge calibration is usually not very high and the effort required to justify the longer intervals (with the risk that an auditor or customer won't accept the justification) and the risk of the pressure gauge failing as-found calibration after a longer interval (and possible requirement to notify customers that the parts they received were tested with a gauge that failed calibration) can make extending the interval not worth it.


Hi Jmech, Thanks for the response. As per the first paragraph of your response only we have extended but the Auditor raised NCR that we have not considered Repeatability and usage for extending the frequency. Currently on our procedure we have mentioned we perform 3 month 2 cycles (If results are satisfactory - No adjustment and Error with in Acceptable range) we extend frequency to 6 months / 9 months ( 2 cycles each with same concept) and finally reach to 12 months. I would be intersted to know further more regarding Repeatability as stated on API 6A and how to comply to the same.

Charles Wathen

Involved - Posts
I'm retired now, but when I was working at Abbott, I would perform an interval analysis every other year in order to capture equipment that can be extended or even reduced due to failures. The software I used is free: Method A3 Interval Tester Calibration Interval Analysis Freeware from Integrated Sciences Group

It passed all of our audits with no issues by any auditor. I would extract data from our equipment database on the model type, then feed the information into the calculator. We also had a policy that if the interval got reduced to 1 month, we would no longer certify it because there was too much risk in it falling out of calibration.

lanley liao

Lingli Liao
Determining the repeatability and usage degree of an instrument involves a series of tests and assessments. Here are some suggested steps:

1. Determining Repeatability:​

1)Repeatability Testing:​

Conduct a series of repeated tests under the same conditions, recording the results of each test. This may include multiple measurements of the same standard value or condition.

2)Statistical Analysis:​

Use statistical methods (such as mean and variance) to analyze the results of repeated tests. Assess the repeatability by observing the distribution and variability of the data.

3) Variance Analysis:​

Identify factors that may cause variations in results through variance analysis, such as changes in environmental conditions or differences among operators.

4)Control Charts:​

Create control charts to monitor variations in measurement values. Control charts can help identify special causes of variation.

2. Determining Usage degree:​

1)Usage Frequency Recording:​

Record the frequency of instrument usage, including the number of times used per day, week, or month, and under what conditions.

2)Environmental Monitoring:​

Monitor the environmental conditions in which the instrument is used, such as temperature, humidity, and pressure. These conditions may impact the instrument's performance.

3)Operating Time Recording:​

Record the total operating time of the instrument within a specific time frame. This can help assess the level of usage.

4)User Feedback:​

Collect feedback and experiences from users. Users may provide valuable information about the instrument's performance and any issues encountered during actual use.

3. Adjusting Calibration Intervals Based on Results:​

1)Based on Repeatability:​

If repeatability tests indicate that the instrument's measurement results remain stable over a certain period, consider extending the calibration interval.

2)Based on Usage Level:​

Adjust the calibration interval based on the stability of the instrument's usage frequency and environmental conditions.

3)Record Calibration History:​

Maintain records of all calibration history, including any adjustments and issues. These records will aid in future calibration decisions.


When conducting these steps, it is advisable to refer to relevant standards and specifications, such as API 6A, to ensure compliance with industry best practices and requirements. Additionally, professional calibration services and expertise may be needed as necessary.
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