Calibration work seems to always be arranged at the most critical moments of the project, such as when the work team is busy preparing for the annual trade show. As an example to illustrate the impact of calibration on the project, we assume that the calibration cycle for a test device is 6 months. In the fifth month, the design engineer launched a test project with a duration of two months. If the instrument is recalibrated during the test, the accumulated drift or error will be larger in the first 5 months, which will result in the need to retest. Is it better to calibrate the device before launching a large project that requires the device? Or do you postpone the calibration to prevent accidents? As with any other event, a calibration plan should be developed in the project planning software and should be in a critical path. If the calibration is ignored, the project will be delayed.
What is calibration?
Some people consider two instruments with the same reading (such as an oscilloscope and a multimeter) to be "calibrated." However, this method has problems, at least not completely unscientific. This is like having an accountant audit his own accounts. If you calibrate in this way, there are three obvious situations that cannot be explained: First, if one instrument is correct and the other is wrong, who is wrong? Second, if the two instruments are in the wrong way, how can the engineer explain that the two are not correct? Finally, if the two instruments are in the same way, the result is wrong and the engineer is unaware of it. Without a true traceable external standard, one cannot say that an instrument is correct.
For this task, the calibration standard (the parts in the circle) is critical. Thanks to NASA/JPL-Caltec for providing pictures.
The accuracy of the calibration standard must be much higher than the accuracy of the instrument being tested. Don't forget, there are tolerances in the standard. If the tolerance range of the device under test (DUT) overlaps the standard tolerance range, full calibration cannot be achieved. This is why the accuracy of the standard is usually required to be at least 10 times higher than the accuracy of the DUT. If the standard tolerance is very certain and small enough, the DUT can be adjusted to prevent readings from exceeding the standard due to normal drift during the two calibrations. When calibrating instruments with the latest technology, the standard accuracy cannot be 10 times higher. Based on practical experience, standards that are four times more accurate than precision can be used, but with more complex processes, including cross-checking with other standards.
Metrology laboratory
Rigorous engineers buy top-of-the-line instruments and expect high precision from these electronic test instruments. As the usage time lengthens and some parameters drift, the engineer sends the instrument to the metrology lab for calibration. But what about the real situation of the measurement laboratory?
Most metrology laboratories are good and effective. Unfortunately, some are unqualified. Some companies choose the lowest-priced laboratory, and do not investigate their qualifications. The engineer is responsible for verifying the authenticity of the laboratory. A good lab is happy to let you visit and be proud of its standard traceability. You can take the instrument's calibration manual and sit with the lab staff to verify that the lab has the equipment necessary to perform a high-precision calibration.
You must also ensure that the laboratory's instruments are properly calibrated and traceable. Most countries have their own national standards laboratories, such as the National Institute of Standards and Technology (NIST) in the United States1. Good metrology laboratories have records that their instruments are correctly aligned with the standard chain and traceable to the main standards maintained by NIST or other national standards.
Internal parts of the test instrument (such as voltage references, input voltage dividers, amplifier gains, and offsets) drift over time. A good calibration method ensures that a typical small drift does not affect the measurement. Calibration is to find and correct this drift. But sometimes there are accidents: the instrument may fall or the operator may slip and detect higher voltages. For example, a digital multimeter (DMM) can be overloaded, resulting in large errors. Since the input of the digital multimeter is protected by a fuse or Circuit Breaker, some people mistakenly believe that this situation will not cause it to exceed the standard. However, high voltages may skip the input protection device or cause damage to the circuit immediately before the protection device functions.
When we send the instrument to calibration, we want the metrology lab to return the instrument to its calibration state. Engineers should also receive a calibration report that lists the instrument's offset before and after calibration. If the calibration report indicates that the instrument has a large calibration error, it may be necessary to re-execute the test project that has been completed using the instrument for a new measurement.
What are the requirements for the frequency of calibration?
This issue cannot be generalized because it varies from instrument to instrument to environment and application. The test instrument manufacturer will give the recommended calibration cycle under typical conditions; extreme conditions and critical measurements may require more frequent calibration. Here are some general rules about the calibration cycle:
1. Routine calibration must be performed according to user contract, quality standards organization, military specifications, or other industry requirements. Prior to testing, applicable requirements must be considered to ensure calibration or verification of the test equipment is met.
2. Before and after the key measurement project. For example, after completing a trial run of a new product, the design engineer will perform a feature analysis on the product to ensure that the technical requirements are met and the test steps are optimized. The final test adjustments made at this time will fundamentally reduce test time and affect revenue. Complete and reliable testing requires verification of the state of the instrument before and after the test cycle.
3. Suspected measurement error, or when the instrument is overloaded or dropped. It is important to check that the calibration and safety status (eg, the conductor is shorted to the case when the instrument is dropped).
High-precision calibration is not an optional luxury – calibration ensures the reliability of the test instrument and even the safety of the person. For example, an engineer might measure the meter voltage to ensure it is safe before using the device. If the meter is damaged or the information provided is inaccurate, it can cause personal injury or even death. In addition, calibration is a quality guarantee that ensures the accuracy of the final test results of the product and meets the specifications when delivered to the customer.
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