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INTELLIGENT PROCESS MEASUREMENT
INSTRUMENTS |
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| 1. Calibration ≠ Verification |
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- Calibration: Comparison and correction to confirm the accuracy of the instrument and issue a calibration certificate; it does not determine pass/fail.
- Verification: Mandatory determination of pass/fail according to national regulations; it has legal effect and is mostly used for mandatory measuring instruments.
- In industrial settings, calibration is sufficient for the vast majority of instruments; only those listed in the mandatory verification catalog require verification.
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| 2. Calibration isn't just for when something breaks down; it should be done periodically. |
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- Instruments experience temperature drift, aging, wear, and vibration, causing their accuracy to gradually decline.
- Common instruments such as pressure, temperature, flow, and level gauges: generally calibrated once a year.
- Critical process/safety control points: this can be shortened to every six months.
- After new installation, repair, or relocation: calibration is mandatory before use.
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| 3. Incorrect calibration environment renders all results useless. |
Temperature, humidity, vibration, and unstable power supply all affect accuracy.
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- Aim for a temperature close to 20℃.
- Avoid strong electromagnetic interference and severe vibration.
- Let the instrument warm up and stabilize before starting calibration.
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| If the environment is not up to standard, even the most impressive calibration certificate is useless. |
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| 4. Calibration points should not be chosen arbitrarily; they must cover commonly used measurement ranges. |
| Don't just calibrate to full range or zero point; select points according to the actual usage range: |
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- Generally, at least: 0%, 25%, 50%, 75%, 100%
- Focus on the accuracy of commonly used operating points
- Measure both forward and reverse strokes to avoid hysteresis errors.
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| 5. Choosing the Right Calibration Method Ensures Sufficient Accuracy (New) |
| Different types of instruments require different calibration methods. The core principles are "standard comparison and error correction." Common methods include: |
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- Direct Comparison Method (Most Common): Use a standard instrument with 3-10 times higher accuracy than the instrument being calibrated to directly measure the same measurand. Compare the readings, calculate the error, and correct it. (Suitable for conventional instruments such as temperature, pressure, and voltage.)
- Substitution Method: Alternately connect the instrument being calibrated and the standard instrument to the same measurement system. Compare the measurement results to eliminate system errors. (Suitable for high-precision instruments and complex measurement scenarios.)
- Zero/Full-Scale Calibration Method (Quick Calibration): First calibrate the instrument's zero point (connect to the zero-value measurand), then calibrate the full-scale range (connect to the full-scale standard quantity). Verify frequently used points proportionally in between. Suitable for rapid on-site calibration and non-critical instruments.
- Closed-Loop Calibration Method: Connect the instrument being calibrated to the actual working loop. Use a standard instrument to monitor the loop output and correct the instrument parameters based on loop feedback. (Suitable for instruments linked to the system, such as flow and level meters.)
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| Note: After calibration, the correction value must be recorded to ensure that the instrument reading is consistent with the standard value. |
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| 6. Records and labels must be made after calibration. |
| Calibration is not finished once the adjustment is complete: |
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- Retain original data and calibration certificate.
- Affix calibration labels: date, expiration date, calibrator.
- Instruments exceeding tolerance: Immediately stop use, isolate, and rework; operation with defects is strictly prohibited.
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| 7. Calibration isn't always better the more expensive it is; matching accuracy is what makes it worthwhile. |
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- Calibration equipment accuracy ≥ 3~10 times that of the instrument being calibrated
- Ordinary industrial instruments don't need ultra-high precision standards; it's a waste of cost.
- Critical safety instruments: must be calibrated using compliant, qualified organizations/equipment.
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