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When conducting field tests for pressure transmitters, measurement accuracy can be affected by various factors, and it's crucial to minimize these impacts within acceptable limits. To reduce measurement errors, appropriate measures should be taken based on the source and nature of the error.
Firstly, addressing static pressure errors is essential. Differential pressure transmitters are typically calibrated at low pressures, but they're often used under conditions where both high and low-pressure chambers experience static pressure. The primary cause of static pressure-induced changes in output is mechanical deformation of the pressure-receiving components. Even though the deformation caused by silicone oil inside the housing is minimal, highly sensitive meters can still exhibit noticeable shifts. Two methods are commonly used to measure static pressure errors:
1) **Single Static Pressure Test**: Apply the nominal pressure to both high and low-pressure chambers separately, then measure the basic and return errors after releasing the pressure to ensure compliance with qualification standards.
2) **Static Pressure Test**: Simultaneously apply the rated pressure to both chambers, ensuring the output lower limit post-pressure release meets verification regulations.
Secondly, managing zero position changes is vital. The installation position of the differential pressure transmitter significantly affects zero output, particularly for devices with small measurement ranges. The weight of the sensing element can also impact output. It’s normal to observe zero shifts during installation. To address this, align the pressure-sensitive component axially perpendicular to gravitational forces. If constraints prevent ideal installation, adjust the transmitter's zero position post-installation to match standard values. Additionally, ensure the differential pressure chamber has a water collector installed in the negative pressure chamber to regularly drain accumulated water, preventing accuracy loss.
Thirdly, optimizing liquid level measurement through migration principles enhances performance. For accurate liquid level indication, technical adjustments—termed "migration"—are necessary. Migration types include no migration, negative migration, and positive migration. The relationship between measurement range, range, and migration is: Measurement Range = Range Range + Migration Amount. For example:
- A range of 30 kPa with no migration results in a measurement range of 30 kPa.
- A range of 30 kPa with -30 kPa migration yields a range of -30 kPa to 0 kPa.
- A range of 30 kPa with +30 kPa migration results in a range of 30 kPa to 60 kPa.
The input characteristic curve shifts horizontally based on migration amounts. Positive migration shifts right, while negative migration shifts left. This adjustment modifies the range's upper and lower limits without altering its size. Conceptually, positive migration adds pressure to the low-pressure chamber, creating additional pressure (P = Ïgh), while negative migration reduces it. Understanding these principles allows for more precise field adjustments tailored to specific processes and environments.
Fourthly, temperature characteristics play a critical role. Differential pressure transmitters are prone to significant temperature-related errors. When ambient temperatures deviate significantly from standard conditions, these errors can become substantial. During installation, aim to position the pressure transmitter openings as low as possible to mitigate temperature-induced errors. Additional temperature compensation measures might be necessary.
Lastly, damping adjustments help stabilize output fluctuations. Field adjustments can be made by observing the time it takes for output current to drop from 20 mA to 10 mA when the input pressure is rapidly reduced to zero. This period represents the damping time constant. Using a small screwdriver, rotate the damping adjustment clockwise to increase damping time. Typically, selecting the shortest damping time constant ensures quick stabilization of meter output, ideally transitioning from 0.2 to 1.67 seconds.
These strategies not only enhance the reliability of differential pressure transmitters but also apply similarly to pressure transmitters due to their shared applications. By analyzing common issues during field verification and providing practical guidance, measurement errors can be effectively minimized, ensuring optimal performance across diverse environments.
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