Common problems and solutions in the on-site calibration of pressure transmitters

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This post was last edited by yuchenchf on 2017-3-16 at 15:26. Common problems and solutions in the field calibration of pressure transmitters: During the field calibration of pressure transmitters, the measurement accuracy may decline due to various influencing factors; it is necessary to minimize the impact on the accuracy of the measurement results so that they remain within acceptable limits. To reduce measurement errors, corresponding measures and methods should be adopted based on the source and nature of the errors. 1. Reducing the impact of static pressure errors: Differential pressure transmitters are usually calibrated with the low-pressure side exposed to atmospheric pressure, but in almost all applications involving differential pressure transmitters, there is a certain amount of static pressure acting on the ports at the high and low pressure chambers of the transmitter. The main reason why the differential pressure transmitter changes its output under static pressure is the mechanical deformation of the pressure-taking element. Although both the cast housing of the transmitter and the silicone oil filled in the diaphragm box deform very little, the sensitivity of the instrument being so high means that these changes are still significant. The static pressure error is measured using the following two methods: 1) Individual static pressure tests – Nominal pressures are applied to the high and low pressure chambers respectively; after depressurization, the basic error and return error are measured, and their values must meet the requirements specified in the qualification procedures. 2) Hydrostatic test: The rated pressure is applied simultaneously to the high-pressure and low-pressure chambers; after pressure release, the change in the lower limit value should meet the requirements specified in the calibration regulations. 2. Reducing the impact of zero-point drift: In differential pressure transmitters, the installation location of the transmitter has the greatest influence on the zero-point output. This is especially true for micro-differential pressure transmitters; due to their very narrow measurement range, the weight of the sensing elements within the transmitter can also affect its output. Therefore, zero-point drift in micro-differential pressure transmitters is a normal phenomenon. The method to eliminate zero drift is to ensure that the pressure-sensitive element of the transmitter is axially perpendicular to the direction of gravity during installation. If the installation conditions are limited, the transmitter should be installed and its zero point adjusted to the standard value. Additionally, a water collector must be installed in the negative pressure chamber of the differential pressure transmitter during installation, and drainage should be carried out regularly to prevent accumulated water from affecting accuracy. 3. Improving the liquid level measurement method using the offset principle: In order to accurately indicate the height of the liquid level, the differential pressure transmitter must undergo certain technical adjustments—namely offsetting. Migration is divided into no migration, negative migration, and positive migration. The relationship between the measurement range, the scale range, and the offset is: measurement range = scale range + offset. For example. The range is 30 kPa, with no drift; the measurement range is equal to the range of 30 kPa ； The b range is 30 kPa, the migration amount is -30 kPa, and the measurement range is from -30 kPa to 0 kPa ； The C range is 30 kPa, the migration amount is 30 kPa, and the measurement range is 30 kPa to 60 kPa. It can be seen that the output and input characteristic curves for positive and negative migration are those without migration, shifted along the horizontal axis representing the input value. Positive migration moves in the positive direction, while negative migration moves in the negative direction; the distance it moves is the amount of migration. The essence of positive and negative migration is to adjust the upper and lower limits of the range by calibrating the differential pressure transmitter, while keeping the size of the range unchanged. From the perspective of the low-pressure chamber, it can also be simply understood as positive migration; it’s as if an additional pressure P=ρgh is generated by increasing the height difference of the pressure-transmitting medium in the low-pressure chamber (where p represents the additional pressure, in Pa) ； ρ is the local acceleration due to gravity, in m/s2 ； h is the height difference of the pressure-transmitting medium, in meters; it represents the amount of migration, with negative migration corresponding to a reduction in the amount of migration caused by ρgh in the chamber at lower pressure. By understanding the principle behind using differential pressure transmitters to measure liquid level changes, it is possible to apply this knowledge flexibly, taking into account the operational conditions of the production facility, the usage requirements of the instruments, and the surrounding environment. This allows for appropriate improvements to the liquid level measurement methods, enabling timely and accurate resolution of any faults that occur in the field instruments. 4. Temperature characteristic compensation: Differential pressure transmitters exhibit significant temperature-induced errors; when the ambient temperature differs greatly from the standard temperature, these temperature errors can become considerable. During installation, it should be considered to place the pressure tapping for the differential pressure transmitter as low as possible in order to eliminate errors caused by temperature changes; temperature compensation should be introduced if necessary. 5. Damping adjustment: Generally, the damping can be adjusted on-site, based on the fluctuations in the output of the differential pressure transmitter. Since adjusting the damping does not affect the static accuracy of the transmitter, it is best to choose the shortest damping time constant in order to stabilize the fluctuations in the instrument’s output as quickly as possible; the time constant can be varied directly between 0.2 and 1.67 seconds. The method for adjusting the on-site damping of a differential pressure transmitter is as follows: the input pressure is suddenly reduced from the maximum value of its range to 0, and simultaneously a stopwatch is used to measure the time it takes for the output current to drop from 20 mA to 10 mA; this time value represents the damping time constant. During adjustment, a small screwdriver can be inserted into the damping adjustment hole (marked with D); turning it clockwise will increase the damping time ； Conversely, it decreases. It usually analyzes the problems that arise during the field calibration of differential pressure transmitters, providing some experience to guide such calibration work and reduce measurement errors. In fact, due to the similarity in measurement applications between differential pressure transmitters and pressure transmitters, the methods discussed are also applicable to the field calibration of pressure transmitters. Everyone, please continue to add explanations for the incorrect or incomplete parts, so that those who read it next will learn more. What you know is exactly what everyone needs.