What are the Factors that Affect the Measurement Accuracy of Differential Pressure Transmitters

With the advancement of instrumentation technology, various brands of differential pressure transmitters are evolving towards high precision and intelligence. However, this technological progress has led to a misconception among designers: when selecting differential pressure transmitters, people often focus on the precision of the transmitter itself, while neglecting the factors that affect its measurement accuracy.

In reality, what users truly need is a transmitter that accurately reflects the true value of the measured variable, not just the precision of the transmitter itself. There are many factors that affect the measurement accuracy of differential pressure transmitters, some of which are related to environmental conditions and installation methods. What are these factors that affect the measurement accuracy of differential pressure transmitters?

What are the Factors that Affect the Measurement Accuracy of Differential Pressure Transmitters

1. Influence of Turndown Ratio on Measurement Accuracy

The turndown ratio of a differential pressure transmitter refers to the ratio of the maximum to the minimum value that the transmitter can measure while meeting accuracy requirements. Generally, the larger the turndown ratio, the lower the measurement accuracy. A high turndown ratio has many benefits, such as allowing a single instrument to be calibrated for use in multiple different scenarios. However, in practice, setting the turndown ratio too high can lead to numerous measurement stability issues.

2. Influence of Temperature on Measurement Accuracy

In process applications, the process temperature and ambient temperature can vary significantly, which will certainly differ from the reference conditions during transmitter testing. This means that the performance will differ from the stated reference accuracy. The impact of temperature on the transmitter is minimal when close to the maximum range (i.e., small turndown ratio), but it becomes significant near the minimum range (i.e., large turndown ratio), sometimes even beyond our expectations. According to the formulas provided in some product manuals for the combined effect of temperature on zero-point accuracy and span accuracy, if the maximum turndown ratio input is 100, the impact can approach 10%.

Some modern intelligent differential pressure transmitters include internal temperature sensors to correct for thermal effects caused by changes in ambient temperature.

3. Influence of Static Pressure on Measurement Accuracy

Similar to the effects of temperature and turndown ratio, static pressure also significantly impacts the zero-point and span accuracy of differential pressure transmitters. For instance, capacitive differential pressure transmitters are almost always affected by static pressure during the operation of production processes.

When the operating static pressure exceeds a certain value, the error due to static pressure can surpass the transmitter’s own accuracy. When calculating the channel accuracy of the measurement path, if the error due to static pressure is included, it may not meet the channel accuracy requirements. Therefore, it is necessary to correct for the impact of static pressure when calibrating capacitive differential pressure transmitters.

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4. Influence of Installation Angle on the Measurement Accuracy of Differential Pressure Transmitter

The installation tilt angle refers to the angle between the transmitter axis and the vertical line after the transmitter is installed on-site.

In differential pressure measurement, the sensing element senses the pressure difference on both sides by detecting the deformation displacement of the sensing diaphragm at the center of the measurement. Since the central measuring chamber is filled with filling silicone oil, when the differential pressure transmitter tilts along a direction perpendicular to the plane of the measuring diaphragm, the filling silicone oil itself exerts pressure on the measuring diaphragm, causing diaphragm deformation and resulting in output value fluctuations.

However, when the differential pressure transmitter tilts along a direction parallel to the plane of the measuring diaphragm, the filling silicone oil will not press the measuring diaphragm, hence no effect. Transmitters with different ranges are affected to varying degrees by tilting, with smaller ranges experiencing greater impacts. When the differential pressure transmitter tilts forward or backward, its output linearity and value change are not significant, thus not affecting its normal operation. When the differential pressure transmitter tilts to the left or right, its output value will drift unidirectionally based on the tilt angle, with greater angles resulting in greater impacts, but the linearity remains unchanged.

5. Influence of Height Difference between Measurement Point and Installation Point on the Measurement Accuracy of Differential Pressure Transmitter

Due to the overall design requirements of industrial sites, differential pressure transmitters often cannot be installed near the measurement points in actual use, but are pressurized to a relatively centralized location through impulse pipes for maintenance and management convenience.

This leads to a height difference between the measurement point and the installation point of the differential pressure transmitter, causing additional pressure in the pressure-transmitting medium within the impulse pipe due to the height difference, thereby introducing additional errors. In process industrial applications, since the density of the medium is often high, even small height differences introduce significant additional errors.

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