With advances in instrumentation technology, differential pressure transmitters from various manufacturers are increasingly trending toward higher accuracy and greater intelligence. However, this technological progress has led designers to a common misconception: when selecting a differential pressure transmitter, attention is often focused solely on its inherent accuracy, while the factors that influence its overall measurement accuracy are overlooked.

In fact, what users really need is the measurement accuracy that accurately reflects the true value of the measured quantity—rather than the inherent accuracy of the differential pressure transmitter itself. Many factors influence the measurement accuracy of a differential pressure transmitter, and some of these factors are also related to environmental conditions and installation methods. So what exactly are the factors that affect the measurement accuracy of a differential pressure transmitter? Electrical Automated Control The editor has compiled the following points for everyone.

1. Influence of Span Ratio on the Measurement Accuracy of Differential Pressure Transmitters
The span ratio of a differential pressure transmitter refers to the ratio of the maximum to the minimum measurable value under specified accuracy requirements. In general, the larger the span ratio, the lower the measurement accuracy. A large span ratio offers several advantages: once an instrument is calibrated, it can be used in multiple different applications. However, in practice, setting the span ratio too high can lead to significant issues with measurement stability.

For most differential pressure transmitters, within a span ratio of 10:1, the accuracy remains the reference accuracy and is unaffected by the span ratio. However, a significant number of differential pressure transmitters specify a reference accuracy only for span ratios below 10:1—some even as low as 3:1 or 2:1. Moreover, for the same model, different span codes may result in substantially different span ratios that still meet the reference accuracy requirement. These factors must be given special consideration by users when calculating the accuracy after accounting for the influence of the span ratio. This article is from www.eadianqi.com.

2. Influence of Temperature on the Measurement Accuracy of Differential Pressure Transmitters
In process applications, differential pressure transmitters are subject to significant variations in both process temperature and ambient temperature, which inevitably differ from the reference conditions used during transmitter calibration. Consequently, the instrument’s performance will deviate from the specified reference accuracy. The impact of temperature on differential pressure transmitters is minimal when operating near the upper end of the span (i.e., at low turndown ratios), but becomes substantial as the range approaches its lower limit (i.e., at high turndown ratios)—in some cases even exceeding expectations. According to the combined temperature-effect formulas for zero-point and span accuracy provided in certain product manuals, an input turndown ratio of 100 can result in an error approaching 10%.

Some modern smart differential pressure transmitters have incorporated internal temperature compensation. Sensor , used to compensate for the thermal effects caused by ambient temperature variations.

3. Influence of Static Pressure on the Measurement Accuracy of Differential Pressure Transmitters
Similar to the effects of temperature and span, static pressure also has a substantial impact on the zero-point and span accuracy of differential-pressure transmitters. For example, capacitive differential-pressure transmitters are almost invariably subject to static-pressure effects during operation in process control systems.

When the static pressure under operating conditions exceeds a certain threshold, the error introduced by static-pressure effects will surpass the transmitter’s inherent accuracy. Consequently, if this static-pressure-induced error is not accounted for in the channel accuracy calculation, the channel may fail to meet the required accuracy specifications. Therefore, during calibration of capacitive differential-pressure transmitters, it is essential to correct for the influence of static pressure.

4. Influence of Installation Tilt Angle on the Measurement Accuracy of Differential Pressure Transmitters
The installation tilt angle refers to the angle between the transmitter’s central axis and the plumb line after the transmitter has been installed on site.

In differential pressure measurement, the sensing element detects the pressure difference between the two sides by measuring the deformation displacement of the central measuring diaphragm. Since the central measuring chamber is filled with silicone oil, when the differential pressure transmitter is tilted in a direction perpendicular to the plane of the measuring diaphragm, the silicone oil itself exerts pressure on the diaphragm, causing it to deform and resulting in fluctuations in the output signal.

When a differential pressure transmitter is tilted in a direction parallel to the plane of the measuring diaphragm, the filled silicone oil does not exert any pressure on the diaphragm, so there is no effect. The degree of influence due to tilting varies with the transmitter’s span: the smaller the span, the greater the impact. When the transmitter is tilted forward or backward, its output linearity and output value change only slightly, and normal operation is not affected. However, when the transmitter is tilted to the left or to the right, its output value will drift unidirectionally in proportion to the tilt angle; the larger the angle, the greater the effect, although the linearity remains essentially unchanged.

5. Influence of the Elevation Difference Between the Measurement Point and the Installation Point on the Measurement Accuracy of Differential Pressure Transmitters
Due to the requirements of overall industrial site design, differential pressure transmitters are often not installed directly at the field measurement point; instead, pressure is transmitted via impulse lines to a centralized location for easier maintenance and management.

This results in a height difference between the measurement point and the installation location of the differential pressure transmitter, causing the pressure-transmitting medium in the impulse line to develop an additional pressure due to this height difference, which in turn introduces an additional error. In process industry applications, because the density of the process fluid is often relatively high, even a small height difference can lead to a significant additional error.