For Differential Pressure Transmitters with Low Error, Accuracy Class Cannot Be Judged by a Single Number

Differential pressure transmitters are used to measure the pressure difference between two points and are commonly used in airflow monitoring, filter blockage detection, liquid level conversion, furnace negative pressure, pipeline flow, and pressure control in closed vessels. For low differential pressure ranges, such as 0–1 kPa, 0–5 kPa, or 0–10 kPa, a deviation of several tens of pascals may alter control decisions. Therefore, “high accuracy” should not be understood solely as the 0.1, 0.25, or 0.5 accuracy class shown on the nameplate.

When selecting a model, first confirm the reference basis of the accuracy class. The commonly used “0.25 class” in the industry generally means that the basic permissible error does not exceed ±0.25% of the upper range limit. For a range of 0–1 kPa, the basic error is approximately ±2.5 Pa. For a range of 0–10 kPa, the same 0.25 class results in a basic error of approximately ±25 Pa. The smallest pressure changes that can actually be identified are therefore different.

Therefore, the first principle for evaluating a low-error differential pressure transmitter is to convert percentage accuracy into actual pascal values and then assess it in conjunction with the allowable fluctuation range of the measured object. If the system control threshold is only 20 Pa, simply selecting a 10 kPa range and a 0.5-class product will usually make it difficult to achieve reliable alarm and control performance.

I. First Understand Accuracy Class, Range, and Total Error

1. Accuracy Class Should Be Calculated Based on Full Scale

The reference error of a differential pressure transmitter is generally based on the upper range limit URL or calibrated span, rather than on the current actual measured value. When the device operates near 10% of its range for an extended period, the absolute error may still approach the full-scale error. When converted into a percentage of reading, it increases significantly.

For example, the basic permissible error of a 0–2 kPa, 0.25-class transmitter is approximately ±5 Pa. When the actual differential pressure is 100 Pa, ±5 Pa is equivalent to ±5% of the reading; when the actual differential pressure is 1.5 kPa, it is approximately ±0.33% of the reading. This is also a primary reason why the range should not be arbitrarily enlarged in low differential pressure applications.

A reasonable approach is to keep the normal operating point within 30%–80% of the range as much as possible, while reserving margins for pressure fluctuations, start-up and shutdown impacts, and overpressure protection. For applications such as slight duct pressure and cleanroom pressure differentials, dedicated low-range, high-resolution differential pressure measurement solutions may be prioritized.

2. Basic Accuracy Must Not Be Treated as Total Field Error

The accuracy class in product specifications is generally the basic error obtained under specified reference temperature, specified static pressure, stable power supply, and standard installation conditions. Actual field error is also affected by temperature changes, common-mode static pressure, installation orientation, accumulated liquid in impulse lines, electromagnetic interference, and zero shift.

During procurement or acceptance, the supplier should be required to clarify the accuracy expression: whether it is “±0.1%FS” or “±0.1%URL,” and whether repeatability, hysteresis, and long-term stability are included. Different expressions correspond to different calculation references and cannot be compared horizontally based solely on descriptions such as “0.1 class” or “high accuracy.”

For safety interlocks, energy consumption accounting, or critical quality control points, an error budget method is recommended. Basic accuracy, temperature drift, static pressure effects, installation effects, and display/acquisition errors should be evaluated cumulatively, with the design margin controlled within 70% of the allowable error.

II. Static Pressure Effects: Low Differential Pressure Is More Difficult to Measure Under High Common-Mode Pressure

1. Distinguish Between Differential Pressure and Static Pressure

Differential pressure is the pressure difference between the high-pressure side and the low-pressure side, while static pressure is the pressure level jointly sustained by both sides. For example, if the pressures at two points of a pipeline are 1.002 MPa and 1.000 MPa respectively, the actual differential pressure is only 2 kPa, while both sides of the instrument simultaneously withstand approximately 1 MPa of static pressure.

When low differential pressure is combined with high static pressure, the sensor isolation diaphragm, sensing element, and sealing structure are subject to higher mechanical loads. If the static pressure characteristics are insufficient, the zero point will shift as common-mode pressure changes. Even when the differential pressure itself remains unchanged, the output signal may drift.

When reviewing technical parameters, particular attention should be paid to “static pressure effect” or “common-mode pressure effect.” Its unit may be %FS/MPa, %URL/MPa, or Pa directly. The smaller the value, the weaker the effect of high static pressure variation on differential pressure zero point and range accuracy.

2. Field Conversion Is Required for High Static Pressure Conditions

Assume that a low differential pressure transmitter has a static pressure effect of ±0.05%FS/MPa and a range of 0–2 kPa. If the field static pressure changes from 0.5 MPa to 2.5 MPa, the change is 2 MPa. The additional effect caused by the static pressure change alone is approximately ±2 Pa. This may be acceptable in ordinary applications, but it must be included in the error budget for control targets at the 10 Pa level.

During selection, maximum working static pressure, one-sided overpressure capability, and bidirectional overpressure capability should also be confirmed. Maximum working static pressure determines whether the instrument can withstand medium pressure over the long term, while overpressure capability relates to transient risks caused by valve switching, water hammer, or blockage. These three indicators cannot replace one another.

In steam, liquid, or high-pressure gas pipelines, compatible valve manifolds, impulse lines, and condensation measures should be provided to ensure consistent medium conditions on both sides. Differences in temperature, liquid column height, or pressure-guiding paths on the two sides can also create additional static pressure differences and affect zero-point stability.

III. Temperature Drift Sensor Error: Essential Calculation for Low-Range Applications

1. Temperature Drift Mainly Includes Zero Drift and Span Drift

The physical properties of sensor sensing elements, circuit components, filling fluids, and metal structures all change with temperature. Zero temperature drift appears as an overall upward or downward shift in output when the actual differential pressure remains unchanged; span temperature drift appears as a change in the proportional relationship between input and output. These two types of error need to be identified separately.

Low differential pressure measurement is more susceptible to zero temperature drift. For example, for a 0–1 kPa product with a zero temperature drift of ±0.05%FS/10°C, when the ambient temperature deviates by 20°C from the calibration temperature, the theoretical additional zero error can reach ±1 Pa. If the actual control target is only 30 Pa, 1 Pa already has a noticeable effect.

In addition to ambient temperature, medium temperature and day-to-night temperature differences should also be considered. For transmitters installed on outdoor equipment, near boilers, in pipe racks exposed to direct sunlight, or in low-temperature storage areas, actual temperature changes may exceed 30°C. Performance must not be assessed based solely on laboratory room-temperature accuracy.

2. Temperature Drift Parameters Must Be Converted to Actual Temperature Differences

Technical documents often express temperature drift as “%FS/10°C” or “%URL/°C.” Quality control personnel should use the calibration temperature as the starting point and calculate the maximum deviation between the highest and lowest field temperatures and the calibration temperature, rather than considering only the average daily temperature. The greater the temperature difference, the less the accumulated temperature drift can be ignored.

For example, for a product with a range of 0–500 Pa and zero temperature drift of ±0.1%FS/10°C, if the field temperature range is -10°C to 40°C, the maximum deviation relative to the 20°C calibration point is 30°C. The additional error caused by temperature drift is approximately ±1.5 Pa, which must also be evaluated together with basic error, static pressure effects, and acquisition module error.

For sites with large temperature differences, low differential pressure sensors with temperature compensation may be used. Avoid mounting transmitters directly against high-temperature pipe walls; where necessary, add sunshades, thermal insulation, or instrument protection enclosures. Installation measures cannot replace high-quality temperature compensation, but they can significantly reduce fluctuations during actual operation.

Simplified Method for Temperature Drift Calculation

A preliminary estimate may be made using “additional temperature drift error = temperature drift coefficient × range × actual temperature difference factor.” If the parameter is 0.05%FS/10°C, the range is 2 kPa, and the temperature difference is 25°C, the additional error is approximately 0.0005 × 2000 × 2.5, namely about ±2.5 Pa.

When a product provides both zero temperature drift and span temperature drift, zero temperature drift should be prioritized in low-reading regions; when operating close to full scale, both indicators should be considered simultaneously. For ultra-low-range products such as 0–100 Pa and 0–250 Pa, it is recommended to request the complete temperature compensation range and factory calibration data from the manufacturer.

Field acceptance should not be limited to verification at a single room-temperature point. Zero and full-scale outputs may be recorded under typical temperature conditions. If the equipment cannot undergo temperature chamber testing, at minimum, a seasonal comparison log should be established to observe zero-point change trends under the same operating conditions in winter and summer.

IV. From Datasheet to Field: How to Select a Reliable Low Differential Pressure Transmitter

1. Establish a Complete Selection Verification Sequence

First, determine the normal differential pressure, maximum differential pressure, allowable error, and alarm threshold; second, determine the highest static pressure on both sides, medium type, temperature range, and corrosiveness; third, compare the accuracy class, static pressure effect, temperature drift, overload capability, output method, and protection rating. This sequence avoids selecting a model first based on price or a single accuracy class.

For products with 4–20 mA output, the accuracy of the control cabinet input module, loop power supply stability, and grounding and shielding conditions must also be considered. Even if the transmitter itself has an error of only ±0.25%FS, the final system performance may not meet expectations if the acquisition module introduces an additional ±0.2%FS error.

Long-term stability also deserves attention. Some products indicate “annual stability ±0.1%FS” or similar specifications, which reflect the drift level after long-term operation. For continuous production facilities, calibration intervals should be determined in conjunction with annual stability and operating condition severity, rather than by applying a single fixed interval.

2. Acceptance and Maintenance Determine Final Measurement Performance

Upon delivery acceptance, verify the range, accuracy, pressure connection, diaphragm material, output signal, power supply range, and static pressure rating, and retain the factory calibration records. For critical points, conduct comparison tests at no fewer than five points of 0%, 25%, 50%, 75%, and 100% of range, while also checking return error.

During operation, if slow output drift occurs, the zero point cannot return, or differential pressure fluctuations do not correspond to process conditions, first investigate impulse line blockage, leakage, condensate imbalance, incompletely opened valve manifolds, and poor electrical contact before determining whether the sensor itself is out of calibration.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide targeted configuration recommendations based on the actual differential pressure range, static pressure rating, medium temperature, installation method, and signal requirements. Before procurement, it is recommended to organize field operating parameters and have technical personnel complete error conversion and range matching to ensure that low differential pressure measurement maintains verifiable accuracy and stability under complex operating conditions.