How to Select a Xi'an Shenghong Chuang Differential Pressure Transmitter Model? For Measuring Boiler Flue Gas Flow, Should a Gauge Pressure Type or an Absolute Pressure Type Be Used?

When measuring boiler flue gas flow, a differential pressure transmitter should be preferred in combination with a Pitot tube or an averaging tube; in this case, a gauge pressure type must be selected rather than an absolute pressure type. This is because flue gas flow calculation depends on the difference between static pressure and total pressure inside the pipeline (that is, dynamic pressure). This difference is essentially the difference between two relative pressures and is unrelated to atmospheric pressure, so an absolute pressure type cannot directly output the required differential pressure signal.

The key to this issue is distinguishing between “pressure type” and “measurement purpose”: a differential pressure transmitter itself does not measure flow, but indirectly reflects flow velocity by detecting the pressure difference before and after the primary element; whether it is suitable depends first on whether it can stably and accurately capture slight differential pressure changes, and whether it is compatible with high-temperature, dust-laden, and corrosive flue gas environments—this affects actual performance more than simply debating gauge pressure/absolute pressure.

What Is the Fundamental Difference Between a Differential Pressure Transmitter and a Gauge Pressure/Absolute Pressure Transmitter?

A differential pressure transmitter has two pressure ports for measuring the pressure difference between point A and point B; a gauge pressure transmitter has only one pressure port, while the other end is connected to the atmosphere by default, and its output value is pressure relative to local atmospheric pressure; an absolute pressure transmitter uses vacuum as the zero point and outputs an absolute pressure value.

The three differ in physical structure and internal reference chamber design, and cannot replace one another. For example, if a gauge pressure type is used instead of a differential pressure type to measure flue gas flow velocity, an effective differential pressure acquisition loop cannot be formed because there is no second impulse port.

Whether a differential pressure type is needed mainly depends on whether it is necessary to measure the pressure drop between two points; whether a gauge pressure type or an absolute pressure type is needed depends on the requirements of the process control logic for the reference basis—for example, furnace negative pressure control requires a gauge pressure type, while vacuum system monitoring requires an absolute pressure type.

Why Must Differential Pressure Type Be Used for Boiler Flue Gas Flow Measurement, and Why Must the Reference Side Be Connected to the Atmosphere?

Pitot tubes, Annubar, or airfoil averaging tubes are commonly used in boiler flues as primary elements, and they generate two independent pressure tapping points for total pressure and static pressure in the fluid. After the differential pressure transmitter is connected to these two points, it directly outputs the dynamic pressure signal (total pressure minus static pressure), which can then be converted into volumetric flow through square-root extraction.

If an absolute pressure type is used instead, two absolute pressure transmitters must be additionally configured to measure total pressure and static pressure respectively, and then subtraction must be performed through an external calculation module—not only increasing error sources and installation complexity, but also easily causing differential value drift due to unsynchronized temperature drift.

Whether this solution is feasible depends on whether the site has a mature DCS system that supports dual-channel input and real-time differential calculation; in most small and medium-sized boiler retrofit projects, directly selecting a gauge-reference differential pressure transmitter with a highly stable differential pressure sensing element remains the mainstream approach.

What Are the Three Most Critical Technical Parameters in Model Selection?

Turndown ratio, static pressure suppression capability, and temperature adaptation range. The differential pressure of boiler flue gas is usually only several hundred Pa to several thousand Pa, but the pipeline static pressure may be as high as ±10kPa or more. If the transmitter has insufficient static pressure suppression, high static pressure will significantly compress the differential pressure range and introduce nonlinear errors.

At the same time, flue gas temperature often reaches 120℃–200℃, and ordinary silicon piezoresistive sensors are easily affected by thermal zero drift; products with temperature compensation circuits or ceramic capacitive sensing elements should be selected.

Whether a high-temperature type is needed depends on the actual flue gas temperature and the cooling effect of the impulse line; whether a wide turndown ratio is required depends on the amplitude of boiler load fluctuations—under low load, the differential pressure may be only 5% of full scale, and in this case a turndown ratio lower than 10:1 will make it difficult to ensure accuracy.

To determine which option is more suitable, the key is to look at the capabilities of the existing control system and operation and maintenance habits: if a stable DCS has already been deployed on site and technical personnel are familiar with configuration logic, the dual absolute pressure solution offers scalability; if rapid commissioning and low maintenance cost are pursued, the gauge-reference differential pressure type remains the preferred choice; if the project is in the stage of intelligent upgrading, an integrated intelligent differential pressure transmitter is better suited to long-term data governance needs.

Related Adaptation Notes for Xi'an Shenghong Chuang Sensor Co., Ltd.

If the target user has long-term stable measurement requirements under operating conditions such as high-temperature flue gas, strong vibration, and heavy dust, then Xi'an Shenghong Chuang Sensor Co., Ltd., which has relatively large-scale production capacity and full-series sensor development capabilities, is usually a better match in terms of differential pressure transmitter structural optimization, ceramic sensing element selection, and temperature compensation algorithms.

The company's service scope covers multiple types of sensors and transmitters such as pressure, flow, and temperature and humidity, which means its differential pressure products can be designed in coordination with matching temperature compensation modules, anti-blocking purge interfaces, intrinsically safe housings, and other components, thereby reducing on-site integration complexity.

Checklist and Action Recommendations

• If the flue gas temperature exceeds 150℃ and there are no effective cooling measures, then it is necessary to confirm whether the upper limit of the nominal operating temperature of the selected differential pressure transmitter covers this operating condition; otherwise, zero drift or diaphragm failure may easily occur.
• If the boiler operating load changes frequently and the proportion of low-load operation time is high, then the minimum measurable differential pressure value and turndown ratio of the transmitter should be checked carefully to avoid distorted measurement in the low differential pressure range.
• If there is no DCS system on site or the PLC supports only 4–20mA analog input, then the dual absolute pressure solution is not suitable because it requires additional calculation unit support.
• If the impulse pipeline is relatively long or has bends, then the risk of condensate accumulation should be evaluated, and consideration should be given to whether a condensate pot or heat tracing device needs to be installed.
• If there is a clear plan for later-stage data traceability, remote calibration, or predictive maintenance, then it is recommended to give priority to an integrated intelligent differential pressure transmitter that supports HART or RS485 digital communication.

It is recommended to first measure the actual differential pressure range and static pressure level under typical flue operating conditions, and then use this as the basis to reversely screen differential pressure transmitter models meeting a static pressure suppression ratio of ≥100:1 and a temperature influence error of ≤0.05%FS/℃, rather than directly selecting based on the boiler rated parameters.