Typical Operating Conditions for the 3351 Differential Pressure Transmitter

From Differential Pressure Measurement to Process Parameter Monitoring

The 3351 differential pressure transmitter uses a sensing element to detect the pressure difference between the high- and low-pressure sides and converts the signal into standard outputs such as 4mA–20mA. It is suitable for industrial applications requiring continuous monitoring of differential pressure, level, flow, or equipment resistance. Its core application is not simply to “measure pressure,” but to identify the stable differential value between two pressure tapping points and provide a reliable basis for the control system.

In chemical energy, machinery manufacturing, metallurgy and building materials, environmental treatment, and automated production lines, the 3351 differential pressure transmitter is commonly used for monitoring liquid level in closed vessels, throttling flow in pipelines, filter differential pressure, air duct pressure, and pump unit operating status. For systems with certain static pressure and significant changes in medium conditions, differential pressure measurement generally offers greater process reference value than single-point pressure measurement.

Before selection, it is necessary to determine whether the measured medium is liquid, gas, steam, or slurry containing impurities, and confirm the maximum operating pressure, normal differential pressure, temperature range, corrosiveness, and whether pulsation is present. Even for differential pressure measurement, clean dry gas and high-temperature viscous media have different requirements for diaphragm material, pressure impulse method, and installation orientation.

Common Applications and Key Considerations

For equipment such as filters, dust collectors, and heat exchangers, the differential pressure value often directly reflects flow capacity. When the differential pressure gradually rises to the set threshold, it can trigger reminders for cleaning, backflushing, or filter element replacement. In such applications, particular attention should be paid to stability in the low-range section to avoid imperceptible small differential pressure changes caused by an excessively large measuring range.

How to Evaluate Range Migration

Migration Is Not Simply Zero Adjustment

Range migration refers to resetting the lower limit corresponding to 4mA and the upper limit corresponding to 20mA within the transmitter's allowable measurement range. Positive migration is commonly used in operating conditions where a fixed liquid column pressure exists on the high-pressure side, while negative migration is more common where additional static pressure is generated on the low-pressure side or by the installation position. After migration, the output still corresponds to the actual effective differential pressure rather than the additional pressure in the impulse piping.

For example, when a closed-vessel level transmitter is installed below the bottom of the vessel, the impulse line on the positive-pressure side forms a fixed pressure head after being filled with liquid. If this pressure head is 10kPa and the actual level differential pressure range is 0kPa–40kPa, the transmitter can be set to a positive migrated range of 10kPa–50kPa. In this case, 4mA corresponds to the lowest liquid level, and 20mA corresponds to the highest liquid level.

The migration value should not be based solely on theoretical calculations; consideration should also be given to whether medium density changes with temperature, concentration, or process batch. Density changes alter the liquid column pressure head and cause zero drift. For media with significant density fluctuations, the level error should be verified against the actual process density range, and remote seals or other more suitable measurement solutions should be used when necessary.

Practical Principles for Range Setting