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Tips And Technical Knowledge about Differential Pressure Transmitter

Views: 30 Author: Site Editor Publish Time: 2025-10-24 Origin: Site

1. What is a DP Transmitter?

At its heart, a DP transmitter is a device that measures the difference in pressure between two points. It does not measure absolute pressure at a single point.

Input: Two pressure signals (High Pressure and Low Pressure).
Output: A standardized signal (e.g., 4-20 mA, HART, Foundation Fieldbus) proportional to the calculated difference (ΔP = P_high - P_low).

2. The Power of ΔP

The true power of a DP transmitter lies in its ability to infer other process variables.

Application	How It Works	Key Formula & Notes
Flow Measurement	A primary element (orifice plate, venturi tube) creates a pressure drop. Flow is proportional to the square root of the differential pressure.	*Q = k √ΔP** This is why flow is non-linear with respect to the transmitter's output. Modern transmitters can often perform the square root extraction internally.
Liquid Level in Tanks	The hydrostatic pressure created by the height of the liquid column is measured.	*ΔP = ρ g * h** Where ρ = density, g = gravity, h = height. For open tanks, the low side is vented to atmosphere. For closed tanks, the low side is connected to the vapor space (dry leg) or must compensate for the pressure of the vapor (wet leg).
Filter & Pump Monitoring	Measures the pressure drop across a filter or a pump's discharge and suction pressure.	ΔP = P_in - P_out A rising ΔP across a filter indicates clogging. A changing ΔP across a pump can indicate wear or cavitation.
Density & Interface Level	Used in specialized setups with remote seal systems to measure the pressure difference between two fixed heights in a vessel. The density of the fluid can be derived.	*ρ = ΔP / (g h)**

3. Technical Specifications

When selecting a DP transmitter, consider these specs:

Accuracy: Usually expressed as a percentage of the calibrated span (e.g., ±0.065% of span). This includes combined effects of non-linearity, hysteresis, and repeatability.
Rangeability (Turndown): The ratio of the maximum to minimum flow it can accurately measure. A high turndown (e.g., 100:1) offers great flexibility.
Static Pressure Rating: The maximum line pressure the transmitter's body and isolating diaphragms can withstand without damage. Crucial for high-pressure applications.
Overpressure Protection: The maximum pressure that can be applied to one side without permanently damaging the sensor, even if the other side is at atmospheric pressure.
Wetted Parts: The materials (e.g., 316L Stainless Steel, Hastelloy, Tantalum) that contact the process fluid. They must be compatible to avoid corrosion.
Output & Communication: 4-20 mA analog is standard. Digital protocols like HART, Profibus PA, or Foundation Fieldbus allow for device configuration, diagnostics, and access to multiple variables (e.g., primary variable, secondary variable, static pressure, temperature).