Flow Measurement

Understanding the Working Principle of Multivariable DP Mass Flow Transmitters

  • Precise parameter monitoring and control are essential for safe and effective processes in industrial operations.
  • Multivariable transmitters play a significant role in this regard by measuring multiple parameters simultaneously. 
  • Multivariable transmitters are unique among these transmitters because of their advanced characteristics and precise mass flow measurement. 
  • The purpose of the article is to explore the operation of multivariable transmitters and explain how they enhance industrial processes.
  • Multivariable transmitters are designed to measure multiple process parameters concurrently, offering a comprehensive insight into industrial operations. 
  • These transmitters typically measure variables such as static pressure, differential pressure, and temperature, allowing for accurate calculation of mass flow. 
  • By integrating various sensors into a single design, multivariable transmitters facilitate real-time monitoring and control, essential for industries requiring precise measurements and instant responses to fluctuations
Conventional Flow Calculation Methods
  • Differential Pressure (DP) transmitters are frequently used in traditional flow computation techniques used in flow computers or Distributed Control Systems (DCS). 
  • To determine DP flow, these systems usually use a simplified flow equation. 
Conventional Flow Calculation Methods formula
  • In this method, the unit conversion factors, approach velocity factor, gas expansion factor, and discharge coefficient are all represented by a single constant in the flow calculation. 
  • Although this reduced equation makes computation easier, it cannot sufficiently account for changes in these parameters. 
  • As a result, the equation is unable to adequately account for swings in variables like fluid characteristics or operating conditions, which can result in inaccurate flow rate calculations.
  • This means that there may be differences between the observed and true flow rates, underscoring the drawbacks of depending only on traditional flow computation techniques in some situations.
  • While these methods provide reasonable estimates under certain conditions, they may introduce errors due to assumptions like steady flow and ideal fluid behavior, highlighting the need for more advanced techniques to improve accuracy in flow measurement.
  • Conventional mass flow calculation methods typically involve separate measurements of temperature, pressure, and differential pressure (DP), which are then used in equations to calculate the mass flow rate.
  • In conventional mass flow calculation techniques, these separate measurements (temperature, pressure, and DP) are typically used in simplified flow equations to estimate the mass flow rate. 
  • However, as mentioned earlier, these methods may not always account accurately for changes in fluid properties or operating conditions, leading to potential inaccuracies in the calculated flow rates.
Working Principle of DP Multivariable Mass Flow Transmitters
  • Multivariable flow transmitters are sophisticated instruments utilized in industrial processes to measure multiple parameters simultaneously and calculate mass flow rates accurately.
  • To maximize their efficiency and guarantee accurate control of industrial processes, it is crucial to comprehend how they work, including the calculations that go into the underlying formulas. 
  • The functioning of multivariable flow transmitters is thoroughly explored in this article, with a focus on the formula computations necessary to determine mass flow rate.
  • Multivariable flow transmitters typically integrate three primary sensors for static pressure, differential pressure, and temperature into a single compact unit. 
  • These sensors are strategically positioned to capture essential process variables, enabling comprehensive monitoring and control.
  • The integrated sensors continuously measure static pressure, differential pressure, and temperature within the process environment. 
  • The collected data are transmitted to the transmitter’s processing unit for analysis and calculation.
  • Multivariable flow transmitters utilize complex flow equations to calculate mass flow rates accurately. 
  • These equations involve various coefficients that account for factors such as fluid properties, flow conditions, and sensor characteristics. 
  • The coefficients include:
  1. Discharge Coefficient (Cd): Represents the efficiency of the flow measurement device, accounting for factors such as flow profile and geometry.
  2. Velocity of Approach Factor (E): Adjusts for the velocity profile at the sensor location, ensuring accurate differential pressure measurements.
  3. Gas Expansion Factor (Y1): Accounts for changes in gas density due to variations in temperature and pressure.
  4. Bore Diameter of Differential Producer (d2): Defines the diameter of the flow restriction element, influencing the differential pressure readings.
  • Multivariable flow transmitters employ dynamic calculation algorithms to adjust flow equation coefficients in real-time. 
  • These algorithms continuously analyze sensor data and update coefficient values based on current process conditions, ensuring accurate mass flow rate calculations across a wide range of operating parameters.
  • Many multivariable flow transmitters utilize communication protocols such as Highway Addressable Remote Transducer (HART) to interface with control systems. 
  • This allows for seamless integration and remote monitoring/control of the transmitter’s operation.

The calculation of mass flow rate (Qmass) by a multivariable flow transmitter involves a combination of sensor readings and flow equation coefficients. 

The general formula for mass flow rate determination is as follows:

Working Principle of DP Multivariable Mass Flow Transmitters formula


  • Qmass: Mass flow rate (units: mass per unit time, e.g., kg/s)
  • N: Units conversion factor (accounts for unit conversions, if necessary)
  • Cd: Discharge coefficient (dimensionless)
  • E: Velocity of approach factor (dimensionless)
  • Y1: Gas expansion factor (dimensionless)
  • d2: Bore diameter of differential producer (units: length, e.g., meters)
  • DP: Differential pressure (units: pressure, e.g., Pascals)
  • ρ: Density of the fluid (units: mass per unit volume, e.g., kg/m^3)

The coefficients N, Cd, E, and Y1 are determined based on the specific properties of the fluid being measured, the design characteristics of the flow transmitter, and the operating conditions of the process. 

These coefficients are often calibrated or configured during the setup phase to ensure accurate mass flow rate calculations.

Differences Between Conventional Mass Flow Calculation Methods and Multivariable Flow Transmitter Methods

This table provides a comparative overview of the key differences between conventional mass flow calculation methods and multivariable flow transmitter methods in various aspects such as measurement parameters, instrumentation required, accuracy, complexity, real-time adjustment, installation and maintenance, and flexibility.

AspectConventional Mass Flow Calculation MethodsMultivariable Flow Transmitter Methods
Measurement ParametersTypically separate measurements of temperature, pressure, and differential pressure (DP) are used.Integrated measurements of multiple parameters including static pressure, differential pressure, and temperature are used.
Instrumentation RequiredRequires separate instruments for temperature (RTD), pressure (PT), and differential pressure (DPT).Utilizes a single multivariable flow transmitter device incorporating sensors for multiple parameters.
AccuracyMay be less accurate due to potential inaccuracies in separate measurements and assumptions in simplified flow equations.Generally offers higher accuracy due to integrated measurements and sophisticated algorithms for real-time adjustment.
ComplexityCalculation methods are often based on simplified flow equations, which may not account for all variables accurately.Involves complex algorithms and calculations to account for various factors affecting flow rate accurately.
Real-time AdjustmentLimited capability for real-time adjustment based on changing process conditions.Capable of real-time adjustment of flow equation coefficients based on current process conditions for improved accuracy.
Installation and MaintenanceRequires installation, calibration, and maintenance of multiple instruments, potentially increasing complexity and cost.Simplified installation and maintenance with a single device, potentially reducing overall installation and maintenance costs.
FlexibilityMay lack flexibility in adapting to diverse process conditions and fluid properties.Offers greater flexibility in accommodating different process conditions and fluid properties through dynamic adjustment algorithms.
CostHigher initial cost due to the purchase of multiple separate instruments and potential need for additional infrastructure.Initial cost may be higher for the multivariable flow transmitter unit but could potentially result in cost savings over time due to reduced installation, calibration, and maintenance requirements.
Space RequirementRequires more space for installation of multiple instruments, potentially leading to increased space..Requires less space as it integrates multiple sensors into a single compact unit, minimizing space.
Calibration FrequencyEach instrument may require individual calibration, increasing the frequency and complexity of calibration procedures.Calibration of the multivariable transmitter may be simpler and less frequent due to integration and automated diagnostics features.
Skill Requirement for MaintenanceMaintenance may require specialized skills for each instrument, potentially leading to higher training costs or reliance on external expertise.Maintenance of the multivariable transmitter may require less specialized skills as it involves a single integrated device, potentially reducing training costs.

Multivariable DP (Differential Pressure) flow transmitters offer several advantages over conventional methods.

  • Simultaneous measurement of multiple parameters such as static pressure, differential pressure, and temperature.
  • Enhanced precision in flow rate calculations due to integrated measurements and real-time adjustment algorithms.
  • Simplified installation, calibration, and maintenance processes compared to using multiple separate instruments.
  • Ability to adapt to diverse process conditions and fluid properties through dynamic adjustment algorithms.
  • Compact design minimizes the space required for installation, ideal for environments with limited space availability.
  • Integration with communication protocols like HART enables remote access for monitoring and control, facilitating proactive maintenance and operational efficiency.
  • Compensate flow readings for changes in line pressure and process temperature and Ensures accurate flow measurements by accounting for variations in line pressure and process temperature, enhancing measurement reliability.
  • Provides real-time data updates, allowing for timely adjustments(process conditions with flow calculations) and optimizations in industrial processes.
  • Simplifies installation and reduces costs associated with additional piping and connections, contributing to overall cost savings.
  • Offers high accuracy(±0.8% flow accuracy over 14:1 flow turndown ) across a wide range of flow rates, ensuring reliable performance in various operating conditions
  • Able to measure a variety of variables, including process temperature, differential pressure, mass, volumetric and energy flow, totalized flow, and static pressure (both gauge and absolute). gives extensive measurement capabilities that enable a thorough comprehension of the dynamics and parameters of the operation.
  • Can produce a 4-20 mA HART output signal and supports communication protocols such as HART, enabling seamless integration with control systems and facilitating remote monitoring and control.

Multivariable DP flow transmitters offer several advantages over conventional methods, they also have some limitations and challenges to consider.

  • While the overall cost might be lower due to reduced installation and maintenance needs, the initial cost of a multivariable transmitter itself can be higher than individual instruments.
  • The internal algorithms and calculations used in multivariable transmitters can be more complex, requiring specialized engineers for troubleshooting and maintenance.
  • Depending on the specific model and technology used, not all multivariable transmitters are suitable for all fluids or process conditions. 
  • Factors like high viscosity, corrosive fluids, or extreme temperatures may require specialized devices.
  • While single-point calibration might be possible, some multivariable transmitters may require multi-point calibration procedures that are more involved and time-consuming compared to individual instruments.
  • Integrating multivariable transmitters with existing control systems might require additional configuration or communication protocols compared to traditional setups.
  • Troubleshooting issues with multivariable transmitters can be more complex due to the interplay of multiple sensors and algorithms. Identifying the source of a problem can require advanced diagnostic tools and expertise.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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