How Temperature and Pressure Influence Liquid Flow Measurement Accuracy

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Accurate liquid flow measurement can rarely be determined by the flow sensor alone. Temperature, pressure, viscosity and the physical properties of the measured fluid can all influence a flowmeter’s performance and calibration, particularly in demanding industrial applications. Understanding these variables is therefore critical when specifying a flowmeter and interpreting its output.

For engineers responsible for process control, dosing, batching or transfer applications, selecting the correct liquid flow meter requires more than simply matching the instrument to a nominal flow rate. The operating conditions of the process can have a significant effect on measurement accuracy, repeatability and the long-term performance of the meter.

This is particularly important where liquids operate across a wide range of temperatures or pressures or where viscosity changes significantly during the process. In such applications, understanding how the physical properties of the liquid interact with the flow measurement technology is an important part of achieving reliable results.

Temperature and liquid flow measurement

Temperature is a particularly important consideration. As liquid temperature changes so too can its viscosity and density. Temperature can also cause thermal expansion within the flow measuring device itself, potentially altering the clearances between internal components. In positive displacement meters, where measurement depends on accurately defining a known volume and tolerances are very tight, even small changes in internal geometry can influence the amount of liquid displaced per cycle.

Temperature therefore needs to be considered both as a property of the process fluid and as a potential influence on the flow meter itself. Changes in temperature can alter the behaviour of the liquid while also affecting the dimensions and operating characteristics of the measuring components.

For applications where accurate liquid flow measurement is essential, these effects need to be understood when selecting and specifying the meter. Calibration conditions should also be considered alongside the actual operating conditions rather than treating the meter’s published accuracy as completely independent of the process.

The importance of temperature as an influence on flow measurement is also reflected in UK flow measurement standards and guidance. Current MCERTS requirements, for example, include water temperature among the performance characteristics assessed for flow measurement equipment.

Oval gear flow meters for demanding liquid applications

Titan Enterprises’ Oval Gear (OG) flowmeters are designed to minimise these effects. Two precision-machined oval gears rotate within a closely defined chamber, trapping and displacing discrete volumes of liquid as they turn. Magnets embedded in one of the gears are detected through the chamber wall by a Hall Effect sensor, reed switch or Namur sensor. The pulses generated correspond to the number of rotations of the gear and therefore to a known volume of liquid displacement.

This positive displacement principle provides an important advantage: measurement is based primarily on the volume displaced rather than the velocity profile of the liquid. Consequently, OG meters are highly tolerant of changes in liquid properties and installation effects.

Indeed, unlike many flowmeter technologies, their accuracy and range of measurement generally improve as viscosity increases, moving from around 1% to approximately 0.1% of reading with higher-viscosity liquids. As liquid viscosity increases, slip between the oval gears and the meter body is reduced, allowing the meter to capture a greater proportion of the actual flow and maintain higher measurement accuracy.

This characteristic can be particularly valuable in industrial processes where oils, fuels, chemicals, adhesives, resins or other viscous liquids need to be measured accurately. Rather than treating increasing viscosity solely as a limitation, the operating principle of an oval gear flow meter can provide an advantage as viscosity rises.

Standard OG meters can measure liquids up to around 1000 cSt, with specially profiled gears available for viscosities above this range. Titan also offers high-viscosity options on its OG range, extending the potential application of positive displacement flow measurement for demanding liquid handling processes.

How viscosity affects flow meter accuracy

Viscosity is closely linked to temperature and is therefore an important consideration when specifying a liquid flow meter. A liquid that becomes significantly less viscous as its temperature rises can behave very differently from the same liquid at a lower operating temperature.

For many flow measurement technologies, changes in viscosity can affect the relationship between the measured signal and actual flow. With positive displacement technology, however, the measurement principle is based on repeatedly capturing a defined volume of liquid.

In an oval gear meter, the gears rotate as the liquid passes through the measuring chamber. The geometry of the gears and chamber defines the volume displaced during each rotation. As viscosity increases, the reduction in slip between the gears and the meter body can contribute to improved measurement accuracy.

This makes the technology particularly suited to applications where liquid viscosity is high or where viscosity can vary significantly during operation. It also highlights why flow meter selection should consider the complete viscosity range of the application rather than specifying a meter based solely on the liquid’s nominal viscosity.

Pressure introduces different challenges

Pressure introduces a different set of challenges. Increasing pressure can place mechanical stresses on the meter body, gears, bearings and seals, while pressure drop across the meter can become significant when measuring viscous fluids.

Titan’s OG design uses the differential pressure generated across the meter to drive the gears. The larger effective surface area of the oval gears provides substantial driving force, delivering superior performance at low flow and maintaining a comparatively low pressure drop, helping extend the usable measurement range for viscous liquids.

Pressure drop is an important consideration when specifying any flow measurement system because the meter becomes part of the process pipework. Excessive pressure drop can affect the wider process and may require additional pumping energy or influence the available operating flow rate.

This becomes increasingly relevant when measuring high-viscosity liquids because greater resistance to flow can increase the pressure required to move the fluid through the system. The flow meter therefore needs to provide reliable measurement without imposing an unnecessarily high pressure drop on the process.

High-pressure flow measurement

For high-pressure applications, mechanical integrity becomes just as important as measurement performance. Titan has invested in dedicated high-pressure testing capability to validate meter designs, materials and performance under demanding conditions.

Its high-pressure OG options can operate at pressures of up to 700 bar, with custom-designed versions tested to 950 bar, depending on model and specification.

The ability to specify different materials, seals, pressure ratings and configurations allows the meter to be selected according to the requirements of the individual application. Titan’s current OG product information also identifies high-pressure models up to 700 bar in 316 stainless steel for certain configurations.

For engineers working with high-pressure process systems, this means that the flow meter needs to be considered as part of the overall pressure-containing installation. Material selection, sealing arrangements, operating pressure and the required flow range all need to be assessed together.

Temperature and pressure must be considered together

Temperature and pressure therefore cannot be considered in isolation. A liquid’s viscosity may change substantially with temperature, while pressure requirements influence the mechanical design and pressure drop of the installation.

Accurate specification must consider the complete operating parameters rather than simply selecting a meter based on nominal flow rate.

This application-led approach is particularly important in industrial environments where operating conditions can vary during production. A meter that performs well under one set of conditions may not necessarily deliver the same measurement performance if temperature, pressure, viscosity or flow rate changes substantially.

Flow measurement accuracy is therefore best considered as part of the complete measurement system rather than as an isolated characteristic of the sensor.

Research into flow measurement under elevated operating conditions has similarly demonstrated that temperature, pressure and viscosity can all influence measurement performance and uncertainty, highlighting the importance of considering the actual service conditions when calibrating and operating flow meters.

Matching the flow meter to the application

For engineers, this is where the flexibility of Titan’s OG range becomes particularly valuable. With different gear profiles, materials, pressure ratings and configurations available, the meter can be matched to the liquid and operating environment.

This flexibility is important because there is no single set of operating conditions that defines every industrial liquid flow measurement application. The required flow range, liquid viscosity, operating pressure, temperature, materials compatibility and installation requirements all need to be taken into account.

A compact flow meter may be appropriate for a low-flow dosing application, while a higher-capacity configuration may be required for liquid transfer. Similarly, a standard meter may be suitable for a relatively low-pressure process, whereas a specialist high-pressure version may be required where operating pressures are substantially higher.

Titan’s OG range is designed to accommodate these different requirements while retaining the same fundamental positive displacement measurement principle.

Calibration and repeatable flow measurement

Combined with individual calibration and Titan’s controlled test facilities, this application-led approach helps maintain reliable, repeatable flow measurement where temperature, pressure and fluid properties would otherwise become limiting factors.

Calibration is a particularly important consideration where measurement accuracy is critical. A flow meter’s performance needs to be understood in relation to the conditions under which it will operate and the characteristics of the liquid being measured.

For process engineers, this means that specifying the correct meter should involve consideration of the full operating envelope. Looking at minimum and maximum flow rates alongside temperature, pressure and viscosity can help ensure that the selected flow meter is capable of delivering the required performance throughout the process.

The result is a more robust approach to liquid flow measurement, helping engineers avoid selecting equipment based solely on a nominal flow rate while overlooking the other variables that can influence measurement accuracy.

Choosing the right liquid flow meter

When selecting a liquid flow meter for an industrial application, temperature, pressure and viscosity should therefore be treated as fundamental specification parameters rather than secondary considerations.

Positive displacement technology such as Titan Enterprises’ Oval Gear flowmeters can offer particular advantages where liquid properties vary or where high-viscosity liquids need to be measured. The ability to measure defined volumes of liquid provides a measurement principle that is less dependent on the velocity profile of the fluid, while increasing viscosity can reduce slip and improve accuracy.

At the same time, high-pressure applications require careful consideration of mechanical design, materials, seals and pressure drop. Selecting a meter that has been designed and tested for the required operating pressure is essential.

Ultimately, reliable flow measurement depends on matching the measurement technology to the application. By considering temperature, pressure, viscosity, flow range, materials and calibration together, engineers can make a more informed choice and achieve consistent measurement performance across demanding industrial processes.

For more information, visit www.flowmeters.co.uk.

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Titan Enterprises Limited

01935812790
sales@flowmeters.co.uk
http://www.flowmeters.co.uk
Unit 2 5a Coldharbour Business Park, Sherborne Dorset, DT9 4JW GB

About us

With over 40-years experience in flow meter innovation, Titan Enterprises Ltd are a UK-based manufacturer of high performance solutions such as the Atrato ultrasonic flowmeter, Oval Gear flow meters and the low flow Turbine flow meters and instrument range.

Our knowledgeable team can offer either an off-the-shelf meter or fully bespoke flow system designed for a particular application whether it is a low cost OEM solution or a specialist flowmeter in exotic materials. Titan supplies innovative flow measurement solutions into a broad range of sectors, including medical, industrial, food and drink, laboratory and pharmaceutical, offshire oil & gas industry, power, chemicals and mining.

For higher volume users we can custom design a flowmeter to match exact application requirements and thus not compromise achievable results with a less than ideal sensor.

At Titan we endeavour to produce devices that are inherently reliable as well as manufactured using the latest and most economical production technology. All flow meters produced by Titan are designed and manufactured to ISO9001 and calibrated to an uncertainty of ±0.25%.

We produce chemically resistant, high accuracy digital flow meters that are not only competitively priced but are engineered to give long-term reliable performance.

What we do in a nutshell

Titan Enterprises is a leading UK manufacturer and international supplier of high-performance liquid flow measurement solutions. We specialise in the design and production of small-bore, affordable off-the-shelf flow meters to bespoke flow sensors designed to customer specifications meeting specific flow ranges and environmental conditions.

Where we supply to

Europe, Africa, Asia, Australia, South America, North America

Industries we supply to

Automation, Chemicals, Consultants, Components Electronics, Energy and Power, Food and Beverage, Glass Ceramics Cement, Metals and Minerals, OEM, Paper and Pulp, Pharmaceutical Cosmetics Toiletries, Plastics and Rubber, Recycling, Textiles, Tobacco, Water and Wastewater

Something interesting you may not know about us:

Titan's R&D spend is 20% of our turnover where the industry norm is 8-10%.

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