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Liquid Thermal Mass Flow Meter Working Principle: Thermodynamic Heat Transfer for Low-Flow Fluids
Quick Answer: A liquid thermal mass flow meter uses a heated PT100 sensor and a reference PT100 sensor. The liquid flow absorbs heat from the heated sensor. The meter converts the temperature difference between the two sensors into a mass flow value, usually in kg/h. This method works best for low-flow liquids with stable fluid properties.
Most industrial flow meters measure volume. A thermal mass flow meter measures mass directly. That matters when you dose additives, solvents, catalysts, or flavors by weight. Silver Automation Instruments supplies liquid thermal mass flow meters for flow ranges from 0.005 kg/h up to about 10 kg/h. Typical pipe sizes are DN8, DN15, and DN25. The electronics output 4-20 mA HART or RS485 Modbus RTU.
How Thermodynamic Heat Transfer Works in a Liquid Thermal Mass Meter
The sensor body contains two PT100 RTD elements. One element is heated with a small constant power. The other element measures the reference liquid temperature. When liquid flows past the heated element, the fluid carries heat away. The change in temperature difference between the heated element and the reference element is proportional to mass flow.
In practice, the meter does not measure a simple temperature drop. The electronics maintain either a constant temperature difference or a constant power input. Most liquid thermal meters from Silver Automation Instruments use constant temperature difference control. This method gives a fast response and a wide turndown ratio. The heat transfer equation depends on fluid specific heat capacity, density, and thermal conductivity. For this reason the fluid composition must stay stable.
Here is the thing. If you change from pure water to a 20 percent methanol mix, the specific heat capacity changes. The meter will read wrong unless you recalibrate. For a fixed recipe, the reading stays stable and repeatable.
Where Low-Flow Liquid Thermal Mass Flow Meters Fit Real Installations
Engineers use these meters on small chemical dosing lines, pilot plant reactors, food flavor injection, and laboratory liquid feed systems. A paint manufacturer in Southeast Asia uses a DN8 liquid thermal mass flow meter to measure solvent flow from 0.5 to 3 kg/h. The solvent line runs at 4 bar and 25 °C. The meter sends a 4-20 mA HART signal to a PLC and a local display shows kg/h.
Last year a customer in Vietnam asked us for a meter to measure 0.3 to 1.2 kg/h of a light organic solvent. We supplied a 316L stainless steel body, PTFE O-rings, and a 4-20 mA HART output. The process pressure was 6 bar. The meter was installed in a vertical upward pipe segment after a 20 micron filter.
These meters also appear in water treatment plants for coagulant dosing, in food plants for vitamin premix injection, and in marine fuel additive systems. They work best with clean, low viscosity fluids. Acceptable viscosity is usually below 50 cP. Liquids with high viscosity reduce sensor response and cause measurement lag.
Key Specifications Engineers Check First
Start with flow range. A DN15 liquid thermal mass flow meter may cover 0.01 to 1 kg/h or 0.1 to 10 kg/h depending on the sensor power and calibration. Accuracy is often plus or minus 0.5 percent of reading from 10 to 100 percent of full scale. Below 10 percent flow, the error
Check pressure and temperature. Standard sensor bodies handle 16 bar at 60 °C. High pressure versions handle 40 bar at 100 °C. Wetted parts are typically 316L stainless steel. Hastelloy C22 is available for aggressive chemicals. Process connections include G1/4, NPT 1/4, and flanges for DN25 and larger. Electrical certifications include ATEX Zone 1 for gas environments and IP67 for field mounting.
Output options matter for integration. The standard output is 4-20 mA with HART. Most plants also request RS485 Modbus RTU for digital communication. Some units include a local LCD display with totalizing function. The display can show kg/h, g/min, or total kg.
Installation Notes That Prevent Measurement Drift
Gas bubbles cause large reading errors in liquid thermal mass meters. Install the meter after a filter or degassing device. A vertical upward pipe segment helps bubbles move away from the sensor. Avoid mounting the meter at the lowest point of a pipe where sediment can collect.
Tighten the process connection with proper torque. Excessive force can deform the sensor tube. We have seen this on customer sites many times. A deformed sensor changes the internal flow profile and shifts the calibration. After installation, close the isolation valves and check the zero point. The display should read zero kg/h with no flow.
Most engineers skip this part. But zero point check after installation takes five minutes and prevents a lot of troubleshooting later.
FAQ: Five Questions About Liquid Thermal Mass Flow Meters
Question 1. Can a liquid thermal mass flow meter measure any liquid? Answer: No. It works best with clean, low viscosity fluids that have stable composition. Changing heat capacity or thermal conductivity changes the flow reading. Oils, light solvents, water, and many additives are fine. Slurries and heavy fuel oil are not recommended.
Question 2. What is a typical low-flow range? Answer: For DN8 and DN15 meters, common ranges are 0.005 to 0.5 kg/h, 0.01 to 1 kg/h, and 0.1 to 10 kg/h. The exact range depends on fluid, pressure, and sensor design.
Question 3. Does the output show mass flow or volume flow? Answer: The meter measures mass flow directly. The display can show kg/h or g/min. Volume flow can be derived if the liquid density is entered into the electronics.
Question 4. How do I choose between a thermal mass meter and a Coriolis meter for low liquid flow? Answer: Thermal mass meters cost less and have no moving parts. Coriolis meters handle changing fluid composition and provide higher accuracy. If the recipe is fixed, a thermal mass meter is usually enough. If the liquid properties vary, select a Coriolis meter.
Question 5. What information should I send for a quote? Answer: Send us your fluid name, viscosity in cP, flow range in kg/h, pressure in bar, temperature in °C, pipe size in DN, and preferred output such as 4-20 mA HART or RS485. This helps us recommend the right sensor body and calibration.
Contact Silver Automation Instruments for a liquid thermal mass flow meter selection. Tel: +86-25-68650347. Whatsapp: +86-25-52155837. WeChat: +86 15365082610. Website: flow-meter.com.au. Send us your pressure (bar), temperature (°C), pipe size (DN), and flow range. We will reply with a model and price for your low-flow liquid application.
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