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Mass Flow Sensors: Precision Hot-Wire Thermal Core Modules for Laboratory Gases

Quick Answer

For laboratory gas flow measurement, hot-wire thermal core modules measure mass flow directly in ranges from 0 to 5 mL/min up to 0 to 30 L/min. Silver Automation Instruments supplies these mass flow sensors with 4-20 mA HART, RS485 Modbus, 0-5 VDC, or pulse outputs and stainless steel 316L wetted parts. Send us your gas type, full scale flow in mL/min or L/min, inlet pressure in bar, process temperature in °C, and fitting size for a specific quote.

Why a Hot-Wire Thermal Core Works for Laboratory Gas Flow

Mass flow sensors that use a hot-wire thermal core measure heat loss from a heated element to the moving gas. The output is proportional to gas mass flow, not volume flow. That matters in laboratory gas lines because density changes with pressure and temperature. A 100 mL/min nitrogen flow at 2 bar is not the same gas quantity as 100 mL/min at 1 bar. With a thermal core, the sensor reads mass flow directly and reports units such as mL/min, L/min, g/h, or kg/h.

Hot-wire thermal core modules fit clean gases common in laboratories: nitrogen, oxygen, dry air, CO2, argon, helium, methane, and hydrogen. Because the sensor responds to heat transfer, gas composition affects calibration. The sensor must be calibrated for the actual gas or a defined binary gas mix. Do not use one calibration curve for all gases. We have seen this cause wrong readings on customer sites many times.

Flow Ranges, Accuracy, and Sensor Construction

We configure laboratory mass flow sensors in ranges such as 0 to 10 mL/min, 0 to 100 mL/min, 0 to 1 L/min, 0 to 5 L/min, 0 to 20 L/min, and 0 to 30 L/min. For very low flows below 100 mL/min, a capillary hot-wire thermal core works best. For flows from 0.5 L/min to 30 L/min, an inline small-bore thermal core provides better stability and lower pressure drop.

Typical accuracy is ±1.0% of full scale under calibrated conditions. Repeatability is 0.2% of full scale. Response time is below 1 second for most clean gas lines. Standard pressure rating is 10 bar, with 30 bar optional. Standard process temperature is 0 to 50 °C. The wetted parts use stainless steel 316L. Seals can be Viton, PTFE, or Kalrez depending on gas compatibility. PT100 temperature compensation is available for labs where gas temperature moves more than 5 °C during a batch.

Most engineers skip this part. Low flow gas measurement fails most often because of poor pressure regulation, not because the sensor core is weak. In practice, a loose compression fitting on a 6 mm PTFE tube causes more drift than the sensor core itself. A two-stage regulator and short 6 mm or 1/8 inch PTFE tubing reduce drift and unstable readings. We have observed this on lab benches in Malaysia and Australia where inlet pressure moved by only 0.2 bar and the reading shifted more than the sensor specification.

Signal Outputs and Lab System Integration

Silver Automation Instruments offers analog and digital outputs for hot-wire thermal core modules. Common choices include 4-20 mA with HART, 0-5 VDC, RS485 Modbus RTU, and pulse output for totalized gas volume. A 4-20 mA HART output connects directly to PLC or DCS analog input cards. RS485 Modbus works better for multi-channel lab rigs because it reduces wiring and allows digital readout of flow, temperature, and totalizer data.

Some modules include a local display showing mass flow, totalized flow, and gas temperature. For vacuum pump systems, place the mass flow sensor upstream of the pump. Downstream vacuum changes gas density and reduces sensor stability. For flammable gases such as hydrogen or methane, request ATEX Zone 1 or Zone 2 approval and state your zone classification clearly in the inquiry.

Installation Notes That Prevent Most Field Errors

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