Thermal dispersion flow switch

thermalbiogasfor liquids

Overview
Rheotherm thermal mass flow switches provide low-maintenance flow monitoring and protection for a wide range of liquid and gas services. Using thermal mass sensing with either inline (heated/unheated RTDs mounted on a straight-through tube) or insertion probe sensors (heated/unheated RTD tips), these switches generate a differential temperature signal proportional to flow. With no wetted moving parts, they resist wear and damage during transient or empty-line conditions.

Function
Flow switches detect when flow rate crosses an adjustable set point (minimum or maximum) and output a trip signal (commonly to pumps, valves, or alarms). Devices can be configured to trigger on low-to-high or high-to-low transitions. Typical trip response ranges from about 1–25 seconds depending on sensor type and configuration; time-delay options are available for custom response profiles.

Applications

• Industrial gases: Argon, Helium, Oxygen, Carbon Dioxide, Methane, Natural Gas, Propane, Ethylene
• Chemical process streams: Ammonia, Ferric Chloride, Hydrofluorosilicic Acid, Liquid Chlorine, Sulfur Dioxide
• Fuel and hydrocarbon services: Flare Gas, Fuel Gas, Coal Gas, Coke Gas, Hydrocarbon Vapors
• Specialty services: Digester Gas, Flue Gas, Diesel Exhaust, Tungsten Hexafluoride, Antifoam Systems
• Industries: Aerospace, Chemical & Petrochemical, Natural Gas, Oil & Gas Production, Wastewater Treatment, Pulp & Paper

Installation
Inline flow switches for small lines are typically installed with standard compression tube fittings. Options include flanges, NPT, and sanitary connections. Insertion probe switches for larger pipes (≥1") are mounted with 1" NPT or flanged fittings and may be used in vertical or horizontal runs. Sensor and electronics are commonly integrated; remote electronics options are available for specific site requirements.

Key Features

• No moving parts contacting the fluid
• Inline TU sensor option
• Advanced signal processing electronics
• Field-adjustable switch point
• Low-maintenance operation
• Easy installation
• Choice of wetted materials
• Long-term reliability

Models Available

• Model 100CS – Cost-effective industrial option with integral sensor and electronics; one SPDT relay (10 A @ 120 VAC or 5 A @ 24 Vdc). CE marking available. Inline or insertion probe sensor options.
• Model 100FS – Separate sensor and electronics; one SPDT relay standard with optional second relay for high/low set points. Available with intrinsically isolated sensor for hazardous locations and remote electronics options.
• Model 400 – Integral insertion probe sensor and electronics; configurable with two SPDT relays or single relay plus non-linear analog flow output. Calibrated signal option available.

How They Work
Inline sensors use a heated RTD and an unheated RTD mounted on the tube exterior; insertion probes use a heated RTD tip and an unheated RTD tip. Flow removes heat from the heated element, altering the temperature differential between RTDs; this differential is the primary flow signal (higher flow → lower differential). Trip points are adjustable in the field.

Documentation (selected)
Product brochure and technical PDFs are available with sensor dimensions, method of operation, selection guides, and installation instructions.

Technical specifications

• Process temperature (standard): 0–140 °F (-18–60 °C); Environment: 0–120 °F (-18–50 °C)
• Temperature options: Inline: -20–350 °F (-30–175 °C); Probe: -20–400 °F (-30–200 °C)
• Pressure (standard): 0–1,000 PSI; Options up to 30,000 PSI
• Calibration accuracy: typically ±1% of reading
• Response time: nominal 1 s; typical 3–5 s (63% of range); time constant/delay options available
• Repeatability: ±0.5% of reading
• Line sizes (standard): Inline sensor — capillary up to 1" tube depending on flow range; Probe sensor — pipes/ducts from 1.5" up to 72" (round, square, rectangular)
• Process connections: Inline — plain tube stubs for compression fittings; Probe — 1" MNPT (options: MNPT, Tri‑Clover, SS flange, other fittings)
• Wetted materials (standard): 316 stainless steel; options: Hastelloy C-276®, Monel®, Alloy 20, Titanium, Tantalum, Inconel
• Enclosure: Sensor — Inline (PVC) or Probe (NEMA 4X junction box); Electronics — Integrated NEMA 4X cast aluminum or Remote NEMA 4
• Hazardous location options: Remote electronics with 100FS only; FM approvals available for Class I, Div. 1 Groups A,B,C,D; Class I, Zone 1 IIB+H2; Class II, Div. 1 Groups E,F,G; Class III, Div. 1
• Outputs (standard): Single SPDT Form C relay (10 A, 125 VAC)
• Output options: Second independent SPDT relay, DPDT relay, time delay on switch (remote electronics with 100FS)
• Input power (standard): 110/230 VAC ±10 V; Option: 24 VDC ±4 V (200 mA)
• Flow capacity data: Inline sensor flow tables and probe sensor velocity limits cover typical normal and extended min/max flow rates for liquids (GPM, cc/min) and gases (SCFM), plus probe velocity limits (ft/min).