Explosion-Proof Thermal Flow Switch NK33EX-90 | Thermal Mass Flow Sensor

Brand：NOIKE-AH
Model：NK33EX-90

Specification

Product Overview

The Explosion-Proof Thermal Flow Switch NK33EX-90 is an industrial thermal mass flow sensor designed for reliable flow detection, alarm, and control in hazardous areas. Based on the King’s Law (L·V·King) thermal heat exchange principle, the sensor detects changes in the temperature difference between two probes to determine the flow condition of gases or liquids.

The integrated structure combines the sensing probe and signal processing circuit into one compact unit, simplifying installation and improving long-term operational reliability. Designed with an Ex dⅡCT6 Gb explosion-proof enclosure, the NK33EX-90 is suitable for Zone 1 and Zone 2 hazardous areas containing Class IIA and IIB explosive gases or vapors.

With no moving mechanical parts, temperature compensation circuitry, and relay switch output, the NK33EX-90 provides dependable flow monitoring for industrial process protection and equipment control.

Working Principle

The NK33EX-90 operates using the thermal dispersion (thermal mass flow) principle, which is based on King’s Law of heat transfer.

Two sensing probes are installed at the front of the sensor:

One probe continuously measures the medium temperature.
The second probe is heated while simultaneously measuring its own temperature.

When no fluid is flowing, the temperature difference between the two probes reaches its maximum value. As fluid begins to flow, heat is carried away from the heated probe, causing the temperature difference to decrease.

The temperature difference varies according to:

Flow velocity
Thermal conductivity of the medium
Humidity of the medium

When the measured flow reaches the preset switching threshold, the internal relay changes state and outputs a switching signal for monitoring or control.

Product Features

Thermal dispersion measuring principle
No moving mechanical parts
Integrated sensor and signal processing circuit
Temperature compensation circuit
Stable long-term performance
Minimal maintenance requirements
Relay switch output
Compact installation design
Low power consumption
Suitable for gas and liquid flow detection
Explosion-proof construction (Ex dⅡCT6 Gb)
Suitable for Zone 1 and Zone 2 hazardous locations

Applications

Industries

Petroleum
Petrochemical
Electric Power
Metallurgy

Typical Applications

Flow detection
Flow alarm
Pump protection
Cooling system monitoring
Process automation
Hazardous area flow monitoring

Technical Specifications

Parameter	Specification
Measuring Principle	Thermal Dispersion (King’s Law)
Power Supply	DC 24 V ±15%
Current Consumption	≤100 mA
Medium Temperature	-45°C to 260°C (High-temperature type)
Relative Humidity	5–95% RH
Nominal Pressure	0–5 MPa
Gas Flow Range	0–30 m/s
Liquid Flow Range	0–10 m/s
Protection Rating	IP65
Explosion-proof Rating	Ex dⅡCT6 Gb
Contact Rating	DC24V / 3A
Power-on Stabilization Time	≤3 minutes
Cable	RVV 4 × 1.0 mm², Ø10 ±0.5 mm
Process Connections	G1/2, NPT1/2, G3/4, NPT3/4, Flange

Electrical Wiring

Terminal	Function
1	DC24V (+)
2	DC24V (-)
3	Normally Closed (NC)
4	Common (COM)
5	Normally Open (NO)

Indicator Lights

VC — Power Indicator
VH — Relay Status Indicator

Installation

For optimal performance, install the sensor on a horizontal pipeline whenever possible.

Recommended Installation

Straight pipe length ≥500 mm
Install at the upper-middle section of the horizontal pipe
Weld the supplied mounting boss onto the pipeline
Install the sealing ring before tightening the sensor
For high-temperature pipelines, apply insulation near the installation point to reduce heat transfer to the instrument housing

Supported process connections include:

G1/2
NPT1/2
G3/4
NPT3/4
Flange connections

Instrument Commissioning

After applying 24 VDC power, allow the instrument to stabilize for approximately 3 minutes before calibration.

During commissioning:

COM serves as the common measurement point.
TP indicates the test point corresponding to flow voltage.
W potentiometer adjusts the switching threshold.
HZ is the voltage test point for threshold adjustment.
VC indicates power status.
VH indicates relay operation.

The switching point can be adjusted by measuring the TP voltage at both low and high flow conditions and setting the threshold voltage accordingly using the potentiometer.

Why Choose the NK33EX-90?

Industrial hazardous environments require reliable flow monitoring without frequent maintenance. The NK33EX-90 combines thermal dispersion sensing, explosion-proof construction, and simple relay output into a compact solution for continuous industrial operation.

No Moving Parts

The sensor contains no mechanical moving components, helping reduce wear and improving long-term reliability.

Integrated Design

The sensing probe and electronic processing circuit are integrated into one device, simplifying installation and reducing wiring complexity.

Temperature Compensation

An internal compensation circuit helps minimize the influence of medium and ambient temperature changes on flow detection.

Hazardous Area Operation

The Ex dⅡCT6 Gb enclosure enables installation in Zone 1 and Zone 2 hazardous locations containing Class IIA and IIB explosive gases or vapors.

Easy Commissioning

Built-in test points and adjustable switching thresholds simplify on-site commissioning and flow switch calibration.

Application Case

Flow Protection for Cooling Water Pipelines in Hazardous Industrial Areas

In petrochemical and power generation facilities, continuous monitoring of cooling water or process fluid flow is essential for protecting pumps and maintaining stable process operation. The NK33EX-90 can be installed on process pipelines to detect whether flow exceeds or falls below a preset switching threshold.

Using the thermal dispersion principle, the sensor continuously monitors changes in heat transfer caused by fluid movement. When the flow reaches the configured switching point, the relay changes state and transmits a switching signal to PLCs or control systems for alarm, pump protection, or process interlocking.

Its integrated construction, temperature compensation circuit, and explosion-proof enclosure make the NK33EX-90 suitable for hazardous industrial environments where reliable flow detection and long-term operational stability are required.

Frequently Asked Questions (FAQ)

1. What measuring principle does the NK33EX-90 use?

It uses the thermal dispersion (King’s Law heat exchange) principle.

2. Which media can it detect?

The sensor is designed for gas and liquid flow detection.

3. What explosion-proof rating does it have?

The NK33EX-90 is rated Ex dⅡCT6 Gb.

4. What hazardous areas is it suitable for?

It is suitable for Zone 1 and Zone 2 hazardous areas containing Class IIA and IIB explosive gases or vapors.

5. What are the supported flow ranges?

Gas: 0–30 m/s
Liquid: 0–10 m/s

6. What relay outputs are available?

The device provides:

Normally Open (NO)
Normally Closed (NC)
Common (COM)

7. How long should the sensor stabilize after power-on?

Allow approximately 3 minutes before commissioning or calibration.

8. Which process connections are available?

Available process connections include:

G1/2
NPT1/2
G3/4
NPT3/4
Flange

Optional Services

Multiple process connection options
Customized installation structures
Instrument configuration support
Installation and commissioning assistance
Product customization based on application requirements

Industries Served

The NK33EX-90 Series is suitable for:

Petroleum and Petrochemical
Oil & Gas
Electric Power
Metallurgy
Chemical Processing
Machinery Manufacturing
Environmental Protection

Installation Instruction for Flow Switch

上盖锁丝 Lock screw of upper cover

电缆锁紧螺丝 Lock screw of cable

上盖 Upper cover

防爆壳体 Explosion-proof shell

六棱锁丝 Hexagonal lock screw

焊接短接 Welded shorting

橡胶圈 Rubber ring

压紧螺帽 Gland nut传感器 Sensor

焊接 Weld

DN10-25 流体管线 DN10-25 fluid pipeline