Humidity is easy to measure in an office or a clean warehouse. It gets harder when the air or gas around the sensor can ignite. Solvent-based tablet coating suites, paint and coating lines, solvent printing presses, dryers handling flammable vapours and dusty bulk handling areas all need relative humidity and temperature data, but a standard transmitter cannot simply be screwed onto the wall there. The instrument has to carry a hazardous-area certification that matches the zone, and the wiring has to be designed around it.
This note looks at the Rotronic HygroFlex5-EX (HF520-EX), an intrinsically safe two-channel humidity and temperature transmitter with interchangeable stainless steel probes for explosive gas and dust atmospheres. Figures come from the Rotronic HygroFlex5-EX datasheet (document 59041E/2025-02, the copy hosted on our product page) and the current Tetracon product pages. Where we give general engineering advice, it is labelled as general. No prices are given; probe type, cable length, display and certification set should be confirmed at quotation.
Why hazardous areas need a dedicated humidity transmitter
- The zone decides the instrument. Any electrical device installed in a classified area must be certified for that zone, gas group and temperature class. A standard RH transmitter, however accurate, is not an option there (general).
- Humidity still matters in these areas. Drying end-points, coating quality, static control and product stability all depend on humidity and temperature, including in rooms and ducts that are classified because of solvents or combustible dust (general).
- Service has to be practical. Calibration and adjustment work should happen outside the explosive zone wherever possible. The HygroFlex5-EX is built around interchangeable probes for exactly this reason: the datasheet notes easy calibration of the probes outside the explosive zone.
How the HygroFlex5-EX is put together
The system has two parts. The transmitter is a robust aluminium housing (IP66) with Ex-e terminals, two galvanically isolated 2-wire 4-20 mA outputs and an optional LCD with trend indicators and keypad configuration. The probe is an intrinsically safe Rotronic HC2-EX probe in a stainless steel body (IP66) with a ROTRONIC HYGROMER IN-1 humidity sensor and a Pt100 Class A temperature sensor. The transmitter measures relative humidity and temperature and calculates psychrometric parameters such as dew point, and the two analogue outputs can be freely selected and scaled.
The datasheet lists two transmitter versions and three probe styles:
- HF520-EX-W wall version and HF520-EX-D duct version.
- HC2-SM-EX probe attached to the transmitter, for wall or duct mounting.
- HC2-IMxxx-EX cable probe with 2, 5 or 10 m of cable and a 130 or 280 mm probe, for flexible installation.
- HC2-IExxx-EX screw-in cable probe with a ½" G or NPT thread for a pressurised pipe, also with 2, 5 or 10 m of cable.
The installation drawings in the datasheet show the transmitter in Zone 1/21, with the duct probe and both cable probes reaching into Zone 0/20 through the AC1301-MEX cable gland. Always check the final arrangement against your own area classification drawing (general).
Rotronic HygroFlex5-EX at a glance
| Item | HygroFlex5-EX datasheet (59041E/2025-02) |
|---|---|
| Measurement | Relative humidity and temperature; all psychrometric parameters available as calculated values |
| Humidity | HYGROMER IN-1 sensor; 0-100 %rh; ±0.8 %rh at 23 °C |
| Temperature | Pt100 Class A; -40 to 60 °C (T5) or -40 to 85 °C (T4); ±0.1 °C at 23 °C |
| Outputs | 2 x 4-20 mA, two-wire, freely selectable and scalable; galvanically isolated; max load 500 Ω; <20 µA at 23 °C |
| Power | 10-28 VDC; 2 x 24 mA at start-up, 2 x 20 mA in operation |
| Timing | Start-up: cold <60 s, warm <30 s; measurement interval 20-240 s |
| Transmitter housing | Aluminium (DIN EN 1706 AC-AlSi12(Fe)), IP66; wall version 1030 g, duct version 1140 g |
| Probes | Stainless steel, IP66; cable lengths 2 / 5 / 10 m; steel sinter filter SP-FN15 |
| Display | Optional LCD without backlight, trend indicators, keypad configuration |
| Ambient (housing / electronics) | -40 to 60 °C without display; -10 to 60 °C with display |
| Transmitter Ex marking | II 2(1) G Ex eb mb [ia Ga] IIC T5 Gb; II 2(1) D Ex tb [ia Da] IIIC T80 °C Db |
| Probe Ex marking | HC2-SM-EX: II 1/2 G Ex ia IIC T5 Ga/Gb, II 1/2 D Ex ia IIIC T80 °C Da/Db; HC2-IM-EX / HC2-IE-EX: II 1/2 G Ex ia IIC T5-T4 Ga/Gb, II 1/2 D Ex ia IIIC T80 °C to 110 °C Da/Db |
| North America | Class I, Div 1/2, Groups ABCD T5-T4; Class II, Div 1/2, Groups EFG; CSA22.2, UL508 |
| Approvals | CE, UKCA, ATEX, EMC, RoHS; Ex certification IECEx, UKEX, QPS |
| Wiring | Ex-e terminals 0.2-2.5 mm²; 16 x 1.5 cable gland for 4.5-7 mm cable |
| Configuration | Specified at order, or on site with HygroSoft; probe adjustment only outside the explosive zone (HygroSoft + AC3001) |
Where it fits
- Pharmaceutical hazardous zones. Listed on our product page. Typical examples are solvent-based granulation, tablet coating and drying rooms, where both room conditions and duct air are recorded (general).
- Coating, painting and drying processes. Also listed on our product page. Humidity affects paint flash-off and curing, and solvent vapour often makes the booth or dryer exhaust a classified area (general).
- Chemical processing and explosive-atmosphere process areas. Room, duct or pressurised-pipe measurement using the wall, duct or screw-in cable probe versions.
- Printing, paper and food. The datasheet lists printing and paper, food and pharmaceutical industries and HVAC as applications. Solvent inks in printing and combustible dust in some food and bulk handling areas are common reasons for classification (general).
Installation notes
- Match the markings to the area. Check the gas group (IIC covers all gas groups), the temperature class (T5 or T4 depending on the probe temperature range) and the dust marking against the area classification drawing before ordering (general).
- Intrinsically safe loop design. The two 4-20 mA loops are 2-wire and galvanically isolated, with 500 Ω maximum load. Your Ex engineer should confirm barriers or isolators and cable parameters for the loop (general).
- Display and temperature. Without a display the housing is rated -40 to 60 °C; with the display it is -10 to 60 °C. Avoid direct sun on outdoor enclosures in Pakistan's summer and keep within the ambient limit (general).
- Calibration outside the zone. Adjustment is not possible through the device menu: it is done outside the explosive zone with HygroSoft and the AC3001 interface. Plan a spare probe so one can be calibrated while the other stays in service (general). The UART service interface must likewise only be used outside the explosive zone.
- Ducts and cable probes. Use the AC1301-MEX cable gland for duct mounting and cable probes, as the datasheet specifies.
Other humidity and moisture options from Tetracon's principals
| Duty | Product | From the datasheet / product page |
|---|---|---|
| RH and temperature in explosive gas or dust atmospheres | Rotronic HygroFlex5-EX | Intrinsically safe probes, ±0.8 %rh / ±0.1 °C, 2 x 4-20 mA, ATEX / IECEx / UKEX |
| Hot, harsh non-hazardous processes up to 150 °C | Rotronic DT722 | Stainless steel, IP65, -40 to 150 °C, two 4-20 mA outputs, RH, dew point, absolute humidity or wet bulb |
| General industrial RH with quick sensor swaps | Michell HygroSmart I7000XP | -40 to 85 °C, 0-100 %RH, ±1 %RH (5-95 %RH), ±0.2 K, interchangeable module |
| Trace moisture (dew point) in process gas in hazardous areas | Michell Easidew PRO XP | Ex d flameproof, -110 to +20 °Cdp, 2-wire 4-20 mA, 450 bar rating |
| Checking and adjusting RH probes | Rotronic HygroGen2 | Portable humidity and temperature probe calibrator for on-site or in-lab calibration |
Typical uses in Pakistan
- Pharmaceutical plants in Karachi, Lahore and Islamabad with solvent granulation, coating and drying suites.
- Paint shops and coating lines in automotive, appliance and steel fabrication plants.
- Flexible packaging and printing plants using solvent-based inks.
- Chemical, refining and petrochemical sites needing RH in classified rooms or ducts.
- Food, flour and other bulk handling sites where dust zones are declared.
These are general application areas based on the datasheet, our product page and our experience of the sector; they are not customer references.
What to send with an enquiry
- The area classification at the transmitter and at the probe (zone, gas group, temperature class, and dust zone if any).
- Mounting: wall, duct or pressurised pipe, plus the cable length needed between transmitter and probe.
- Process temperature range at the probe and the ambient range at the transmitter.
- Required outputs: which two values (for example RH and temperature, or dew point and temperature) and their scaling.
- Whether a local display is needed and how the 4-20 mA loops reach the DCS, PLC or BMS.
FAQ
What is the Rotronic HygroFlex5-EX?
An intrinsically safe two-channel humidity and temperature transmitter from Rotronic AG for explosive gas and dust atmospheres, available as the HF520-EX-W wall version and HF520-EX-D duct version with interchangeable stainless steel probes.
How accurate is it?
The datasheet gives ±0.8 %rh and ±0.1 °C at 23 °C, over 0-100 %rh and -40 to 60 °C (T5) or -40 to 85 °C (T4).
What outputs and power does it need?
Two freely selectable and scalable 2-wire 4-20 mA outputs, galvanically isolated, with a 500 Ω maximum load, powered from 10-28 VDC.
Which zones can it be used in?
The transmitter is marked II 2(1) G Ex eb mb [ia Ga] IIC T5 Gb and II 2(1) D Ex tb [ia Da] IIIC T80 °C Db. The datasheet drawings show the transmitter in Zone 1/21 with the duct and cable probes reaching into Zone 0/20. North American Class I and II, Division 1/2 markings are also listed.
How is it calibrated?
The probes are interchangeable and are calibrated outside the explosive zone. Adjustment is done with Rotronic HygroSoft and the AC3001 interface, not through the device menu.
Can it measure in a pressurised pipe?
Yes. The HC2-IExxx-EX screw-in cable probe has a ½" G or NPT thread for a pressure pipe and comes with 2, 5 or 10 m of cable.
Next steps
Tetracon (Pvt.) Ltd. is the sole authorized agent for Rotronic AG in Pakistan and supplies Rotronic humidity instruments with local technical support. Send us your area classification, mounting, temperature range and output requirements, and we will propose the right HygroFlex5-EX transmitter and probe combination. Use the contact page or see our services. Start with the HygroFlex5-EX, compare the DT722, HygroSmart I7000XP and Easidew PRO XP, check probes with the HygroGen2, browse humidity and pressure, or see all Rotronic products and the full product catalogue. Related notes: humidity and temperature monitoring for pharma cleanrooms, humidity probe calibration with the HygroGen2 and fixed gas detectors for hazardous areas. Free technical guides are collected under Guides.