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Testo · Testo 605i Thermohygrometer - Hygrometer & Indoor Thermometer for Humidity Measurement and Indoor Temperature - 90° Bendable In-Duct Temperature and Humidity Monitor with Bluetooth - iOS & Android
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Verdict
The testo 605i is a technician’s instrument rather than a household hygrometer. It measures relative humidity and temperature at the tip of a compact probe head that bends to 90°, which lets it reach into a duct, a register, a crawl space or an environmental chamber, and it sends the readings over Bluetooth to the testo Smart Probes App, which calculates dew point and wet bulb temperature and logs the data. It is supplied calibrated with three AAA batteries. For HVAC and refrigeration work, particularly when used alongside other testo Smart Probes, it is efficient. For someone who wants a number on a shelf, a conventional hygrometer with a display is the better purchase; see choosing a humidity meter for indoor use.
Who It Is For
- HVAC/R technicians measuring supply and return air conditions across an evaporator coil.
- Building inspectors checking humidity in crawl spaces, attics and behind access panels.
- Laboratory and facilities staff verifying chamber or cold-room conditions.
Documented Specifications
| Item | Documentation |
|---|---|
| Measurements | Relative humidity, temperature |
| Calculated values | Dew point, wet bulb temperature |
| Probe head | Compact, bendable to 90° |
| Connectivity | Bluetooth to the testo Smart Probes App (iOS and Android) |
| Display | None on the probe; phone or tablet |
| Power | 3 × AAA (included) |
| Calibration | Supplied calibrated |
The Bendable Head
Duct humidity is measured by inserting a probe through a test hole or a register, and a rigid probe often cannot reach the airstream at a useful angle. The 605i’s head bends to 90°, so it can be pushed through an opening and turned into the flow. The same feature helps in crawl spaces and behind access panels, where the phone stays outside and the probe goes in. Because the sensor is at the tip, the reading reflects the air at the measurement point rather than at the technician’s hand. Our guide to measuring relative humidity correctly explains why placement and settling time matter as much as the instrument.
App, Calculations and Logging
The testo Smart Probes App shows live readings from one or several probes at once, so a technician can view supply and return conditions side by side. Dew point and wet bulb are calculated automatically, which removes the psychrometric chart from a coil evaluation. The app logs measurements over time and produces reports that can be saved and shared. When the 605i is used together with a testo pressure probe, an air-velocity probe or the testo 860i thermal camera, all of them appear in one report, which is the main reason technicians standardise on the system. Two probes placed at supply and return give the enthalpy difference across a coil in the app without a chart, and the logging function allows a probe to be left in a duct for an hour to watch conditions settle after a change. Owners report Bluetooth range of a few rooms in practice, enough to leave the probe in a crawl space and read it from the hallway.
Accuracy and Calibration
The 605i is delivered calibrated. testo publishes the accuracy on its datasheet, and a calibration certificate can be ordered where a traceable record is required. As with any capacitive humidity sensor, accuracy is best in the middle of the humidity range and after the probe has had time to settle in the airstream. For understanding what a certificate does and does not guarantee, see how to read a calibration certificate. Periodic checks against a salt reference are described in how to calibrate a hygrometer.
Strengths
- Bendable probe head reaches ducts and voids.
- Automatic dew point and wet bulb in the app.
- Part of a probe system that produces one combined report.
- Supplied calibrated with batteries.
Limitations
- No display; the phone is required.
- Designed for spot measurements, not continuous room monitoring.
- Higher price than a basic display hygrometer.
How It Compares
The Extech EN300 is a five-in-one meter with humidity and temperature on its own display, plus air velocity, light and sound, which suits a generalist. The Extech MO290 combines a psychrometer with a pinless moisture sensor for restoration work. The 605i wins when the measurement point is hard to reach and the results need to flow into a testo report.
How We Research
This review is based on testo’s published specifications, the Amazon listing and owner reports. We did not run laboratory tests.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
Does the testo 605i have a display?
No. Readings appear in the testo Smart Probes App on a phone or tablet over Bluetooth.
What does it calculate?
The listing states relative humidity and temperature, with dew point and wet bulb temperature calculated automatically.
Is it calibrated?
The listing states it is supplied calibrated and ready to use with three AAA batteries.
Can it be used for home humidity monitoring?
It can, but it is designed for spot measurements by a technician. For continuous room monitoring a wall or desk hygrometer with a display is more practical.
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