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Extech RH300 Review: Dew Point and Wet Bulb Without Slinging Anything

Wet bulb temperature used to require whirling a wet thermometer around your head and reading it quickly. The measurement still matters; the whirling does not.

By the Akermin Editorial Team · 4 min read

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Extech RH300 Review: Dew Point and Wet Bulb Without Slinging Anything

Extech · Extech RH300 Digital Psychrometer, Humidity, Temp, Dew Point, Wet Bulb

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Price and availability are shown on Amazon and can change at any time.

Verdict

The Extech RH300 is a digital psychrometer: it measures relative humidity and temperature and displays dew point or wet bulb alongside them. The listing states simultaneous display of relative humidity, temperature and dew point or wet bulb or probe temperature, and describes wet bulb measurement without slinging. For building surveys, condensation investigations, HVAC work and anything involving drying, those two derived values are the ones that lead to decisions.

Documented Specifications

Item Manufacturer listing
Measures Relative humidity, temperature
Calculates Dew point, wet bulb
Display Simultaneous humidity, temperature and one derived value
Probe temperature Can be displayed in place of a derived value
Method Electronic, no sling required

The listing does not publish ranges, accuracy or response time. Check the datasheet if you need those for documentation.

Dew Point Is the Number That Predicts Damage

Relative humidity is a ratio, and it moves when the temperature moves even if nothing else changes. That makes it a poor predictor on its own. Dew point is an absolute measure of the moisture in the air, expressed as the temperature at which that moisture condenses.

The practical rule is simple and powerful: any surface colder than the dew point will collect water. That is why condensation forms on single glazing, on cold water pipes, on the inside of an uninsulated wall and on a metal lintel. Measure the dew point of the air and the temperature of the surface, compare the two, and you have predicted the problem before it appears. This is the measurement that turns “the wall is damp” into a specific cause with a specific fix.

Wet Bulb and Why the Slinging Went Away

Wet bulb temperature is what a thermometer reads when its bulb is wrapped in a wet wick and air moves over it. Evaporation cools the bulb, and the extent of that cooling depends on how much moisture the air can still absorb. It is the basis of evaporative cooling, of cooling tower performance and of heat stress assessment.

Traditionally it was measured with a sling psychrometer: two thermometers on a handle, one with a wet wick, whirled for a minute and read immediately. It works, and it is slow, awkward, easy to do wrong and impossible in a confined space. Calculating it from an electronic humidity measurement gives the same answer in a second, which is why almost nobody slings any more.

Using It on a Damp Survey

  • Measure air dew point in the affected room, then measure surface temperatures with an infrared thermometer.
  • Any surface at or below the dew point is a condensation risk regardless of what the relative humidity reads.
  • Compare the affected room with an unaffected one. A higher dew point indoors than outdoors means moisture is being generated inside.
  • Repeat at different times of day. Condensation is usually a night-time phenomenon when surfaces are coldest.

Probe Temperature Has a Purpose

The option to display probe temperature instead of a derived value is useful when the probe is somewhere different from the instrument, such as in a duct or through a wall. It lets the same tool report the condition at the measuring point rather than at your hand, which matters whenever the two are not the same temperature.

Strengths

  • Dew point and wet bulb calculated rather than looked up.
  • Simultaneous display of measurement and derived value.
  • No sling psychrometer technique to get wrong.
  • Long established instrument with a clear, narrow purpose.

Limitations

  • No ranges, accuracy or response time published in the bullets.
  • No logging or connectivity stated.
  • Does not measure surface temperature, which the dew point comparison requires.
  • Single purpose compared with a multi-function environmental meter.

How It Compares

The Testo 405i and the Testo 605i cover humidity with a Bluetooth workflow and duct access. A five in one meter such as the Extech EN300 covers more parameters less deeply, as set out in that comparison. Our ranking is in the hygrometer roundup.

How We Research

All figures come from the manufacturer listing. We did not run laboratory tests and publish no measurements of our own.

Extech RH300 Digital Psychrometer, Humidity, Temp, Dew Point, Wet Bulb

Extech RH300 Digital Psychrometer, Humidity, Temp, Dew Point, Wet Bulb

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Testo 605i Thermohygrometer - Hygrometer & Indoor Thermometer for Humidity

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HT607 Temperature Humidity Meter - Large LCD Backlight with Cover Protector

HT607 Temperature Humidity Meter - Large LCD Backlight with Cover

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

What is a psychrometer?

An instrument that determines humidity from temperature measurements. Traditionally it used a dry bulb and a wet bulb thermometer and a chart. A digital psychrometer measures humidity electronically and calculates the same derived values.

Why does dew point matter?

It is the temperature at which moisture condenses out of the air. Any surface colder than the dew point will get wet, which is how condensation, mould and corrosion problems begin. Relative humidity alone does not tell you that.

What is wet bulb used for?

Air conditioning and evaporative cooling calculations, and heat stress assessment. It represents the lowest temperature achievable by evaporating water into the air, which sets the limit of what evaporative cooling can do.

What does simultaneous display mean in practice?

The listing states relative humidity and temperature are shown together with dew point, wet bulb or probe temperature. You see the measurement and the derived value at once instead of switching modes and writing numbers down.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.