Dew Point in Metal Heat Treatment

In a heat treatment furnace, the atmosphere is part of the recipe: annealing, sintering and brazing are performed under protective gases that prevent oxidation and unwanted reactions of the metal. The moisture present in that atmosphere affects the quality and consistency of the process, and with them the properties of the final product — which is why it is specified and controlled as dew point. For the general context of the magnitude, see the dew point instrumentation landing.

This note covers

• Why the moisture in the furnace atmosphere defines process quality
• Which dew point ranges are controlled depending on the process and the metal
• How to measure dew point in high-temperature furnace gases

The Atmosphere as a Process Variable

The water content of the atmosphere takes part in the chemical equilibrium inside the furnace: it determines whether the metal surface tends to oxidize or stay reduced during the thermal cycle. An atmosphere wetter than specified translates into surface oxidation, sintered parts out of specification or defective brazed joints — defects usually discovered only at quality control, with the batch already processed.

That is why the process specification is expressed as a target dew point. Depending on the process and the metal, control values range from −60 °C up to +35 °C or even higher — a range no ambient relative humidity measurement can meaningfully cover.

How to Measure in High-Temperature Gases

The practical difficulty is temperature: furnace gas runs far above the operating range of any humidity sensor. Measurement is normally done on a sampling line that draws a small gas flow and lets it cool before it reaches the dew point probe. Since the absolute water content of the gas does not change as it cools — as long as it does not condense along the way —, the dew point measured in the line still represents the furnace atmosphere.

The instrument has to be rugged, leak-tight, easy to install and maintain, and compatible with different furnace types and protective gases.

What DRYCAP® Technology Brings

Vaisala's DRYCAP® technology is specified for this duty: it stays stable at low dew points, withstands condensation and recovers undamaged if the sensor gets wet, and its sensor purge — a controlled heating cycle that removes residual chemicals — sustains accuracy in contaminant-laden atmospheres between calibrations.

For blanket gases in heat treatment, a probe like the DMP7 — with a short probe head for tight spaces and sensor purge — integrates into the sampling line or the equipment; for your furnace's specific configuration, consult a specialist.

This technical content is based on Vaisala's documentation on metal heat treatment furnace atmospheres. AKRIBIS is an authorized Vaisala partner for distribution and technical support in the region — explore the full Vaisala instrumentation line.