When our earth’s atmosphere (which might contain a significant amount of moisture in it on a warm, moist day) is cooled, such as what happens at night, it will not be able to hold onto the amount of moisture (water) that it could when it was warm. So, some of that moisture will condense out on the grass in the form of dew. Then, when the sun heats the air up once again the following day, the dew will evaporate from the ground.
It is well known that the warmer the gas, the greater the amount of moisture that gas can hold.
At any given point in time, all gases will have what is called a “dew point.” The dew point of any gas is the temperature to which that gas must be cooled to get the first droplet of moisture to condense out of that gas (assumed to be at one standard atmosphere of pressure). The less the amount of moisture in that gas, the cooler must be the temperature of that gas in order to get the first condensation to occur. Thus, the lower the dew point of a gas, the drier (lacking moisture) it is.
Bottom line … the dew point of a gas tells you how much moisture (water) is present in a gas.
Please note, however, that water also represents the presence of oxygen, and oxygen is an element that can oxidize any metal being heated in it. To prevent oxidation of metals during a furnace brazing process, it is important to keep oxygen away from the part being brazed, which means controlling the amount of moisture present in any gaseous atmosphere being used in a furnace atmosphere during that brazing process.
The dew point of a gas can be measured fairly easily by the use of a dew-point meter, which, as the name suggests, is an instrument that will measure the amount of moisture in a gas and provide that information via readouts on the furnace control instruments.
Here are two very important things to know about dew point:
1. Dew point must be measured at the furnace, not at the gas source. Having a certification from your gas supplier that they are filling your liquid tank, or your gas bottles with dry gas, means nothing in the furnace chamber itself where the brazing is taking place. The piping and pipe fittings can have a major effect (either negative or positive) on the dew point of an atmosphere gas being piped from a liquid tank or other external source of gas. This will be the topic of another blog soon.
2. If a gas is being used, you must know its dew point. If a gaseous atmosphere is being used in a furnace brazing process, then the dew point of that gas must be measured and provided to the furnace operator or the brazing process is not being properly controlled! I am often amazed by brazing shops that do atmospheric brazing and do not know the dew point of the gaseous atmosphere being used inside the furnace. They merely “assume” it’s OK. That is a very poor brazing practice.
The dew point of any furnace atmosphere used for brazing must be known in order for the brazing process to be “in control.” There are a number of good dew-point equipment manufacturers available in the marketplace. Investigate the options for that, and be sure that your brazing operations are in control by being able to provide accurate dew-point information about any atmosphere you are using in your brazing furnaces. That includes any gases being used for quenching processes after brazing or for rapid cooling, preheating, etc. If a gaseous atmosphere touches a brazed component at any time during the entire brazing cycle, from loading to unloading, you should know (and control) the dew point of that gas.