The dewpoint (or more correctly, the dewpooint temperature) is the temperature at which air can no longer support the existing quantity of water in vapour form.
Air always has a certain amount of moisture in the form of gas which is therefore normally invisible to the human eye. As the temperature of the air is lowered, the ability of the air to support this moisture is also lowered. At some temperature, a point will be reached where the excess moisture will change to liquid and will either become visible as a mist, or will be deposited on nearby surfaces as condensation. The temperature where this phenomena occurs is known as the dewpoint temperature.
Since the dewpoint temperature is not a real measure of the current air temperature, it is normal that the actual air temperature and the dewpoint temperature will be at different values. For example, in a PET resin dryer, the actual air temperature is typically 160ºC, while the dewpoint temperature is around -20ºC.
The dewpoint measurement is taken on the process air line just after the desiccant chamber(s). Many dryer makers fit an air sampling point with an outlet coming to the front panel of the dryer. When taking an air sample, allow adequate time for a stable result to be obtained and make sure to check all desiccant chambers. Each dewpoint meter may have different operating methods so please refer to the instruction manual. Although dewpoint is important for correct drying, the process airflow is by far the most critical. A dryer with good airflow and poor dewpoint is still partially effective, whereas a dryer with excellent dewpoint but poor airflow is useless.