This method is fairly accurate for non-condensable gases, such as oxygen and nitrogen. However, condensable gases, such as water vapour, ammonia, carbon dioxide, and pump-oil vapors may be in gaseous form in the low pressure of the vacuum chamber, but will condense when compressed by the McLeod gauge. The result is an erroneous reading, showing a pressure much lower than actually present. A cold trap may be used in conjunction with a McLeod gauge to condense these vapors before they enter the gauge.