Similar to GC analysis, the experimental method is subject to potential sources of error. The first error is in the detection of condensation. A key component in chilled mirror dew point measurements is the subtlety with which condensate can be detected — in other words, the thinner the film is when detected, the better. A manual chilled mirror device relies on the operator to determine when a mist has formed on the mirror, and, depending on the device, can be highly subjective. It is also not always clear what is condensing: water or hydrocarbons. Because of the low resolution that has traditionally been available, the operator has been prone to under report the dew point, in other words, to report the dew point temperature as being below what it actually is. This is due to the fact that by the time condensation had accumulated enough to be visible, the dew point had already been reached and passed. The most modern manual devices make possible greatly improved reporting accuracy. There are two manufacturers of manual devices, and each of their devices meet the requirements for dew point measurement apparatus as defined in the ASTM Manual for Hydrocarbon Analysis. However, there are significant differences between the devices – including the optical resolution of the mirror and the method of mirror cooling – depending on the manufacturer.