Why does the unit NOT have an indicator light to indicate that the filter needs replacing?
Short answer – There is not enough value added to unit and user to justify the cost of the device. Long answer – If you were to add such a device, there are three ways to do so. First, a simple timer could be used that would keep track of the length of time the unit has operated and turn on a light after a given length of time. Several companies that provide such a light do it this way. It is the least complicated and least costly method. Unfortunately, it really does not measure anything that tells that the filter needs to be replaced. Second, you could measure the pressure drop across the filter. The higher the pressure drop, the dirtier the filter. A light could be turned on when a certain pressure drop was reached. This method is more costly than the first but does work. However, normally the carbon is used up in the filters long before the high-pressure drop level is reached. Therefore, the filter would really need to be replaced before the light is ever turned on. Third the vapor
