The Filter/Dryer is often fitted with a "sight glass" through which the a/c technician can determine much about the amount of refrigerant contained within the a/c system (the "refrigerant charge"). In fact, one of the important aspects of good a/c system service is to insure that this level of charge is correct for optimum a/c unit operation. Although only an approximation, a good "rule of thumb" to determine if an air conditioner is properly charged, is to add refrigerant to the system just until there are no more bubbles visible in the sight glass. (A more comprehensive method is presented in Appendix I.)

The reason a bubble-free liquid refrigerant emerging from the Condenser is a good indicator of proper refrigerant charge level is that just enough refrigerant has been added to be sure that at least some liquid refrigerant exists in the last stages of the Condenser. If there is not enough liquid in the system (bubbles still visible) then there can be no possibility of subcooling all of the refrigerant; this denies the Condenser the opportunity to deliver its maximum performance. That is, a deficient charge will enable some of the refrigerant to literally 'slip-through' the Condenser. Low refrigerant charge will also cause the Evaporator to operate at lower pressure and temperature than optimum and therefore also decrease system performance.

On the other hand, excess charge levels cause increased refrigerant pressure loss through the Condenser because liquids are generally more resistive to flow than the vapor which is much lighter. This pressure loss must be made up by the Compressor because it has to deliver a higher discharge pressure ("head pressure") for the refrigerant to flow through the loop. This, of course, increases the amount of energy required to operate the air conditioner and, therefore, decreases its efficiency. [Most air conditioning systems operate with about 15 Fº subcooling.]

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