CM guage is quite expensive about price US2000 to US5000,but it very good idea for finding the end of drying.

SJ creator S or Command F models can optional with this CM gauge.

The Pirani vacuum gauge is built in such a way that the gas composition will alter its pressure reading. Pirani vacuum sensors read erroneously high in the presence of water vapor. This means that if a pirani sensor is the only vacuum sensor on the system it will indicate an erroneously high pressure reading and the transfer of that information to a dryer with a capacitance manometer will, in most likelihood, not work as smoothly as possible. A capacitance manometer is designed in such a way that it indicates the absolute vacuum and is not affected by water vapor in the system. If a freeze dryer is equipped with both sensors on the tray dryer the sensors can be utilized to indicate the end of primary drying. The first step would be to characterize the pirani and the capacitance manometer against each other in a dry, empty and refrigerated system. The freeze dryer is turned on, as per the manufacturer’s operator manual, with nothing on the shelf. The shelf temperature can be left at room temperature or set for a lower temperature, the condenser is activated to be fully on and the system vacuum pump is activated. The freeze dryer is allowed a period of time to become stable in this condition and the pirani and capacitance manometers are read. The difference in their pressure readings in a dry, empty and refrigerated freeze dryer will be relatively small (mT to 10’s of mT) when the system is working properly. These two gauges should NOT be calibrated against each other. There may always be slight differences in their readings do to their difference in design. The difference in pressure (ΔP) between these two gauges, when dry, empty and refrigerated, indicates how close they come together when there is little to no water vapor in the gas mix over the product.

The intent of primary drying is to maximize the sublimation rate within the product. As such the gases in the chamber have a high water vapor content. Subsequently the pirani sensor will read higher than the capacitance manometer. As freeze drying progresses and the amount of ice converting to vapor through the sublimation process will decrease as the product becomes lyophilized from top through the bottom of the product. Eventually the pirani and capacitance manometers will approach the ΔP that was established in a dry, empty and refrigerated system. Approaching this ΔP value indicates that the end of primary drying has occurred.

In most freeze dryers this process has been built into the software. The operator must determine the ΔP for the two sensors as indicated above. The user enters an pressure differential that would indicate end of primary drying into the software program. During the primary drying process, when the differential is met, the software program jumps to a finishing sequence and then secondary drying.