What is the C/D Ratio and how is it calculated? - WKB76617

ENVIRONMENT

• Alliance HPLC System
• 2695 Separations Module
• ACQUITY UPLC BSM
• ACQUITY UPLC H-Class Quaternary Solvent Manager (QSM)

ANSWER

C/D Ratio stands for "Compression/Decompression Ratio".  It is a measure of the gas load and/or fluidic integrity of the primary pump head on most of the current solvent managers, such as Alliance or ACQUITY.   

On both ACQUITY and Alliance solvent managers, pulse-free transfer of solvent is accomplished by precompressing the solvent in the primary pump to the same pressure as the solvent being delivered by the accumulator pump. In the illustration below, this is viewed in the smaller "precompression step" preceding the larger "primary delivery" step, where solvent is actually transferred from primary to accumulator. Because of this precompression step, by the time solvent is transferred from primary to accumulator, it is already at system pressure. This is what allows for pulse-free pump transfer with low-pressure ripples.  

The "Compression" in C/D ratio is the number of motor steps required for the primary pump to compress the solvent to the same pressure as the accumulator pump during this precompression step. The "Decompression" in C/D ratio is the number of motor steps required for the primary pump to decompress back to atmospheric pressure after transfer is completed.

The number of motor steps required to decompress will not change too much over time. However, the number of motor steps required to compress will change based on several factors:

• The pump used (i.e., Alliance versus ACQUITY)
• The system pressure
• The flow rate being delivered
• The stroke volume of the pump
• The solvent or solvent mixture used as mobile phase
• The efficiency of the degassing of that mobile phase
• The fluidic integrity of the pump (i.e., no leaks)

There is no official specification as to what the C/D ratio should be. However, Alliance systems will produce the error "Bubble found on compression" if the C/D ratio goes above 1.8.  

ADDITIONAL INFORMATION

For a 2695, the C/D ratio can be found under the Diagnostics tab from the Main screen