6.4 The Comyn oil/water Separater Slightly open both mixture inlet test valve and high-level test valve. Leave these two valves slightly open during the working of the separator. mixture inlet test valve will indicate the nature of the mixture entering the separator. while high-level test valve will free the separator of air and also denote the presence of oil within the oil recovery dome6.4 The Comyn oil/water Separater • Slightly open both mixture inlet test valve and high-level test valve. Leave these two valves slightly open during the working of the separator. Mixture inlet test valve will indicate the nature of the mixture entering the separator, while high-level test valve will free the separator of air and also denote the presence of oil within the oil recovery dome