The test gas used is the same as the actual gas specified. The pressures are essentially the same as those specified. In general, this class of test can be applied only to air compressors with an atmospheric suction for strict conformance to the ASME code. The test piping would be an open loop with an atmospheric inlet and the discharge vented to atmosphere downstream of a control valve. The test conditions of Class I tests more closely duplicate actual operating conditions than do the test conditions used in the other two classes of ASME tests. Modified open-loop Class I tests that do not meet the Code are sometimes run to quickly approximate the performance.