When taking a pressure measurement at a built-in test fitting, when should the test valve be opened?

Opening the test valve after attaching the test device is the appropriate step because it ensures that the measurement equipment is securely connected and calibrated correctly before pressure readings are taken. This sequence prevents any accidental loss of pressure or contaminants from entering the system during the attachment process.

By first securing the test device, you ensure that any pressure measured will be accurate and representative of the system's true state. If the test valve were to be opened before the test device was attached, it could lead to erroneous readings and potential safety hazards, particularly if there are any leaks or malfunctions in the test device.

Additionally, opening the valve after a secure connection helps create a controlled environment where the pressure can stabilize, allowing for an accurate and reliable reading. This is critical in fluid power systems where pressure can vary considerably based on each component's operation and the overall system dynamics.