Test blocks may include facilities that test relief valves with a liquid test medium such as water. Water is nontoxic and
inexpensive and may allow a close simulation of operating conditions. Because very small water leaks are not readily
detected, a water test is usually limited to measuring the set pressure. Leakage or tightness tests are usually made
with air (see 6.2). However, leakage tests may instead be made with water as described in API 527.