译文：
临界压力比rcr是指在最小流道截面处，气流流速达到当地音速时的出进口压力之比。喷管的临界压力比在理论上xx可以由公式计算确定。当喷管出进口压力比低于或等于喷管临界压力比时，由于出口截面上已是音速流，出进口压力比的扰动不能超过音速面，所以扰动不能影响喷管内的流动。出口截面上的气流压力维持p2/p1= rcr不变，出口截面上气流仍是音速流，相对排量也维持不变，即W/Wmax=1，此时，喷管处于临界或超临界流动状态。除了喷管以外，其它结构的临界压力比往往需要由试验确定，而以试验确定的临界压力比称第二临界压力比，以资区别。 由于安全阀结构的复杂性，很难测定安全阀最小流道截面积处气流流速，从而无法根据最小流道截流面积处是否达到音速而准确确定安全阀临界压力比。目前，判断安全阀是否达到临界流动状态的方法是测定安全阀的排量系数，认为只要排量系数不随压力比变化 ，安全阀就达到临界流动状态。实测结果是安全阀排量总是随压力比的变化而变化 ，只不过当安全阀压力比低于0.2～0.3时，安全阀排量随压力比的变化较小，而人们认为这种较小的变化是由于测量误差引起的，从而判断全启式安全阀临界压力比约为0.2～0.3。这一试验测定安全阀临界压力比的方法，其理论依据是在临界和超临界流动状态，压力比扰动不能超过音速面，而使喷管相对排量维持不变，即W/Wmax=1。然而，在临界或超临界流动状态，则喷管出口截面的流动已是音速流而使相对排量W/Wmax维持不变，反之，若相对排量维持不变，就判断出口截面的流动已是音速流，安全阀处于临界流动状态则缺乏理论依据和试验证明。

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原文：
Critical pressure ratio rcr is the minimum flow cross section in the Department, the air flow to the local speed of sound when the inlet pressure ratio. The critical nozzle pressure ratio can theoretically be calculated and determined by a formula. When the nozzle out of the inlet pressure is less than or equal to the nozzle than the critical pressure ratio, due to export of the cross section is the speed of sound flow, the inlet pressure disturbance can not exceed the speed of sound than the surface, so disturbances can not affect the flow nozzle. Exit section of the air pressure to maintain p2/p1 = rcr unchanged, the export section of the stream flow is still sound, and the relative displacement remain unchanged, that is, W / Wmax = 1, this time, the nozzle is critical or super critical flow state. In addition to the nozzle, other structure, often require the critical pressure ratio determined by the test, but a trial to determine the critical pressure ratio, said second critical pressure ratio, to distinguish it. As the valve structure, complexity, difficult to measure the minimum flow valve air flow cross-sectional area office, which can not be closure according to the minimum flow area office to determine whether the speed of sound and accurate safety valve critical pressure ratio. At present, determine whether the safety valve critical flow condition is a safety valve discharge coefficient determination, that as long as the discharge coefficient does not change with the pressure ratio safety valve to achieve the critical flow state. The result is always measured with the pressure relief valve displacement ratio change, but when the safety valve pressure ratio less than 0.2 ~ 0.3, the safety valve displacement changes with the pressure ratio lower, and people think that such small The change is due to measurement error caused safety valve to determine the critical pressure ratio of about 0.2 to 0.3. This test determined the critical pressure ratio safety valve approach, the theory is based on the critical and super critical flow condition, pressure ratio can not exceed the speed of sound surface disturbance, Er Shi nozzle relative displacement remain unchanged, that is, W / Wmax = 1. However, critical or supercritical flow state, the nozzle exit section of the flow is sonic flow Ershi relative displacement W / Wmax remain unchanged, the other hand, if the relative displacement remains unchanged, the export section of the flow on the judge has is the speed of sound flow, critical flow safety valve in a state of the lack of theoretical basis and experimental results show.