diff --git a/open_loop.py b/open_loop.py
index d624983..ae3138d 100644
--- a/open_loop.py
+++ b/open_loop.py
@@ -6,8 +6,8 @@
 2. 逐个开度：写入电机行程 → 以固定周期采集控流阀前后 4 个传感器
    （前压力、前流量、后压力、后流量）→ 等待“控流阀后流量”进入稳态后
    切换到下一个开度；
-3. 全程逐周期写 CSV，结束后在单个窗口内输出两张子图（前后压力、前后流量），
-   保存到工程目录下的 open_loop_data/。
+3. 全程逐周期写 CSV，结束后在单个窗口内输出三张子图（前后压力、前后流量、
+   绝对压力比），保存到工程目录下的 open_loop_data/。
 
 示例：
     python open_loop.py
@@ -71,6 +71,11 @@ PLOT_DPI = config.PLOT_DPI
 ANNOTATE_STEPS = True                                  # 曲线上标注开度步
 
 
+# 绝对压力 = 表压读数 + 大气压；临界压力比用于判断是否出现壅塞（choked）流动。
+ATMOSPHERIC_PRESSURE_KPA = 101.325   # 标准大气压（kPa）
+CRITICAL_PRESSURE_RATIO = 0.528      # 临界压力比（阀后绝对压力 / 阀前绝对压力）
+
+
 CSV_FIELDS = (
     "time_s",
     "step_index",
@@ -80,6 +85,8 @@ CSV_FIELDS = (
     "flow_after_slm",
     "pressure_before_kpa",
     "pressure_after_kpa",
+    "P_abs_ratio",
+    "is_ratio_smaller_than_0.528",
 )
 
 
@@ -140,7 +147,7 @@ class SteadyStateDetector:
 # ---------------------------------------------------------------------------
 
 class OpenLoopRecorder:
-    """逐周期写 CSV，结束时输出两张子图（前后压力、前后流量）。"""
+    """逐周期写 CSV，结束时输出三张子图（前后压力、前后流量、绝对压力比）。"""
 
     def __init__(self, output_directory, *, plot_dpi=160):
         output_dir = Path(output_directory)
@@ -176,6 +183,18 @@ class OpenLoopRecorder:
         if self._closed:
             raise RuntimeError("记录器已关闭")
 
+        p_abs_ratio = self._absolute_pressure_ratio(
+            pressure_before_kpa, pressure_after_kpa
+        )
+        if p_abs_ratio is None:
+            p_abs_ratio_text = ""
+            is_smaller_text = ""
+        else:
+            p_abs_ratio_text = f"{p_abs_ratio:.6f}"
+            is_smaller_text = (
+                "Y" if p_abs_ratio < CRITICAL_PRESSURE_RATIO else "N"
+            )
+
         self._writer.writerow(
             {
                 "time_s": self._fmt(time_s),
@@ -186,6 +205,8 @@ class OpenLoopRecorder:
                 "flow_after_slm": self._fmt(flow_after_slm),
                 "pressure_before_kpa": self._fmt(pressure_before_kpa),
                 "pressure_after_kpa": self._fmt(pressure_after_kpa),
+                "P_abs_ratio": p_abs_ratio_text,
+                "is_ratio_smaller_than_0.528": is_smaller_text,
             }
         )
         self._file.flush()
@@ -195,6 +216,21 @@ class OpenLoopRecorder:
             self.steps.append((float(time_s), float(opening_pct)))
             self._last_step_index = step_index
 
+    @staticmethod
+    def _absolute_pressure_ratio(pressure_before_kpa, pressure_after_kpa):
+        """阀后绝对压力 / 阀前绝对压力；读数缺失或分母非法时返回 None。"""
+        if pressure_before_kpa is None or pressure_after_kpa is None:
+            return None
+        try:
+            before_abs = float(pressure_before_kpa) + ATMOSPHERIC_PRESSURE_KPA
+            after_abs = float(pressure_after_kpa) + ATMOSPHERIC_PRESSURE_KPA
+        except (TypeError, ValueError):
+            return None
+        if before_abs <= 0:
+            return None
+        ratio = after_abs / before_abs
+        return ratio if math.isfinite(ratio) else None
+
     def finalize(self):
         """关闭 CSV 并生成 PNG；无采样点时只保留 CSV。"""
         self.close()
@@ -221,6 +257,7 @@ class OpenLoopRecorder:
         flow_after = []
         pressure_before = []
         pressure_after = []
+        p_abs_ratio = []
 
         with self.csv_path.open("r", newline="", encoding="utf-8-sig") as file:
             for row in csv.DictReader(file):
@@ -233,11 +270,12 @@ class OpenLoopRecorder:
                 pressure_after.append(
                     self._float_or_nan(row["pressure_after_kpa"])
                 )
+                p_abs_ratio.append(self._float_or_nan(row["P_abs_ratio"]))
 
         figure, axes = plt.subplots(
-            2,
+            3,
             1,
-            figsize=(12, 9),
+            figsize=(12, 12),
             sharex=True,
             constrained_layout=True,
         )
@@ -264,10 +302,25 @@ class OpenLoopRecorder:
             "Flow after valve", "#6A1B9A",
         )
         axes[1].set_ylabel("Flow (SLM)")
-        axes[1].set_xlabel("Time (s)")
         axes[1].set_title("Flow across control valve")
         axes[1].legend(loc="best")
 
+        self._plot_series(
+            axes[2], time_s, p_abs_ratio,
+            "P_abs_ratio (after/before)", "#EF6C00",
+        )
+        axes[2].axhline(
+            CRITICAL_PRESSURE_RATIO,
+            color="#B71C1C",
+            linestyle="--",
+            linewidth=1.2,
+            label=f"Critical ratio {CRITICAL_PRESSURE_RATIO:.3f}",
+        )
+        axes[2].set_ylabel("P_abs_ratio")
+        axes[2].set_xlabel("Time (s)")
+        axes[2].set_title("Absolute pressure ratio across control valve")
+        axes[2].legend(loc="best")
+
         if ANNOTATE_STEPS and self.steps:
             y_top = axes[1].get_ylim()[1]
             for step_time, opening in self.steps: