message: Add debug var for tidal volume.
  GPC: tv-debug-var
diff: @@ -31,13 +31,14 @@ static DebugVar d2("dp_exhale", &dp_exhale, "Exhale diff pressure, cm/h2O",
  "%.4f");
  static DebugVar d3("pressure", &pressure, "Patient pressure, cm/h2O", "%.4f");
  -static float flow_inhale, flow_exhale, flow_delta;
  +static float flow_inhale, flow_exhale, flow_delta, volume;
  static DebugVar d4("flow_inhale", &flow_inhale, "Inhale flow rate, cc/sec",
  "%.3f");
  static DebugVar d5("flow_exhale", &flow_exhale, "Exhale flow rate, cc/sec",
  "%.3f");
  static DebugVar d6("flow_delta", &flow_delta,
  "Inhale - Exhale flow rate, cc/sec", "%.3f");
  +static DebugVar d7("volume", &volume, "Tidal volume, ml", "%0.3f");
  //@TODO: Potential Caution: Density of air slightly varies over temperature and
  // altitude - need mechanism to adjust based on delivery? Constant involving
  @@ -161,12 +162,13 @@ SensorReadings Sensors::GetSensorReadings() {
  VolumetricFlow inflow = PressureDeltaToFlow(inflow_delta);
  VolumetricFlow outflow = PressureDeltaToFlow(outflow_delta);
  VolumetricFlow flow = inflow - outflow;
  + tv_integrator_.AddFlow(Hal.now(), flow);
  flow_inhale = inflow.ml_per_sec();
  flow_exhale = outflow.ml_per_sec();
  flow_delta = flow.ml_per_sec();
  + volume = tv_integrator_.GetTV().ml();
  - tv_integrator_.AddFlow(Hal.now(), flow);
  return {
  .patient_pressure_cm_h2o = patient_pressure.cmH2O(),
  .volume_ml = tv_integrator_.GetTV().ml(),
  