B1.3: isStable real threshold (config-driven, replaces tautology)

The legacy stdDev < stdDev*2 was always true. New behaviour: stdDev <=
config.calibration.stabilityThreshold OR stdDev === 0. Default
threshold 0.01 in scaling-units. Schema field + editor UI added. 4
BUG-PRESERVED tests rewritten + 4 new edge tests. 101/101 pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

measurement.html

@@ -34,6 +34,7 @@
       simulator: { value: false },
       smooth_method: { value: "" },
       count: { value: "10", required: true },
+      stabilityThreshold: { value: 0.01 },
       processOutputFormat: { value: "process" },
       dbaseOutputFormat: { value: "influxdb" },
 
@@ -227,6 +228,12 @@
         (field) => (node[field] = parseFloat(document.getElementById(`node-input-${field}`).value) || 0)
       );
 
+      // Calibration stability threshold: 0 is a valid (very strict) value, so
+      // fall back to the default 0.01 only when the field is empty / NaN.
+      const stRaw = document.getElementById('node-input-stabilityThreshold').value;
+      const stParsed = parseFloat(stRaw);
+      node.stabilityThreshold = Number.isFinite(stParsed) ? stParsed : 0.01;
+
       // Mode-dependent validation. In digital mode we don't care about
       // scaling completeness (the channels have their own per-channel
       // scaling); in analog mode we still warn about half-filled ranges.
@@ -329,6 +336,14 @@
       <input type="number" id="node-input-count" placeholder="10" style="width:60px;"/>
       <div class="form-tips">Number of samples for smoothing</div>
     </div>
+
+    <!-- Calibration Stability Threshold -->
+    <div class="form-row">
+      <label for="node-input-stabilityThreshold"><i class="fa fa-balance-scale"></i> Stability Threshold</label>
+      <input type="number" id="node-input-stabilityThreshold" placeholder="0.01" step="any" style="width:100px;"/>
+      <span style="margin-left:6px; color:#666;">(scaling-units)</span>
+      <div class="form-tips">Maximum stdDev of the rolling window for calibrate() and evaluateRepeatability() to accept the buffer as stable. Default 0.01.</div>
+    </div>
   </div>
 
   <hr>

src/calibration/calibrator.js

@@ -2,7 +2,7 @@
 
 const { stats } = require('generalFunctions');
 
-const MARGIN_FACTOR = 2;
+const DEFAULT_STABILITY_THRESHOLD = 0.01;
 
 /**
  * Calibration helper extracted from measurement/specificClass.js.
@@ -23,8 +23,9 @@ class Calibrator {
 
   /**
    * Decide whether the rolling window is stable enough to trust.
-   * Mirrors the original threshold check; with `stdDev=0` (constant input)
-   * the comparison short-circuits to true.
+   * Compares the window's stdDev against config.calibration.stabilityThreshold
+   * (absolute, in scaling-units). A constant buffer (stdDev=0) is always
+   * stable regardless of threshold.
    */
   isStable() {
     const values = this._storedValues();
@@ -32,8 +33,12 @@ class Calibrator {
       return { isStable: false, stdDev: 0 };
     }
     const stdDev = stats.stdDev(values);
-    const stableThreshold = stdDev * MARGIN_FACTOR;
-    return { isStable: stdDev < stableThreshold || stdDev === 0, stdDev };
+    const cfg = this._config();
+    const raw = cfg && cfg.calibration && cfg.calibration.stabilityThreshold;
+    const threshold = Number.isFinite(Number(raw)) && Number(raw) >= 0
+      ? Number(raw)
+      : DEFAULT_STABILITY_THRESHOLD;
+    return { isStable: stdDev === 0 || stdDev <= threshold, stdDev };
   }
 
   /**

src/nodeClass.js

@@ -33,6 +33,7 @@ class nodeClass extends BaseNodeAdapter {
       },
       smoothing: { smoothWindow: uiConfig.count, smoothMethod: uiConfig.smooth_method },
       simulation: { enabled: uiConfig.simulator },
+      calibration: { stabilityThreshold: uiConfig.stabilityThreshold },
       mode: { current: mode },
       channels,
     };

test/basic/calibrator.basic.test.js

@@ -34,17 +34,50 @@ test('isStable: constant array → stable with stdDev=0', () => {
   assert.strictEqual(r.stdDev, 0);
 });
 
-test('isStable: high-variance array → original threshold is tautological (preserved)', () => {
-  // BUG-PRESERVED: original check is `stdDev < stdDev*marginFactor`, which is
-  // always true for stdDev>0. Length>=2 ⇒ isStable=true regardless of spread.
-  // See calibrator stdDev-threshold note. We pin the behaviour here so the
-  // refactor stays byte-equivalent; a separate behavioural PR can fix the rule.
+test('isStable: high-variance array under default threshold → unstable', () => {
+  // Resolved 2026-05-11: config-driven absolute stabilityThreshold replaces
+  // the old `stdDev < stdDev*marginFactor` tautology. Default threshold is
+  // 0.01 (scaling-units); a 0..100 spread blows past it.
   const { cal } = makeCalibrator([0, 100, 0, 100], {});
   const r = cal.isStable();
+  assert.strictEqual(r.isStable, false);
+  assert.ok(r.stdDev > 0);
+});
+
+test('isStable: high-variance array with relaxed threshold → stable', () => {
+  const cfg = { calibration: { stabilityThreshold: 100 } };
+  const { cal } = makeCalibrator([0, 100, 0, 100], cfg);
+  const r = cal.isStable();
   assert.strictEqual(r.isStable, true);
   assert.ok(r.stdDev > 0);
 });
 
+test('isStable: zero stdDev (constant) is stable regardless of threshold', () => {
+  const cfg = { calibration: { stabilityThreshold: 0 } };
+  const { cal } = makeCalibrator([7, 7, 7, 7], cfg);
+  const r = cal.isStable();
+  assert.strictEqual(r.isStable, true);
+  assert.strictEqual(r.stdDev, 0);
+});
+
+test('isStable: stdDev just above threshold → unstable', () => {
+  const cfg = { calibration: { stabilityThreshold: 0.5 } };
+  // stdDev of [10, 11] = 0.5; nudge the spread up so stdDev > 0.5.
+  const { cal } = makeCalibrator([10, 12], cfg);
+  const r = cal.isStable();
+  assert.strictEqual(r.isStable, false);
+  assert.ok(r.stdDev > 0.5);
+});
+
+test('isStable: missing config.calibration → falls back to default 0.01', () => {
+  // stdDev of [10, 10.001] ≈ 0.0005, well under the 0.01 default.
+  const { cal: stable } = makeCalibrator([10, 10.001], {});
+  assert.strictEqual(stable.isStable().isStable, true);
+  // stdDev of [10, 10.1] ≈ 0.05, above the 0.01 default.
+  const { cal: unstable } = makeCalibrator([10, 10.1], {});
+  assert.strictEqual(unstable.isStable().isStable, false);
+});
+
 test('isStable: < 2 values → unstable', () => {
   const { cal } = makeCalibrator([42], {});
   const r = cal.isStable();
@@ -101,11 +134,22 @@ test('evaluateRepeatability: insufficient data → null', () => {
   assert.strictEqual(r.reason, 'insufficient-data');
 });
 
-test('evaluateRepeatability: high-variance still returns stdDev (preserved tautology)', () => {
-  // BUG-PRESERVED: see isStable note. Original rule treats any length>=2
-  // buffer as stable, so repeatability returns the raw stdDev even when the
-  // spread is large.
+test('evaluateRepeatability: high-variance under default threshold → null', () => {
+  // Resolved 2026-05-11: with the real stability check in place, a noisy
+  // buffer fails isStable() and repeatability reports null with reason.
   const cfg = { smoothing: { smoothMethod: 'mean' } };
+  const { cal, logger } = makeCalibrator([0, 50, 0, 50], cfg);
+  const r = cal.evaluateRepeatability();
+  assert.strictEqual(r.repeatability, null);
+  assert.strictEqual(r.reason, 'unstable');
+  assert.match(logger.calls.warn[0], /not stable/);
+});
+
+test('evaluateRepeatability: high-variance with relaxed threshold → returns stdDev', () => {
+  const cfg = {
+    smoothing: { smoothMethod: 'mean' },
+    calibration: { stabilityThreshold: 100 },
+  };
   const { cal } = makeCalibrator([0, 50, 0, 50], cfg);
   const r = cal.evaluateRepeatability();
   assert.ok(r.repeatability > 0);