succd: eke out a few extra bits of precision by oversampling ADC

succd/http.go

@@ -22,7 +22,7 @@ var (
 )
 
 func formatVolts(v float32) string {
-	return fmt.Sprintf("%.3f V", v)
+	return fmt.Sprintf("%.4f V", v)
 }
 
 // formatMbar formats a millibar value using scientific notation and returns a

succd/main.go

@@ -23,8 +23,9 @@ type daemon struct {
 
 	// mu guards state variables below.
 	mu sync.RWMutex
-	// adcPiraniVolts is the last readout of adcPirani.
-	adcPiraniVolts float32
+	// adcPiraniVolts is a moving window of read ADC values, used to calculate a
+	// moving average.
+	adcPiraniVolts []float32
 }
 
 // process runs the pain acquisition and control loop of succd.
@@ -55,7 +56,11 @@ func (d *daemon) processOnce(_ context.Context) error {
 		return fmt.Errorf("when reading ADC: %w", err)
 	}
 	d.mu.Lock()
-	d.adcPiraniVolts = v
+	d.adcPiraniVolts = append(d.adcPiraniVolts, v)
+	trim := len(d.adcPiraniVolts) - 100
+	if trim > 0 {
+		d.adcPiraniVolts = d.adcPiraniVolts[trim:]
+	}
 	d.mu.Unlock()
 
 	return nil
@@ -64,7 +69,13 @@ func (d *daemon) processOnce(_ context.Context) error {
 // pirani returns the Pirani gauge voltage and pressure.
 func (d *daemon) pirani() (volts float32, mbar float32) {
 	d.mu.RLock()
-	volts = d.adcPiraniVolts
+	volts = 0.0
+	for _, v := range d.adcPiraniVolts {
+		volts += v
+	}
+	if len(d.adcPiraniVolts) != 0 {
+		volts /= float32(len(d.adcPiraniVolts))
+	}
 	d.mu.RUnlock()
 
 	// Per Pirani probe docs.