pH cal: correct borate to 9.18, add NIST buffer temp correction

main/eis.c

@@ -656,7 +656,43 @@ float eis_get_ph_offset(void) { return ph_offset_cached; }
 
 /* ---- 3-buffer × 3-temperature pH calibration ---- */
 
-static const float PH_BUFFERS[PH_CAL_BUFFERS] = {4.0f, 6.86f, 9.0f};
+/* nominal 25 C pH of the three NIST primary buffers (phthalate, phosphate, borate) */
+static const float PH_BUFFERS[PH_CAL_BUFFERS] = {4.01f, 6.86f, 9.18f};
+
+/* NIST SRM buffer pH vs temperature, 0-50 C in 5 C steps */
+#define PH_NIST_T_MIN   0
+#define PH_NIST_T_STEP  5
+#define PH_NIST_N      11
+
+static const float PH_NIST_PHTHALATE[PH_NIST_N] = {
+    4.003f, 3.999f, 3.998f, 3.999f, 4.002f, 4.008f,
+    4.015f, 4.024f, 4.035f, 4.047f, 4.060f
+};
+static const float PH_NIST_PHOSPHATE[PH_NIST_N] = {
+    6.984f, 6.951f, 6.923f, 6.900f, 6.881f, 6.865f,
+    6.853f, 6.844f, 6.838f, 6.834f, 6.833f
+};
+static const float PH_NIST_BORATE[PH_NIST_N] = {
+    9.464f, 9.395f, 9.332f, 9.276f, 9.225f, 9.180f,
+    9.139f, 9.102f, 9.068f, 9.038f, 9.011f
+};
+
+static const float * const PH_NIST_TABLES[PH_CAL_BUFFERS] = {
+    PH_NIST_PHTHALATE, PH_NIST_PHOSPHATE, PH_NIST_BORATE
+};
+
+float eis_ph_buffer_at_temp(uint8_t buf, float temp_c)
+{
+    if (buf >= PH_CAL_BUFFERS) return 0.0f;
+    const float *tbl = PH_NIST_TABLES[buf];
+    if (temp_c <= (float)PH_NIST_T_MIN) return tbl[0];
+    float t_max = (float)(PH_NIST_T_MIN + PH_NIST_T_STEP * (PH_NIST_N - 1));
+    if (temp_c >= t_max) return tbl[PH_NIST_N - 1];
+    float f = (temp_c - (float)PH_NIST_T_MIN) / (float)PH_NIST_T_STEP;
+    int i = (int)f;
+    float frac = f - (float)i;
+    return tbl[i] + frac * (tbl[i + 1] - tbl[i]);
+}
 
 #define NVS_PH_CAL_PTS_KEY "ph_cal9"
 
@@ -682,7 +718,7 @@ static void ph_cal_recalculate(void)
         int bit = i * PH_CAL_TEMPS + PH_TEMP_BASE;
         if (!(ph_cal.valid & (1 << bit))) continue;
         float x = ph_cal.s[i][PH_TEMP_BASE].ocp_mv;
-        float y = PH_BUFFERS[i];
+        float y = eis_ph_buffer_at_temp(i, ph_cal.s[i][PH_TEMP_BASE].temp_c);
         sx += x; sy += y; sxx += x * x; sxy += x * y;
         n++;
     }

main/eis.h

@@ -90,5 +90,6 @@ void  eis_ph_cal_clear_all(void);
 bool  eis_ph_cal_get_point(uint8_t buf, uint8_t tslot, float *ocp_mv, float *temp_c);
 int   eis_ph_cal_count(void);
 float eis_ph_cal_buffer_ph(uint8_t buf);
+float eis_ph_buffer_at_temp(uint8_t buf, float temp_c);
 
 #endif

main/eis4.c

@@ -254,8 +254,8 @@ void app_main(void)
             uint8_t bid = cmd.ph_cal_point.buffer_id;
             uint8_t tsl = cmd.ph_cal_point.temp_slot;
             if (bid >= PH_CAL_BUFFERS || tsl >= PH_CAL_TEMPS) break;
-            float buf_ph = eis_ph_cal_buffer_ph(bid);
+            printf("pH cal: buffer %u slot %u (nominal pH %.2f)\n",
-            printf("pH cal: buffer %u slot %u (pH %.2f)\n", bid, tsl, buf_ph);
+                   bid, tsl, eis_ph_cal_buffer_ph(bid));
 
             PhConfig ph_cfg;
             ph_cfg.stabilize_s = cmd.ph_cal_point.stabilize_s;
@@ -266,12 +266,14 @@ void app_main(void)
 
             eis_ph_cal_set_point(bid, tsl, ph_result.v_ocp_mv, ph_result.temp_c);
 
+            float buf_ph = eis_ph_buffer_at_temp(bid, ph_result.temp_c);
+
             int baseline_n = 0;
             for (int i = 0; i < PH_CAL_BUFFERS; i++)
                 if (eis_ph_cal_get_point(i, PH_TEMP_BASE, NULL, NULL)) baseline_n++;
 
-            printf("pH cal: [%u][%u] OCP=%.1f mV T=%.1f C (%d/%d)\n",
+            printf("pH cal: [%u][%u] OCP=%.1f mV T=%.1f C pH=%.3f (%d/%d)\n",
-                   bid, tsl, ph_result.v_ocp_mv, ph_result.temp_c,
+                   bid, tsl, ph_result.v_ocp_mv, ph_result.temp_c, buf_ph,
                    baseline_n, eis_ph_cal_count());
             send_ph_cal_point(bid, tsl, ph_result.v_ocp_mv, ph_result.temp_c,
                               buf_ph, (uint8_t)baseline_n);