diff --git a/cue-ios/CueIOS/AppState.swift b/cue-ios/CueIOS/AppState.swift
index 074d94f..fa6aec9 100644
--- a/cue-ios/CueIOS/AppState.swift
+++ b/cue-ios/CueIOS/AppState.swift
@@ -323,6 +323,9 @@ final class AppState {
             phSlope = Double(slope)
             phOffset = Double(offset)
             status = String(format: "pH cal: slope=%.4f offset=%.4f", slope, offset)
+
+        case .keepalive:
+            break
         }
     }
 
diff --git a/cue-ios/CueIOS/Models/Protocol.swift b/cue-ios/CueIOS/Models/Protocol.swift
index ab4581d..34c3664 100644
--- a/cue-ios/CueIOS/Models/Protocol.swift
+++ b/cue-ios/CueIOS/Models/Protocol.swift
@@ -31,6 +31,7 @@ let RSP_REFS_DONE:    UInt8 = 0x22
 let RSP_REF_STATUS:   UInt8 = 0x23
 let RSP_CL_FACTOR:    UInt8 = 0x24
 let RSP_PH_CAL:       UInt8 = 0x25
+let RSP_KEEPALIVE:    UInt8 = 0x50
 
 // Cue -> ESP32
 let CMD_SET_SWEEP:     UInt8 = 0x10
@@ -131,6 +132,7 @@ enum EisMessage {
     case cellK(Float)
     case clFactor(Float)
     case phCal(slope: Float, offset: Float)
+    case keepalive
 }
 
 // MARK: - Response parser
@@ -271,6 +273,9 @@ func parseSysex(_ data: [UInt8]) -> EisMessage? {
             offset: decodeFloat(p, at: 5)
         )
 
+    case RSP_KEEPALIVE:
+        return .keepalive
+
     default:
         return nil
     }
diff --git a/cue/src/app.rs b/cue/src/app.rs
index 2ba5db2..f1a9a0d 100644
--- a/cue/src/app.rs
+++ b/cue/src/app.rs
@@ -866,6 +866,7 @@ impl App {
                     self.ph_offset = Some(offset);
                     self.status = format!("pH cal: slope={:.4} offset={:.4}", slope, offset);
                 }
+                EisMessage::Keepalive => {}
             },
             Message::TabSelected(t) => {
                 if t == Tab::Browse {
diff --git a/cue/src/protocol.rs b/cue/src/protocol.rs
index 1aa589e..a0c2548 100644
--- a/cue/src/protocol.rs
+++ b/cue/src/protocol.rs
@@ -28,6 +28,7 @@ pub const RSP_CELL_K: u8 = 0x11;
 pub const RSP_REF_STATUS: u8 = 0x23;
 pub const RSP_CL_FACTOR: u8 = 0x24;
 pub const RSP_PH_CAL: u8 = 0x25;
+pub const RSP_KEEPALIVE: u8 = 0x50;
 
 /* Cue → ESP32 */
 pub const CMD_SET_SWEEP: u8 = 0x10;
@@ -266,6 +267,7 @@ pub enum EisMessage {
     CellK(f32),
     ClFactor(f32),
     PhCal { slope: f32, offset: f32 },
+    Keepalive,
 }
 
 fn decode_u16(data: &[u8]) -> u16 {
@@ -432,6 +434,7 @@ pub fn parse_sysex(data: &[u8]) -> Option<EisMessage> {
                 offset: decode_float(&p[5..10]),
             })
         }
+        RSP_KEEPALIVE => Some(EisMessage::Keepalive),
         _ => None,
     }
 }
diff --git a/main/echem.c b/main/echem.c
index cb20334..964029f 100644
--- a/main/echem.c
+++ b/main/echem.c
@@ -327,7 +327,14 @@ int echem_clean(float v_mv, float duration_s)
     AD5940_LPDAC0WriteS(code, VZERO_CODE);
 
     printf("Clean: %.0f mV for %.0f s\n", v_mv, duration_s);
-    vTaskDelay(pdMS_TO_TICKS((uint32_t)(duration_s * 1000.0f)));
+
+    uint32_t remain_ms = (uint32_t)(duration_s * 1000.0f);
+    while (remain_ms > 0) {
+        uint32_t chunk = remain_ms > 3000 ? 3000 : remain_ms;
+        vTaskDelay(pdMS_TO_TICKS(chunk));
+        remain_ms -= chunk;
+        if (remain_ms > 0) send_keepalive();
+    }
 
     echem_shutdown_lp();
     AD5940_AFECtrlS(AFECTRL_ALL, bFALSE);
@@ -497,7 +504,15 @@ static uint32_t sample_phase(float v_mv, float t_dep_ms, float t_meas_ms,
     AD5940_LPDAC0WriteS(mv_to_vbias_code(v_mv), VZERO_CODE);
 
     /* settling — no samples recorded */
-    vTaskDelay(pdMS_TO_TICKS((uint32_t)t_dep_ms));
+    {
+        uint32_t remain_ms = (uint32_t)t_dep_ms;
+        while (remain_ms > 0) {
+            uint32_t chunk = remain_ms > 3000 ? 3000 : remain_ms;
+            vTaskDelay(pdMS_TO_TICKS(chunk));
+            remain_ms -= chunk;
+            if (remain_ms > 0) send_keepalive();
+        }
+    }
 
     /* measurement — sample at ~50ms intervals */
     uint32_t n_samples = (uint32_t)(t_meas_ms / 50.0f + 0.5f);
@@ -541,7 +556,15 @@ int echem_chlorine(const ClConfig *cfg, ClPoint *out, uint32_t max_points,
 
     printf("Cl: conditioning at %.0f mV for %.0f ms\n", cfg->v_cond, cfg->t_cond_ms);
     AD5940_LPDAC0WriteS(mv_to_vbias_code(cfg->v_cond), VZERO_CODE);
-    vTaskDelay(pdMS_TO_TICKS((uint32_t)cfg->t_cond_ms));
+    {
+        uint32_t remain_ms = (uint32_t)cfg->t_cond_ms;
+        while (remain_ms > 0) {
+            uint32_t chunk = remain_ms > 3000 ? 3000 : remain_ms;
+            vTaskDelay(pdMS_TO_TICKS(chunk));
+            remain_ms -= chunk;
+            if (remain_ms > 0) send_keepalive();
+        }
+    }
 
     printf("Cl: free chlorine at %.0f mV\n", cfg->v_free);
     idx = sample_phase(cfg->v_free, cfg->t_dep_ms, cfg->t_meas_ms,
@@ -579,7 +602,14 @@ int echem_ph_ocp(const PhConfig *cfg, PhResult *result)
     AD5940_ADCBaseCfgS(&adc);
 
     printf("pH: stabilizing %0.f s\n", cfg->stabilize_s);
-    vTaskDelay(pdMS_TO_TICKS((uint32_t)(cfg->stabilize_s * 1000.0f)));
+
+    uint32_t remain_ms = (uint32_t)(cfg->stabilize_s * 1000.0f);
+    while (remain_ms > 0) {
+        uint32_t chunk = remain_ms > 3000 ? 3000 : remain_ms;
+        vTaskDelay(pdMS_TO_TICKS(chunk));
+        remain_ms -= chunk;
+        if (remain_ms > 0) send_keepalive();
+    }
 
     /* average N readings of V(SE0) and V(RE0) */
     #define PH_AVG_N 10
diff --git a/main/protocol.c b/main/protocol.c
index bf81691..8e53997 100644
--- a/main/protocol.c
+++ b/main/protocol.c
@@ -144,6 +144,14 @@ static int send_sysex(const uint8_t *sysex, uint16_t len)
     return wifi_send_sysex(sysex, len);
 }
 
+/* ---- outbound: keepalive ---- */
+
+int send_keepalive(void)
+{
+    uint8_t sx[] = { 0xF0, 0x7D, RSP_KEEPALIVE, 0xF7 };
+    return send_sysex(sx, sizeof(sx));
+}
+
 /* ---- outbound: EIS ---- */
 
 int send_sweep_start(uint32_t num_points, float freq_start, float freq_stop)
diff --git a/main/protocol.h b/main/protocol.h
index fa57d3c..77dece0 100644
--- a/main/protocol.h
+++ b/main/protocol.h
@@ -62,6 +62,7 @@
 #define RSP_REF_STATUS  0x23
 #define RSP_CL_FACTOR   0x24
 #define RSP_PH_CAL      0x25
+#define RSP_KEEPALIVE   0x50
 
 /* Session sync responses (0x4x) */
 #define RSP_SESSION_CREATED  0x40
@@ -152,6 +153,9 @@ int send_ref_lp_range(uint8_t mode, uint8_t low_idx, uint8_t high_idx);
 int send_refs_done(void);
 int send_ref_status(uint8_t has_refs);
 
+/* keepalive (sent during long blocking ops) */
+int send_keepalive(void);
+
 /* session management */
 const Session *session_get_all(uint8_t *count);
 uint8_t session_get_current(void);
diff --git a/main/wifi_transport.c b/main/wifi_transport.c
index 277e995..2df1eb0 100644
--- a/main/wifi_transport.c
+++ b/main/wifi_transport.c
@@ -18,7 +18,9 @@
 
 #define UDP_PORT           5941
 #define UDP_BUF_SIZE       128
-#define MAX_UDP_CLIENTS    10
+#define UDP_CLIENTS_MAX    16
+#define CLIENT_TIMEOUT_MS  30000
+
 static int udp_sock = -1;
 static esp_netif_t *ap_netif;
 
@@ -26,7 +28,7 @@ static struct {
     struct sockaddr_in addr;
     uint8_t mac[6];
     bool active;
-} clients[MAX_UDP_CLIENTS];
+} clients[UDP_CLIENTS_MAX];
 
 static int client_count;
 
@@ -38,7 +40,7 @@ static void client_touch(const struct sockaddr_in *addr)
             return;
     }
 
-    if (client_count < MAX_UDP_CLIENTS) {
+    if (client_count < UDP_CLIENTS_MAX) {
         clients[client_count].addr = *addr;
         clients[client_count].active = true;
         memset(clients[client_count].mac, 0, 6);
@@ -56,7 +58,7 @@ static void client_touch(const struct sockaddr_in *addr)
         }
 
         client_count++;
-        printf("UDP: client added (%d/%d)\n", client_count, MAX_UDP_CLIENTS);
+        printf("UDP: client added (%d)\n", client_count);
     }
 }
 
@@ -65,7 +67,7 @@ static void client_remove_by_mac(const uint8_t *mac)
     for (int i = 0; i < client_count; ) {
         if (memcmp(clients[i].mac, mac, 6) == 0) {
             clients[i] = clients[--client_count];
-            printf("UDP: client removed by MAC (%d/%d)\n", client_count, MAX_UDP_CLIENTS);
+            printf("UDP: client removed by MAC (%d)\n", client_count);
         } else {
             i++;
         }