diff --git a/Software/MainBoard/rust/Cargo.lock b/Software/MainBoard/rust/Cargo.lock
index cfaea11..098ebf3 100644
--- a/Software/MainBoard/rust/Cargo.lock
+++ b/Software/MainBoard/rust/Cargo.lock
@@ -1901,6 +1901,7 @@ dependencies = [
  "chrono",
  "chrono-tz",
  "crc",
+ "critical-section",
  "deranged",
  "ds323x",
  "edge-dhcp",
diff --git a/Software/MainBoard/rust/Cargo.toml b/Software/MainBoard/rust/Cargo.toml
index bd3cbeb..27bf35e 100644
--- a/Software/MainBoard/rust/Cargo.toml
+++ b/Software/MainBoard/rust/Cargo.toml
@@ -101,6 +101,7 @@ chrono-tz = { version = "0.10.4", default-features = false, features = ["filter-
 heapless = { version = "0.7.17", features = ["serde"] } # stay in sync with mcutie version
 static_cell = "2.1.1"
 portable-atomic = "1.11.1"
+critical-section = "1"
 crc = "3.3.0"
 bytemuck = { version = "1.24.0", features = ["derive", "min_const_generics", "pod_saturating", "extern_crate_alloc"] }
 deranged = "0.5.5"
diff --git a/Software/MainBoard/rust/src/hal/water.rs b/Software/MainBoard/rust/src/hal/water.rs
index cdc642d..44bec81 100644
--- a/Software/MainBoard/rust/src/hal/water.rs
+++ b/Software/MainBoard/rust/src/hal/water.rs
@@ -1,8 +1,6 @@
 use crate::bail;
 use crate::fat_error::FatError;
 use crate::hal::{ADC1, TANK_MULTI_SAMPLE};
-use core::cell::RefCell;
-use embassy_sync::blocking_mutex::CriticalSectionMutex;
 use embassy_time::Timer;
 use esp_hal::analog::adc::{Adc, AdcCalLine, AdcConfig, AdcPin, Attenuation};
 use esp_hal::delay::Delay;
@@ -19,14 +17,13 @@ use portable_atomic::{AtomicUsize, Ordering};
 unsafe impl Send for TankSensor<'_> {}
 
 static FLOW_OVERFLOW_COUNTER: AtomicUsize = AtomicUsize::new(0);
-static FLOW_UNIT: CriticalSectionMutex<RefCell<Option<Unit<'static, 1>>>> =
-    CriticalSectionMutex::new(RefCell::new(None));
 
 pub struct TankSensor<'a> {
     one_wire_bus: OneWire<Flex<'a>>,
     tank_channel: Adc<'a, ADC1<'a>, Async>,
     tank_power: Output<'a>,
     tank_pin: AdcPin<GPIO5<'a>, ADC1<'a>, AdcCalLine<ADC1<'a>>>,
+    flow_unit: Unit<'static, 1>,
 }
 
 impl<'a> TankSensor<'a> {
@@ -64,61 +61,47 @@ impl<'a> TankSensor<'a> {
 
         pcnt1.listen();
 
-        FLOW_UNIT.lock(|refcell| {
-            refcell.borrow_mut().replace(pcnt1);
-        });
-
         Ok(TankSensor {
             one_wire_bus,
             tank_channel,
             tank_power,
             tank_pin,
+            flow_unit: pcnt1,
         })
     }
 
     pub fn reset_flow_meter(&mut self) {
-        FLOW_OVERFLOW_COUNTER.store(0, Ordering::SeqCst);
-        FLOW_UNIT.lock(|refcell| {
-            if let Some(unit) = refcell.borrow_mut().as_mut() {
-                unit.pause();
-                unit.clear();
-            }
+        // Pause, clear counter, clear any pending interrupt, then reset the overflow counter —
+        // all inside a single critical section to prevent a race where the interrupt fires
+        // between the overflow reset and the pause.
+        critical_section::with(|_| {
+            self.flow_unit.pause();
+            self.flow_unit.clear();
+            self.flow_unit.reset_interrupt();
+            FLOW_OVERFLOW_COUNTER.store(0, Ordering::SeqCst);
         });
     }
 
     pub fn start_flow_meter(&mut self) {
-        FLOW_UNIT.lock(|refcell| {
-            if let Some(unit) = refcell.borrow_mut().as_mut() {
-                unit.resume();
-            }
-        });
-    }
-
-    fn get_flow_meter_value(&mut self) -> i16 {
-        FLOW_UNIT.lock(|refcell| {
-            refcell.borrow_mut().as_mut().map_or(0, |unit| unit.value())
-        })
+        self.flow_unit.resume();
     }
 
     pub fn stop_flow_meter(&mut self) -> i16 {
-        FLOW_UNIT.lock(|refcell| {
-            let mut borrowed = refcell.borrow_mut();
-            if let Some(unit) = borrowed.as_mut() {
-                let val = unit.value();
-                unit.pause();
-                val
-            } else {
-                0
-            }
+        critical_section::with(|_| {
+            let val = self.flow_unit.value();
+            self.flow_unit.pause();
+            val
         })
     }
 
     pub fn get_full_flow_count(&self) -> u32 {
-        let current = FLOW_UNIT.lock(|refcell| {
-            refcell.borrow().as_ref().map_or(0, |unit| unit.value() as u32)
-        });
-        let overflowed = FLOW_OVERFLOW_COUNTER.load(Ordering::SeqCst) as u32;
-        overflowed * (i16::MAX.wrapping_add(1) as u32) + current
+        // Read both values inside a single critical section so an overflow interrupt cannot
+        // fire between the two reads and produce an inconsistent result.
+        critical_section::with(|_| {
+            let overflowed = FLOW_OVERFLOW_COUNTER.load(Ordering::SeqCst) as u32;
+            let current = self.flow_unit.value() as u32;
+            overflowed * (i16::MAX as u32 + 1) + current
+        })
     }
 
     pub async fn water_temperature_c(&mut self) -> Result<f32, FatError> {
@@ -218,15 +201,12 @@ impl<'a> TankSensor<'a> {
 
 #[esp_hal::handler]
 pub fn flow_interrupt_handler() {
-    FLOW_UNIT.lock(|refcell| {
-        if let Some(unit) = refcell.borrow_mut().as_mut() {
-            if unit.interrupt_is_set() {
-                let events = unit.events();
-                if events.high_limit {
-                    FLOW_OVERFLOW_COUNTER.fetch_add(1, Ordering::SeqCst);
-                }
-                unit.reset_interrupt();
-            }
+    use esp_hal::peripherals::PCNT;
+    let pcnt = PCNT::regs();
+    if pcnt.int_raw().read().cnt_thr_event_u(1).bit() {
+        if pcnt.u_status(1).read().h_lim().bit() {
+            FLOW_OVERFLOW_COUNTER.fetch_add(1, Ordering::SeqCst);
         }
-    });
+        pcnt.int_clr().write(|w| w.cnt_thr_event_u(1).set_bit());
+    }
 }