diff --git a/ModbusThermometer.cpp b/ModbusThermometer.cpp
index 3ff9fe2..e63a819 100644
--- a/ModbusThermometer.cpp
+++ b/ModbusThermometer.cpp
@@ -135,12 +135,16 @@ void loop() {
 
       switch (calibrationState) {
         case e_CAL_IDLE:
+          for (uint8_t i = 0; i < NUM_OF_CHANNELS; i++) {
+            ads1210[i].setCalOffset(0.0);
+            ads1210[i].setCalFactor(1.0);
+          }
           calibrationState = e_CAL_RUNNING;
           break;
         case e_CAL_RUNNING:
           calibrationCycleCnt++;
           for (uint8_t i = 0; i < NUM_OF_CHANNELS; i++) {
-            calibrationValueSum[i] += ads1210[i].getRRaw();
+            calibrationValueSum[i] += ads1210[i].getR();
           }
           if (calibrationCycleCnt >= CALIBRATION_CYCLES) {
             calibrationState = e_CAL_SET;
diff --git a/ads1210.cpp b/ads1210.cpp
index d1278e6..6f54d23 100644
--- a/ads1210.cpp
+++ b/ads1210.cpp
@@ -94,7 +94,7 @@ void ADS1210::exec() {
 		
 		m_u = (((float)m_value) / ((float)vMax)) * U_REF;
 		m_rRaw = ((((float)vMax) / ((float)m_value)) - 1.0) * R_REF;
-		m_r = (m_rRaw + m_calOffset) * m_calFactor;
+		m_r = (m_rRaw - m_calOffset) * m_calFactor;
 	}
 }