diff --git a/lambda/lambda_function.py b/lambda/lambda_function.py
index 1755798..15ecce0 100644
--- a/lambda/lambda_function.py
+++ b/lambda/lambda_function.py
@@ -8,7 +8,7 @@
 SUPABASE_KEY = os.environ.get("SUPABASE_KEY")
 
 supabase: Client = create_client(SUPABASE_URL, SUPABASE_KEY)
-PAST_RECORDS_DURATION = 5 # in minutes 
+PAST_RECORDS_DURATION = 13 # in minutes TODO: change such that we can sample >= 25 past records (e.g. sampling interval * 25)
 
 def lambda_handler(event, context):
     rowId = event.get('rowId')
@@ -37,13 +37,24 @@ def lambda_handler(event, context):
     
     r_value = None
     fire_probability = 0
-    if len(temp_arr) != 0 or len(humidity_arr) != 0:
+    
+    if len(temp_arr) >= 0 and len(humidity_arr) > 0:
         r_value = get_r_value(temp_arr, humidity_arr)
+    
+    # if less than 25 records, dont calculate r value and return default fire probability
+    else:
+        return json.dumps({
+        'statusCode': 200,
+        'body': json.dumps({
+            'fire_probability': fire_probability,
+            'r_value': r_value,
+        })
+    })
 
     if len(aq_arr) != 0 and r_value != None:
         fire_probability = get_fire_probability(temp, aq_arr, flame, r_value)
     
-    #update row in supabase table with r_value and fire_probability
+    # TODO: update row in supabase table with r_value and fire_probability
     try:
         data, count = supabase.table('firecloud') \
                                 .update({
diff --git a/node.py b/node.py
index 4a75fa8..64c3e3d 100644
--- a/node.py
+++ b/node.py
@@ -18,7 +18,7 @@
 _DEVICE_NAME = _DEVICE_NAME_PREFIX + str(_NODE_ID)
 
 # Sampling intervals
-_SAMPLING_INTERVAL_LOW = 60 # change to 30s in demo
+_SAMPLING_INTERVAL_LOW = 30 # Actual: 60s, Demo: 30s
 _SAMPLING_INTERVAL_HIGH = 5
 
 # Frequencies
diff --git a/sensors/lib/mq135.py b/sensors/lib/mq135.py
index 730aa7e..94f68c3 100644
--- a/sensors/lib/mq135.py
+++ b/sensors/lib/mq135.py
@@ -18,7 +18,6 @@ class MQ135(object):
     RLOAD = 10.0
     # Calibration resistance at atmospheric CO2 level
     RZERO = 76.63
-    RZERO = 10
     # Parameters for calculating ppm of CO2 from sensor resistance
     PARA = 116.6020682
     PARB = 2.769034857
@@ -73,8 +72,7 @@ def get_corrected_ppm(self, temperature, humidity):
         corrected for temperature/humidity"""
         corrected_rzero = self.get_corrected_rzero(temperature,humidity)
         corrected_resistance = self.get_corrected_resistance(temperature, humidity)
-#         return self.PARA * math.pow((self.get_corrected_resistance(temperature, humidity)/ self.RZERO), -self.PARB)
-        return round(self.PARA * math.pow((corrected_resistance/self.RZERO), -self.PARB),3)
+        return self.PARA * math.pow((self.get_corrected_resistance(temperature, humidity)/ self.RZERO), -self.PARB)
 
     def get_rzero(self):
         """Returns the resistance RZero of the sensor (in kOhms) for calibratioin purposes"""