diff --git a/src/transformers/data_transformer.py b/src/transformers/data_transformer.py
index dfc6294..3b10045 100644
--- a/src/transformers/data_transformer.py
+++ b/src/transformers/data_transformer.py
@@ -113,9 +113,20 @@ class DataTransformer:
         Returns:
             Measurement dictionary for this node with state, calc_err, and measurement fields
         """
+        from decimal import Decimal
+
+        # Helper to convert MySQL Decimal to float for JSON serialization
+        def convert_value(val):
+            if isinstance(val, Decimal):
+                return float(val)
+            elif isinstance(val, str):
+                return float(val)
+            else:
+                return val
+
         measurement = {
             "state": mysql_row.get("State"),
-            "calc_err": mysql_row.get("calcerr", 0),
+            "calc_err": convert_value(mysql_row.get("calcerr", 0)),
         }
 
         # Create nested structure for measurement categories
@@ -131,7 +142,7 @@ class DataTransformer:
         for mysql_col, (category, pg_key) in ELABDATADISP_FIELD_MAPPING.items():
             value = mysql_row.get(mysql_col)
             if value is not None:
-                categories[category][pg_key] = float(value) if isinstance(value, str) else value
+                categories[category][pg_key] = convert_value(value)
 
         # Merge categories into measurement, keeping only non-empty categories
         for category, values in categories.items():