diff --git a/custom_field_list.json b/custom_field_list.json
index b7da8d7..df761b7 100644
--- a/custom_field_list.json
+++ b/custom_field_list.json
@@ -58,13 +58,13 @@
 },
 "ON4IR": {
     "comment": "ON4IR - UBA/IRM balloon custom fields",
-    "struct": "<HHBHHH",
+    "struct": "<hhBHH",
     "fields": [
-        ["geiger_cpm", "none"],
-        ["ext_temperature", "divide_by_10"],
+        ["ascent_rate", "divide_by_100"],
+	["ext_temperature", "divide_by_10"],
         ["ext_humidity", "none"],
         ["ext_pressure", "divide_by_10"],
         ["pulse_counts","none"]
     ]
 }
-}
\ No newline at end of file
+}