diff --git a/pyglider/seaexplorer.py b/pyglider/seaexplorer.py
index 2c98c41..1ede7b1 100644
--- a/pyglider/seaexplorer.py
+++ b/pyglider/seaexplorer.py
@@ -287,6 +287,20 @@ def _interp_pld_to_pld(pld, ds, val, indctd):
     return val
 
 
+def _remove_fill_values(df, fill_value=9999):
+    """
+    For input dataframe df, this function converts all Float values equaling fill_values to null. Columns of other
+    datatypes are not affected.
+    """
+    df = df.with_columns(
+        pl.when(pl.col(pl.Float64) == fill_value)
+        .then(None)
+        .otherwise(pl.col(pl.Float64))
+        .keep_name()
+    )
+    return df
+
+
 def raw_to_timeseries(indir, outdir, deploymentyaml, kind='raw',
                       profile_filt_time=100, profile_min_time=300):
     """
@@ -303,6 +317,7 @@ def raw_to_timeseries(indir, outdir, deploymentyaml, kind='raw',
     gli = pl.read_parquet(f'{indir}/{id}-rawgli.parquet')
     _log.info(f'Opening combined payload file {indir}/{id}-{kind}pld.parquet')
     sensor = pl.read_parquet(f'{indir}/{id}-{kind}pld.parquet')
+    sensor = _remove_fill_values(sensor)
 
     # build a new data set based on info in `deploymentyaml.`
     # We will use ctd as the interpolant
diff --git a/tests/test_seaexplorer.py b/tests/test_seaexplorer.py
index d0ed977..6b7c866 100644
--- a/tests/test_seaexplorer.py
+++ b/tests/test_seaexplorer.py
@@ -1,4 +1,5 @@
 import polars as pl
+import numpy as np
 import pytest
 from pathlib import Path
 import os
@@ -63,6 +64,14 @@ def test_merge_rawnc():
             kind='sub')
     assert result_default is False
     assert result_sub is True
+    
+    
+def test__remove_fill_values():
+    # This should convert values equallling 9999 in the original df to nan
+    df_in = pl.read_parquet('tests/data/realtime_rawnc/sea035.0012.pld1.sub.0036.parquet')
+    df_out = seaexplorer._remove_fill_values(df_in)
+    assert (df_in.select("GPCTD_DOF").to_numpy()[:, 0] == 9999).all()
+    assert np.isnan(df_out.select("GPCTD_DOF").to_numpy()[:, 0]).all()
 
 
 def test__interp_gli_to_pld():