diff --git a/odml/tools/format_converter.py b/odml/tools/format_converter.py
index 02ffb2d..3ae8d91 100644
--- a/odml/tools/format_converter.py
+++ b/odml/tools/format_converter.py
@@ -6,6 +6,7 @@ import sys
 import odml
 from .rdf_converter import RDFWriter
 from .version_converter import VersionConverter
+from .utils import ConversionFormats
 
 try:
     unicode = unicode
@@ -15,23 +16,6 @@ except NameError:
 
 class FormatConverter(object):
 
-    _conversion_formats = {
-        'v1_1': '.xml',
-        'odml': '.odml',
-        # rdflib version "4.2.2" serialization formats
-        'xml': '.rdf',
-        'pretty-xml': '.rdf',
-        'trix': '.rdf',
-        'n3': '.n3',
-        'turtle': '.ttl',
-        'ttl': '.ttl',
-        'ntriples': '.nt',
-        'nt': '.nt',
-        'nt11': '.nt',
-        'trig': '.trig',
-        'json-ld': '.jsonld'
-    }
-
     @classmethod
     def convert(cls, args=None):
         """
@@ -50,7 +34,7 @@ class FormatConverter(object):
         """
         parser = argparse.ArgumentParser(description="Convert directory with odml files to another format")
         parser.add_argument("input_dir", help="Path to input directory")
-        parser.add_argument("result_format", choices=list(cls._conversion_formats),
+        parser.add_argument("result_format", choices=list(ConversionFormats),
                             help="Format of output files")
         parser.add_argument("-out", "--output_dir", help="Path for output directory")
         parser.add_argument("-r", "--recursive", action="store_true",
@@ -70,11 +54,11 @@ class FormatConverter(object):
                            Possible choices: "v1_1" (converts to version 1.1 from version 1 xml)
                                              "odml" (converts to .odml from version 1.1 .xml files)
                                              "turtle", "nt" etc. (converts to rdf formats from version 1.1 .odml files)
-                                             (see full list of rdf serializers in FormatConverter._conversion_formats)
+                                             (see full list of rdf serializers in utils.ConversionFormats)
         """
-        if res_format not in cls._conversion_formats:
+        if res_format not in ConversionFormats:
             raise ValueError("Format for output files is incorrect. "
-                             "Please choose from the list: {}".format(cls._conversion_formats.keys()))
+                             "Please choose from the list: {}".format(list(ConversionFormats)))
 
         cls._check_input_output_directory(input_dir, output_dir)
         input_dir = os.path.join(input_dir, '')
@@ -114,11 +98,14 @@ class FormatConverter(object):
                 p, _ = os.path.splitext(output_path)
                 output_path = p + ".odml"
             odml.save(odml.load(input_path), output_path)
-        elif res_format in cls._conversion_formats:
-            if not output_path.endswith(cls._conversion_formats[res_format]):
+        elif res_format in ConversionFormats:
+            if not output_path.endswith(ConversionFormats[res_format]):
                 p, _ = os.path.splitext(output_path)
-                output_path = p + cls._conversion_formats[res_format]
+                output_path = p + ConversionFormats[res_format]
             RDFWriter(odml.load(input_path)).write_file(output_path, res_format)
+        else:
+            raise ValueError("Format for output files is incorrect. "
+                             "Please choose from the list: {}".format(list(ConversionFormats)))
 
     @staticmethod
     def _create_sub_directory(dir_path):
diff --git a/odml/tools/rdf_converter.py b/odml/tools/rdf_converter.py
index 91873af..143691e 100644
--- a/odml/tools/rdf_converter.py
+++ b/odml/tools/rdf_converter.py
@@ -13,6 +13,7 @@ from ..format import Format, Document, Section, Property
 from .dict_parser import DictReader
 from .parser_utils import ParserException
 from ..info import FORMAT_VERSION
+from .utils import RDFConversionFormats
 
 try:
     unicode = unicode
@@ -33,7 +34,7 @@ class RDFWriter(object):
 
     def __init__(self, odml_documents):
         """
-        :param odml_documents: list of odml documents 
+        :param odml_documents: list of odml documents
         """
         self.docs = odml_documents if not isinstance(odml_documents, odml.doc.BaseDocument) else [odml_documents]
         self.hub_root = None
@@ -66,9 +67,9 @@ class RDFWriter(object):
     def save_element(self, e, node=None):
         """
         Save the current element to the RDF graph
-        :param e: current element 
+        :param e: current element
         :param node: A node to pass the earlier created node to inner elements
-        :return: the RDF graph 
+        :return: the RDF graph
         """
         fmt = e.format()
 
@@ -180,21 +181,27 @@ class RDFWriter(object):
     def __unicode__(self):
         return self.convert_to_rdf().serialize(format='turtle').decode("utf-8")
 
-    def get_rdf_str(self, rdf_format):
+    def get_rdf_str(self, rdf_format="turtle"):
         """
-        Get converted into one of the supported formats data 
-        :param rdf_format: possible formats: 'xml', 'n3', 'turtle', 
-                                             'nt', 'pretty-xml', 'trix', 
+        Get converted into one of the supported formats data
+        :param rdf_format: possible formats: 'xml', 'n3', 'turtle',
+                                             'nt', 'pretty-xml', 'trix',
                                              'trig', 'nquads', 'json-ld'.
-               Full lists see in odml.tools.format_converter.FormatConverter._conversion_formats
+               Full lists see in utils.RDFConversionFormats
         :return: string object
         """
+        if rdf_format not in RDFConversionFormats:
+            raise ValueError("odml.RDFWriter.get_rdf_str: Format for output files is incorrect. "
+                             "Please choose from the list: {}".format(list(RDFConversionFormats)))
         return self.convert_to_rdf().serialize(format=rdf_format).decode("utf-8")
 
-    def write_file(self, filename, rdf_format):
+    def write_file(self, filename, rdf_format="turtle"):
         data = self.get_rdf_str(rdf_format)
-        with open(filename, "w") as file:
-            file.write(data)
+        filename_ext = filename
+        if filename.find(RDFConversionFormats.get(rdf_format)) < 0:
+            filename_ext += RDFConversionFormats.get(rdf_format)
+        with open(filename_ext, "w") as wFile:
+            wFile.write(data)
 
 
 class RDFReader(object):
diff --git a/odml/tools/utils.py b/odml/tools/utils.py
new file mode 100644
index 0000000..d981365
--- /dev/null
+++ b/odml/tools/utils.py
@@ -0,0 +1,22 @@
+import copy
+
+RDFConversionFormats = {
+    # rdflib version "4.2.2" serialization formats
+    'xml': '.rdf',
+    'pretty-xml': '.rdf',
+    'trix': '.rdf',
+    'n3': '.n3',
+    'turtle': '.ttl',
+    'ttl': '.ttl',
+    'ntriples': '.nt',
+    'nt': '.nt',
+    'nt11': '.nt',
+    'trig': '.trig',
+    'json-ld': '.jsonld'
+}
+
+ConversionFormats = copy.deepcopy(RDFConversionFormats)
+ConversionFormats.update({
+    'v1_1': '.xml',
+    'odml': '.odml'
+})

Add default RDFWriter format
Use 'turtle' as default RDFWriter format.
diff --git a/test/test_rdf_writer.py b/test/test_rdf_writer.py
index 0d1d30a..dc8b33d 100644
--- a/test/test_rdf_writer.py
+++ b/test/test_rdf_writer.py
@@ -195,3 +195,9 @@ class TestRDFWriter(unittest.TestCase):
         self.assertEqual(len(list(w.g.subjects(predicate=odmlns.hasDtype, object=Literal(p_dtype)))), 1)
         self.assertEqual(len(list(w.g.subjects(predicate=odmlns.hasValueOrigin, object=Literal(p_value_origin)))), 1)
         self.assertEqual(len(list(w.g.subjects(predicate=odmlns.hasReference, object=Literal(p_ref)))), 1)
+
+    def test_get_rdf_string(self):
+        w = RDFWriter([self.doc1])
+        w.get_rdf_str()
+        with self.assertRaises(ValueError):
+            w.get_rdf_str("abc")

pip install -e .
parse_log_pytest
pytest
pytest pytest-cov
apt-get update apt-get install -y libxml2-dev libxslt1-dev lib32z1-dev
3.6
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs @ file:///opt/conda/conda-bld/attrs_1642510447205/work
certifi==2021.5.30
coverage==6.2
docopt==0.6.2
importlib-metadata @ file:///tmp/build/80754af9/importlib-metadata_1631916693255/work
iniconfig @ file:///home/linux1/recipes/ci/iniconfig_1610983019677/work
isodate==0.6.1
lxml==5.3.1
more-itertools @ file:///tmp/build/80754af9/more-itertools_1637733554872/work
-e git+https://github.com/G-Node/python-odml.git@0c6b5141cd756e93ee993516a196ee1add49baea#egg=odML
packaging @ file:///tmp/build/80754af9/packaging_1637314298585/work
pathlib==1.0.1
pluggy @ file:///tmp/build/80754af9/pluggy_1615976315926/work
py @ file:///opt/conda/conda-bld/py_1644396412707/work
pyparsing @ file:///tmp/build/80754af9/pyparsing_1635766073266/work
pytest==6.2.4
pytest-cov==4.0.0
PyYAML==4.2b4
rdflib==5.0.0
six==1.17.0
toml @ file:///tmp/build/80754af9/toml_1616166611790/work
tomli==1.2.3
typing_extensions @ file:///opt/conda/conda-bld/typing_extensions_1647553014482/work
zipp @ file:///tmp/build/80754af9/zipp_1633618647012/work

name: python-odml
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - attrs=21.4.0=pyhd3eb1b0_0
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - importlib-metadata=4.8.1=py36h06a4308_0
  - importlib_metadata=4.8.1=hd3eb1b0_0
  - iniconfig=1.1.1=pyhd3eb1b0_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - more-itertools=8.12.0=pyhd3eb1b0_0
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - packaging=21.3=pyhd3eb1b0_0
  - pip=21.2.2=py36h06a4308_0
  - pluggy=0.13.1=py36h06a4308_0
  - py=1.11.0=pyhd3eb1b0_0
  - pyparsing=3.0.4=pyhd3eb1b0_0
  - pytest=6.2.4=py36h06a4308_2
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - toml=0.10.2=pyhd3eb1b0_0
  - typing_extensions=4.1.1=pyh06a4308_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zipp=3.6.0=pyhd3eb1b0_0
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - coverage==6.2
      - docopt==0.6.2
      - isodate==0.6.1
      - lxml==5.3.1
      - pathlib==1.0.1
      - pytest-cov==4.0.0
      - pyyaml==4.2b4
      - rdflib==5.0.0
      - six==1.17.0
      - tomli==1.2.3
prefix: /opt/conda/envs/python-odml

test/test_rdf_writer.py::TestRDFWriter::test_get_rdf_string
test/test_rdf_writer.py::TestRDFWriter::test_adding_doc_to_the_hub test/test_rdf_writer.py::TestRDFWriter::test_adding_other_entities_properties test/test_rdf_writer.py::TestRDFWriter::test_adding_properties test/test_rdf_writer.py::TestRDFWriter::test_adding_repository test/test_rdf_writer.py::TestRDFWriter::test_adding_sections test/test_rdf_writer.py::TestRDFWriter::test_adding_values test/test_rdf_writer.py::TestRDFWriter::test_convert_to_rdf test/test_rdf_writer.py::TestRDFWriter::test_section_subclass
odml/tools/format_converter.py odml/tools/utils.py odml/tools/rdf_converter.py
odml/tools/format_converter.py odml/tools/utils.py odml/tools/rdf_converter.py