<xs:simpleType name="scaleType">
  <xs:annotation>
    <xs:documentation>The scale units (can be decimal, ppm, ppb, denominator)</xs:documentation>
  </xs:annotation>
  <xs:restriction base="xs:NMTOKEN">
    <xs:enumeration value="decimal"/>
    <xs:enumeration value="ppm"/>
    <xs:enumeration value="ppb"/>
    <xs:enumeration value="denominator"/>
  </xs:restriction>
</xs:simpleType>