diff --git a/src/blackmisc/collection.h b/src/blackmisc/collection.h
index a3ea6b783..10d4949ab 100644
--- a/src/blackmisc/collection.h
+++ b/src/blackmisc/collection.h
@@ -265,10 +265,10 @@ namespace BlackMisc
         void detach() { m_impl.detach(); }
 
         //! Test for equality.
-        bool operator ==(const CCollection &other) const { return m_impl == other.m_impl; }
+        friend bool operator ==(const CCollection &a, const CCollection &b) { return a.m_impl == b.m_impl; }
 
         //! Test for inequality.
-        bool operator !=(const CCollection &other) const { return m_impl != other.m_impl; }
+        friend bool operator !=(const CCollection &a, const CCollection &b) { return a.m_impl != b.m_impl; }
 
     private:
         QOrderedSet<T> m_impl;
diff --git a/src/blackmisc/pq/measurementunit.cpp b/src/blackmisc/pq/measurementunit.cpp
index c6dbcd893..3f710bb4b 100644
--- a/src/blackmisc/pq/measurementunit.cpp
+++ b/src/blackmisc/pq/measurementunit.cpp
@@ -16,17 +16,6 @@ namespace BlackMisc
 {
     namespace PhysicalQuantities
     {
-        bool CMeasurementUnit::operator ==(const CMeasurementUnit &other) const
-        {
-            if (this == &other) return true;
-            return m_data->m_name == other.m_data->m_name;
-        }
-
-        bool CMeasurementUnit::operator !=(const CMeasurementUnit &other) const
-        {
-            return !(other == *this);
-        }
-
         double CMeasurementUnit::convertFrom(double value, const CMeasurementUnit &unit) const
         {
             if (this->isNull() || unit.isNull()) return 0;
diff --git a/src/blackmisc/pq/measurementunit.h b/src/blackmisc/pq/measurementunit.h
index 2b6bfbd9f..78ff39464 100644
--- a/src/blackmisc/pq/measurementunit.h
+++ b/src/blackmisc/pq/measurementunit.h
@@ -272,10 +272,17 @@ namespace BlackMisc
             }
 
             //! Equal operator ==
-            bool operator == (const CMeasurementUnit &other) const;
+            friend bool operator == (const CMeasurementUnit &a, const CMeasurementUnit &b)
+            {
+                if (&a == &b) return true;
+                return a.m_data->m_name == b.m_data->m_name;
+            }
 
             //! Unequal operator !=
-            bool operator != (const CMeasurementUnit &other) const;
+            friend bool operator != (const CMeasurementUnit &a, const CMeasurementUnit &b)
+            {
+                return !(a == b);
+            }
 
             //! \copydoc CValueObject::qHash
             friend uint qHash(const CMeasurementUnit &unit)
diff --git a/src/blackmisc/pq/physicalquantity.cpp b/src/blackmisc/pq/physicalquantity.cpp
index 29eeefd72..77bf11f64 100644
--- a/src/blackmisc/pq/physicalquantity.cpp
+++ b/src/blackmisc/pq/physicalquantity.cpp
@@ -76,7 +76,7 @@ namespace BlackMisc
         }
 
         template <class MU, class PQ>
-        bool CPhysicalQuantity<MU, PQ>::operator ==(const CPhysicalQuantity<MU, PQ> &other) const
+        bool CPhysicalQuantity<MU, PQ>::equals(const CPhysicalQuantity<MU, PQ> &other) const
         {
             if (this == &other) return true;
 
@@ -87,12 +87,6 @@ namespace BlackMisc
             return diff <= m_unit.getEpsilon();
         }
 
-        template <class MU, class PQ>
-        bool CPhysicalQuantity<MU, PQ>::operator !=(const CPhysicalQuantity<MU, PQ> &other) const
-        {
-            return !((*this) == other);
-        }
-
         template <class MU, class PQ>
         CPhysicalQuantity<MU, PQ> &CPhysicalQuantity<MU, PQ>::operator +=(const CPhysicalQuantity<MU, PQ> &other)
         {
@@ -100,14 +94,6 @@ namespace BlackMisc
             return *this;
         }
 
-        template <class MU, class PQ>
-        PQ CPhysicalQuantity<MU, PQ>::operator +(const PQ &other) const
-        {
-            PQ copy(other);
-            copy += *this;
-            return copy;
-        }
-
         template <class MU, class PQ>
         void CPhysicalQuantity<MU, PQ>::addValueSameUnit(double value)
         {
@@ -127,14 +113,6 @@ namespace BlackMisc
             return *this;
         }
 
-        template <class MU, class PQ>
-        PQ CPhysicalQuantity<MU, PQ>::operator -(const PQ &other) const
-        {
-            PQ copy = *derived();
-            copy -= other;
-            return copy;
-        }
-
         template <class MU, class PQ>
         bool CPhysicalQuantity<MU, PQ>::isZeroEpsilonConsidered() const
         {
@@ -241,7 +219,7 @@ namespace BlackMisc
         }
 
         template <class MU, class PQ>
-        bool CPhysicalQuantity<MU, PQ>::operator <(const CPhysicalQuantity<MU, PQ> &other) const
+        bool CPhysicalQuantity<MU, PQ>::lessThan(const CPhysicalQuantity<MU, PQ> &other) const
         {
             if (*this == other) return false;
             if (this->isNull() || other.isNull()) return false;
@@ -249,26 +227,6 @@ namespace BlackMisc
             return (m_value < other.value(m_unit));
         }
 
-        template <class MU, class PQ>
-        bool CPhysicalQuantity<MU, PQ>::operator >(const CPhysicalQuantity<MU, PQ> &other) const
-        {
-            return other < *this;
-        }
-
-        template <class MU, class PQ>
-        bool CPhysicalQuantity<MU, PQ>::operator >=(const CPhysicalQuantity<MU, PQ> &other) const
-        {
-            if (*this == other) return true;
-            return *this > other;
-        }
-
-        template <class MU, class PQ>
-        bool CPhysicalQuantity<MU, PQ>::operator <=(const CPhysicalQuantity<MU, PQ> &other) const
-        {
-            if (*this == other) return true;
-            return *this < other;
-        }
-
         template <class MU, class PQ>
         PQ &CPhysicalQuantity<MU, PQ>::switchUnit(const MU &newUnit)
         {
diff --git a/src/blackmisc/pq/physicalquantity.h b/src/blackmisc/pq/physicalquantity.h
index 3e709022d..c3138cf06 100644
--- a/src/blackmisc/pq/physicalquantity.h
+++ b/src/blackmisc/pq/physicalquantity.h
@@ -160,10 +160,10 @@ namespace BlackMisc
             PQ operator /(double divide) const;
 
             //! Equal operator ==
-            bool operator==(const CPhysicalQuantity &other) const;
+            friend bool operator==(const CPhysicalQuantity &a, const CPhysicalQuantity &b) { return a.equals(b); }
 
             //! Not equal operator !=
-            bool operator!=(const CPhysicalQuantity &other) const;
+            friend bool operator!=(const CPhysicalQuantity &a, const CPhysicalQuantity &b) { return !a.equals(b); }
 
             //! Plus operator +=
             CPhysicalQuantity &operator +=(const CPhysicalQuantity &other);
@@ -172,22 +172,22 @@ namespace BlackMisc
             CPhysicalQuantity &operator -=(const CPhysicalQuantity &other);
 
             //! Greater operator >
-            bool operator >(const CPhysicalQuantity &other) const;
+            friend bool operator>(const CPhysicalQuantity &a, const CPhysicalQuantity &b) { return b.lessThan(a); }
 
             //! Less operator <
-            bool operator <(const CPhysicalQuantity &other) const;
+            friend bool operator<(const CPhysicalQuantity &a, const CPhysicalQuantity &b) { return a.lessThan(b); }
 
             //! Less equal operator <=
-            bool operator <=(const CPhysicalQuantity &other) const;
+            friend bool operator<=(const CPhysicalQuantity &a, const CPhysicalQuantity &b) { return !b.lessThan(a); }
 
             //! Greater equal operator >=
-            bool operator >=(const CPhysicalQuantity &other) const;
+            friend bool operator>=(const CPhysicalQuantity &a, const CPhysicalQuantity &b) { return !a.lessThan(b); }
 
             //! Plus operator +
-            PQ operator +(const PQ &other) const;
+            friend PQ operator +(const PQ &a, const PQ &b) { PQ copy(a); copy += b; return copy; }
 
             //! Minus operator -
-            PQ operator -(const PQ &other) const;
+            friend PQ operator -(const PQ &a, const PQ &b) { PQ copy(a); copy -= b; return copy; }
 
             //! Quantity value <= epsilon
             bool isZeroEpsilonConsidered() const;
@@ -281,6 +281,12 @@ namespace BlackMisc
             //! Implementation of compare
             static int compareImpl(const PQ &, const PQ &);
 
+            //! Private implementation of equality operators
+            bool equals(const CPhysicalQuantity &other) const;
+
+            //! Private implementation of comparison operators
+            bool lessThan(const CPhysicalQuantity &other) const;
+
             //! Easy access to derived class (CRTP template parameter)
             PQ const *derived() const;