A measuring device for determining a hydrocarbon concentration in compressed air, comprising a housing and the following components disposed in the housing : - a main gas line with an inlet for the compressed air, - a first gas line for a compressed air to be measured branching off from the main gas line , wherein the first gas line connects the main gas line to a first switchable valve , - a second gas line for a reference compressed air branching off from the main gas line , wherein the second gas line connects the main gas line to the first switchable valve , - a third gas line made from copper connecting the first switchable valve to a sensor unit , so that compressed air flowing through the third gas line assumes the temperature prevailing in the interior of the housing , wherein the sensor unit has a photoionization detector with a measurement chamber, - a reference gas unit arranged in the second gas line , containing an oxidation catalyst, in which the reference compressed air is generated from the compressed air, - a pressure regulator arranged in the main gas line for ensuring a constant through-flow of compressed air in the range from 3 to 16 bar, - a pressure measuring device arranged upstream of the sensor unit for determining a pressure of the respective compressed air flowing into the sensor unit , - a temperature sensor arranged downstream of the sensor unit for determining a temperature of the respective compressed air leaving the sensor unit , and - a cooling device for cooling the reference gas unit , wherein the cooling device for cooling the reference gas unit is configured as a fan, so that ambient air flows into the housing and heated air flows out of the interior of the housing . - a main gas line with an inlet for the compressed air, - a first gas line for a compressed air to be measured branching off from the main gas line , wherein the first gas line connects the main gas line to a first switchable valve , - a second gas line for a reference compressed air branching off from the main gas line , wherein the second gas line connects the main gas line to the first switchable valve , - a third gas line made from copper connecting the first switchable valve to a sensor unit , so that compressed air flowing through the third gas line assumes the temperature prevailing in the interior of the housing , wherein the sensor unit has a photoionization detector with a measurement chamber, - a reference gas unit arranged in the second gas line , containing an oxidation catalyst, in which the reference compressed air is generated from the compressed air, - a pressure regulator arranged in the main gas line for ensuring a constant through-flow of compressed air in the range from 3 to 16 bar, - a pressure measuring device arranged upstream of the sensor unit for determining a pressure of the respective compressed air flowing into the sensor unit , - a temperature sensor arranged downstream of the sensor unit for determining a temperature of the respective compressed air leaving the sensor unit , and - a cooling device for cooling the reference gas unit , wherein the cooling device for cooling the reference gas unit is configured as a fan, so that ambient air flows into the housing and heated air flows out of the interior of the housing . <EOS>