Passive surface wave sensor which can be wirelessly interrogated

In a system including an interrogation device having a transmitting part supplying interrogation signals, a receiving part and an evaluation part,

• a passive-apparatus to be interrogated by radio and used as a measuring sensor, comprising:
• a first passive surface wave structure defining a sensor element supplying sensor output signals upon receiving interrogation signals from the transmitting part of the interrogation device;
• a second passive surface wave structure defining a reference element supplying reference output signals upon receiving the interrogation signals from the transmitting part of the interrogation device;
• the interrogation device interrogating a measured value being formed from a comparison of the output sensor signals of said sensor element and the reference output signals of said reference element resulting from the interrogation signals;
• and
• said sensor element and said reference element having different sensitivities to a variable to be measured.