A thermodynamic instrument is any device for the measurement of thermodynamic systems. In order for a thermodynamic parameter or physical quantity to be truly defined, a technique for its measurement must be specified. For example, the ultimate definition of temperature is "what a thermometer reads". The question follows – what is a thermometer?
There are two types of thermodynamic instruments: the meter and the reservoir. A thermodynamic meter is any device which measures any parameter of a thermodynamic system. A thermodynamic reservoir is a system which is so large that it does not appreciably alter its state parameters when brought into contact with the test system.