A device for continuously fractionating a gas by adsorption, an in-service testing device in a processor-controlled adsorption drier system and a method for testing the operating capability of an adsorption drier system under the control of a processor, including two adsorbent beds connected to one another in a switching system, in which regeneration is performed by a flow of regeneration gas. The device has a gas inlet, a gas outlet and a bypass outlet feeding into a common functional block, which contains a line switching circuit, a first and a second shuttle valve and, optionally, other components. The in-service testing device and the device are so compact due to their respective structures that they can be integrated with one another requiring little space and having low assembly requirements. The testing method is configured to co-operate with the in-service testing device and the device, requiring only a few components for the application thereof. Furthermore, since it needs only a few components, the method has very few requirements in terms of manufacture and space.