A dialysis device comprising: a flow channel that comprises an inlet line, an outlet line, and a disposable flow-through container, wherein the flow channel and the container are configured to contain a fluid, the inlet line, the outlet line, and the container are in fluid communication with each other; a toroid configured to accept, through an aperture of the toroid, at least a portion of the container; an electromagnetic coil wrapped around the toroid, such that the coil passes around an outer surface and an inner surface of the toroid, and covers less than the entire circumference of the toroid, the coil configured to: generate an electromagnetic signal; concentrate the electromagnetic signal on the fluid in the container such that the electromagnetic signal traverses the container in a direction that is substantially parallel to the direction of flow of the flow channel; and read the electromagnetic signal after it has traversed the fluid in the container, characterized in that the container includes a fluid inlet port and a fluid outlet port, wherein the fluid is to enter the container from the inlet line through the fluid inlet port, the fluid is to exit the container to the outlet line through the fluid outlet port, and the container is configured such that, in use, the fluid is to reside substantially stagnantly within an area of the container for a period of time, wherein the flow-through container, the toroid, and the coil are positioned such that, in use, the electromagnetic signal generated by the coil traverses the fluid in the flow-through container in an area of the flow-through container where the fluid resides substantially stagnant for the period of time. a flow channel that comprises an inlet line, an outlet line, and a disposable flow-through container, wherein the flow channel and the container are configured to contain a fluid, the inlet line, the outlet line, and the container are in fluid communication with each other; the flow channel and the container are configured to contain a fluid, the inlet line, the outlet line, and the container are in fluid communication with each other; a toroid configured to accept, through an aperture of the toroid, at least a portion of the container; an electromagnetic coil wrapped around the toroid, such that the coil passes around an outer surface and an inner surface of the toroid, and covers less than the entire circumference of the toroid, the coil configured to: generate an electromagnetic signal; concentrate the electromagnetic signal on the fluid in the container such that the electromagnetic signal traverses the container in a direction that is substantially parallel to the direction of flow of the flow channel; and read the electromagnetic signal after it has traversed the fluid in the container, generate an electromagnetic signal; concentrate the electromagnetic signal on the fluid in the container such that the electromagnetic signal traverses the container in a direction that is substantially parallel to the direction of flow of the flow channel; and read the electromagnetic signal after it has traversed the fluid in the container, characterized in that the container includes a fluid inlet port and a fluid outlet port, wherein the fluid is to enter the container from the inlet line through the fluid inlet port, the fluid is to exit the container to the outlet line through the fluid outlet port, and the container is configured such that, in use, the fluid is to reside substantially stagnantly within an area of the container for a period of time, the container includes a fluid inlet port and a fluid outlet port, wherein the fluid is to enter the container from the inlet line through the fluid inlet port, the fluid is to exit the container to the outlet line through the fluid outlet port, and the container is configured such that, in use, the fluid is to reside substantially stagnantly within an area of the container for a period of time, wherein the flow-through container, the toroid, and the coil are positioned such that, in use, the electromagnetic signal generated by the coil traverses the fluid in the flow-through container in an area of the flow-through container where the fluid resides substantially stagnant for the period of time.