NOVELTY - The monitoring assembly for detecting sulfur containing fuel compounds in a fuel cell system, comprises an indicator assembly (106) including an indicator material and a unit for housing the indicator material for passing a fuel (102), a sensor assembly for detecting a physical property of the indicator material or a change in the physical property of the indicator material, a non-reactive support screen and non-reactive support material disposed adjacent to an inlet and adjacent to an outlet of the housing, and a controller (501) for performing a predetermined action.    USE - The monitoring assembly is useful for detecting sulfur containing fuel compounds in a fuel cell system.    ADVANTAGE - The assembly ensures continuous monitoring of the sulfur concentration without sampling the processed fuel thus providing high accurate detection of the sulfur containing fuel compounds in an economical manner.    DETAILED DESCRIPTION - The monitoring assembly for detecting sulfur containing fuel compounds in a fuel cell system, comprises an indicator assembly (106) including an indicator material and a unit for housing the indicator material for passing a fuel (102), a sensor assembly for detecting a physical property of the indicator material or a change in the physical property of the indicator material, a non-reactive support screen and non-reactive support material disposed adjacent to an inlet and adjacent to an outlet of the housing, and a controller (501) for performing a predetermined action. The housing is adapted to be placed on-line in a main path for receiving all the fuel or a bypass path for receiving only a portion of the fuel in the fuel cell system. When the housing is placed on-line in the fuel cell system, the physical property of the indicator material changes and when the indicator material is exposed to sulfur-containing compounds in the fuel of the fuel cell system, the indicator assembly is adapted to allow detection of the change in the physical property of the indicator material. The physical property includes a color of the indicator material, where a predetermined concentration of the sulfur-containing compounds corresponds to a predetermined color of the indicator material. The indicator material has a volume of 0.3-18.3 in3. The indicator material is retained within the housing by the non-reactive support screen and non-reactive support material. If the fuel comprises HD-5 propane fuel, the fuel is passed through the indicator assembly at a predetermined flow rate of 15 scfm or less, a predetermined superficial velocity of 0.46 ft/s or less and a predetermined space velocity of 0.46/s or less. If the fuel comprises natural gas fuel, the fuel is passed through the indicator assembly at the predetermined flow rate of 40 scfm or less, the predetermined superficial velocity of 1.22 ft/s or less and a predetermined space velocity of 1.22/s or less. The indicator material is adapted to remove the sulfur-containing compounds from the fuel. The controller serves as a controller for controlling the fuel cell system. The monitoring assembly detects sulfur-containing compounds in the fuel after passing the fuel through a desulfurizer assembly (104) of the fuel cell system. The desulfurizer assembly comprises first and second desulfurizers and fuel flow control members, where the predetermined action is activating an alarm (130) or controlling the fuel flow control members. The first desulfurizer is coupled with the second desulfurizer in parallel. The fuel flow control members are controlled the flow of fuel through the first and second desulfurizers such that the fuel passes through one of the desulfurizers while the other of the first and second desulfurizers is in standby mode. The predetermined action is performed by controlling the fuel flow control members to inhibit the flow of fuel through the one of the first and second desulfurizers and to pass the fuel through the other of the first and second desulfurizers of the desulfurizer assembly. The predetermined concentration of sulfur-containing compounds in the fuel is 30 ppbv or greater. The fuel cell system comprises a fuel cell and a fuel supply control member downstream from the monitoring assembly for controlling flow of the fuel to the fuel cell. The monitoring assembly detects sulfur-containing compounds in the fuel after passing the fuel through a desulfurizer assembly of the fuel cell system. The housing has a diameter of 0.25-1.5 inches, and comprises a transparent or translucent quartz material or poly vinyl chloride material to allow the indicator material to be visible through the material, an access way which allows one of a sensor assembly and a human operator to detect the change in the physical properties of the indicator material. The access way comprises a sight glass or window in the housing to allow visibility of the indicator material from outside the housing and an internal sight glass or window formed by a transparent or translucent inner housing layer. Each indicator assembly is adapted to allow detection of the change in the physical property of the indicator material. The indicator material comprises FSK-A (RTM: Adsorbent), Ag400 (RTM: Adsorbent), TOSPIX 94 (RTM: Adsorbent), F8-01 (RTM: Adsorbent), Selectra Sulf-X CNG-2 (RTM: Adsorbent) or SulfaTrap-R6 (RTM: Adsorbent). The controller determines a rate of progression of a concentration of sulfur containing compounds along the length of the housing of the respective indicator assembly based on the detecting by the sensors of each the sensor assembly, and determines the predetermined concentration of sulfur-containing compounds present in the fuel based on the rate of progression within a predetermined time period. INDEPENDENT CLAIMS are included for:    (1) a fuel cell system;    (2) a desulfurizer-indicator assembly for use in a fuel cell system;    (3) a method of monitoring sulfur-containing compounds in fuel used in a fuel cell system; and    (4) a method of operating a monitoring assembly used in a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a monitoring assembly.    Fuel (102)    Desulfurizer assembly (104)    Indicator assembly (106)    Heat exchanger (108)    Alarm (130)    Controller. (501)