A method for operating a fluid-insulated electrical apparatus, in particular gas-insulated medium or high voltage switchgear, wherein an insulation fluid of the electrical apparatus comprises at least a first component and a second component, the method comprising the method elements of: - determining a physical state of the insulation fluid by measuring at least three measurement variables (p, p, T, λ, η, c S) by means of at least one sensor, wherein the measurement variables (p, p, T, λ, η, c S) are indicative of the physical state of the insulation fluid and wherein the measurement variables (ρ, p, T, λ, η, c S) are selected such that at least two characterizing variables indicative of partial pressures (p A, p B) or concentrations (c A, c B) of the first component and the second component of the insulation fluid can be determined using the measurement variables (ρ, p, T, λ, η, c S), - wherein the method comprises a further method element (i) of: deriving at least one trend variable for at least one of the measurement variables (p, p, T, λ, η, c S), wherein the at least one trend variable is indicative of a change over time of the at least one measurement variable (p, p, T, λ, η, c S); and/or a further method element (ii) of: deriving from at least one of the measurement variables (p, p, T, λ, η, c S) at least one of the characterizing variables, wherein the at least one of the characterizing variables is indicative of the physical state of the insulation fluid, and deriving at least one trend variable for the at least one of the characterizing variables, wherein the trend variable is indicative of a change over time of the at least one of the characterizing variables; wherein at least two trend variables are derived for the at least two characterizing variables indicative of the partial pressures (p A, p B) or concentrations (c A, c B) of the first and second components (A, B) of the insulation fluid, and wherein the at least two characterizing variables are selected from the group consisting of: a function f1(p A), a function f2(p A /T), a function f3(p B), a function f4(p B /T), a function f5(p A, p B), a function f6(p A, p B, T), a function f7(p A /p B), a function f8(c A), a function f9(c B), a function f10(c A, c B), and a function f11(c A /c B), with p A and p B being the partial pressures of the first and second components (A, B) of the insulation fluid, with c A and c B being the concentrations of the first and second components (A, B) of the insulation fluid, and with T being a temperature T of the insulation fluid; wherein further the method comprises a method element of: determining, using the at least one trend variable for the at least one of the characterizing variables, an operating state (O, F1) of the electrical apparatus out of a group of at least two possible operating states (0, F1). - determining a physical state of the insulation fluid by measuring at least three measurement variables (p, p, T, λ, η, c S) by means of at least one sensor, wherein the measurement variables (p, p, T, λ, η, c S) are indicative of the physical state of the insulation fluid and wherein the measurement variables (ρ, p, T, λ, η, c S) are selected such that at least two characterizing variables indicative of partial pressures (p A, p B) or concentrations (c A, c B) of the first component and the second component of the insulation fluid can be determined using the measurement variables (ρ, p, T, λ, η, c S), - wherein the method comprises a further method element (i) of: deriving at least one trend variable for at least one of the measurement variables (p, p, T, λ, η, c S), wherein the at least one trend variable is indicative of a change over time of the at least one measurement variable (p, p, T, λ, η, c S); and/or a further method element (ii) of: deriving from at least one of the measurement variables (p, p, T, λ, η, c S) at least one of the characterizing variables, wherein the at least one of the characterizing variables is indicative of the physical state of the insulation fluid, and deriving at least one trend variable for the at least one of the characterizing variables, wherein the trend variable is indicative of a change over time of the at least one of the characterizing variables; determining, using the at least one trend variable for the at least one of the characterizing variables, an operating state (O, F1) of the electrical apparatus out of a group of at least two possible operating states (0, F1).