An infusion fluid warmer comprising: - a DC power supply input, - a first printed circuit board comprising a first surface and a second, opposing, surface, wherein the second surface comprises a first integrally formed electrically resistive pattern (R 1 -R 5), - a second printed circuit board comprising a first surface and a second, opposing, surface, wherein the second surface comprises a second integrally formed electrically resistive pattern (R 6 -R 10), - a heat exchanger comprising an upper wall structure and a lower, opposing, wall structure separated by a fluid channel or passage extending between fluid inlets and outlets (302, 304) of the heat exchanger wherein an outer surface of the upper wall structure is thermally connected to the first integrally formed electrically resistive pattern (R 1 -R 5) and an outer surface of the lower wall structure is thermally connected to the second integrally formed electrically resistive pattern (R 6 -R 10); - a controller, configured for: - connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10), - selectively connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns during a first time period to dissipate power in the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10), characterised in that the controller is further configured for: - determining, during a second time period, a resistance of the first integrally formed electrically resistive pattern (R 1 -R 5) and/or a resistance of the second integrally formed electrically resistive pattern (R 6 -R 10), and - determining a temperature of the first or second integrally formed electrically resistive pattern based on the determined resistance, - providing a predetermined delay time, such as between 10 ms and 200 ms, when switching from the first time period to the second time period, and wherein the DC power supply (V DD) to the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10) is disconnected during the predetermined delay time. - a DC power supply input, - a first printed circuit board comprising a first surface and a second, opposing, surface, wherein the second surface comprises a first integrally formed electrically resistive pattern (R 1 -R 5), - a second printed circuit board comprising a first surface and a second, opposing, surface, wherein the second surface comprises a second integrally formed electrically resistive pattern (R 6 -R 10), - a heat exchanger comprising an upper wall structure and a lower, opposing, wall structure separated by a fluid channel or passage extending between fluid inlets and outlets (302, 304) of the heat exchanger wherein an outer surface of the upper wall structure is thermally connected to the first integrally formed electrically resistive pattern (R 1 -R 5) and an outer surface of the lower wall structure is thermally connected to the second integrally formed electrically resistive pattern (R 6 -R 10); - a controller, configured for: - connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10), - selectively connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns during a first time period to dissipate power in the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10), - connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10), - selectively connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns during a first time period to dissipate power in the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10), - determining, during a second time period, a resistance of the first integrally formed electrically resistive pattern (R 1 -R 5) and/or a resistance of the second integrally formed electrically resistive pattern (R 6 -R 10), and - determining a temperature of the first or second integrally formed electrically resistive pattern based on the determined resistance, - providing a predetermined delay time, such as between 10 ms and 200 ms, when switching from the first time period to the second time period, and wherein the DC power supply (V DD) to the first and/or second integrally formed electrically resistive patterns (R 1 -R 5), (R 6 -R 10) is disconnected during the predetermined delay time.