An underwater gas measuring device for gases dissolved in water comprising a. a pressure vessel with a gas inlet opening in the wall of the pressure vessel b. a semi-permeable, gas-permeable membrane arranged at the gas inlet opening in a watertight manner; c. a support device for the membrane against hydrostatic pressure; d. a measuring chamber in the pressure vessel in gas exchanging communication with the membrane and having a gas outlet; e. a detection device for detecting at least one physical and/or chemical parameter of gas in the measuring chamber; f. an electronic evaluation device designed to detect at least one signal of the detection device representing at least one detected parameter and designed for digitized transmission and/or for non-volatile digital storage of at least one signal and/or at least one derivative signal; g. a vacuum pump connected downstream of the gas outlet of the measuring chamber; h. a non-return valve connected downstream of the vacuum pump and arranged in the flow direction; i. a collecting chamber downstream of the non-return valve for measured gas; j. a pump control device which is designed to cause the vacuum pump to draw gas from the measuring chamber and to supply it to the collecting chamber; characterized in that k. the collection chamber has a variable volume and a gas outlet; l. a pressure relief valve is arranged at the gas outlet of the receiving chamber, which opens when a predetermined gas pressure in the receiving chamber is exceeded; m. mechanical means are provided for changing the volume of the receiving chamber which, when gas is supplied, allow the volume of the receiving chamber to be increased up to a predetermined maximum volume while maintaining the predetermined gas pressure; n. a drive is provided for the mechanical means for changing the volume and is designed to cause the mechanical means, when the predetermined maximum volume is reached, to reduce the volume of the collecting chamber while blowing out the gas content through the pressure relief valve, where o. a gas pipe leads the gas from the outlet of the pressure relief valve to a gas outlet opening in the wall of the pressure vessel. a. a pressure vessel with a gas inlet opening in the wall of the pressure vessel b. a semi-permeable, gas-permeable membrane arranged at the gas inlet opening in a watertight manner; c. a support device for the membrane against hydrostatic pressure; d. a measuring chamber in the pressure vessel in gas exchanging communication with the membrane and having a gas outlet; e. a detection device for detecting at least one physical and/or chemical parameter of gas in the measuring chamber; f. an electronic evaluation device designed to detect at least one signal of the detection device representing at least one detected parameter and designed for digitized transmission and/or for non-volatile digital storage of at least one signal and/or at least one derivative signal; g. a vacuum pump connected downstream of the gas outlet of the measuring chamber; h. a non-return valve connected downstream of the vacuum pump and arranged in the flow direction; i. a collecting chamber downstream of the non-return valve for measured gas; j. a pump control device which is designed to cause the vacuum pump to draw gas from the measuring chamber and to supply it to the collecting chamber; characterized in that k. the collection chamber has a variable volume and a gas outlet; l. a pressure relief valve is arranged at the gas outlet of the receiving chamber, which opens when a predetermined gas pressure in the receiving chamber is exceeded; m. mechanical means are provided for changing the volume of the receiving chamber which, when gas is supplied, allow the volume of the receiving chamber to be increased up to a predetermined maximum volume while maintaining the predetermined gas pressure; n. a drive is provided for the mechanical means for changing the volume and is designed to cause the mechanical means, when the predetermined maximum volume is reached, to reduce the volume of the collecting chamber while blowing out the gas content through the pressure relief valve, where o. a gas pipe leads the gas from the outlet of the pressure relief valve to a gas outlet opening in the wall of the pressure vessel.