Liquid feeding device and method of determining time of exchange of gas control element

A liquid feeding device that feeds liquid through a feeding system to a predetermined mechanism, the liquid feeding device comprising dissolved gas processing part that processes dissolved gas in said liquid,

• said dissolved gas processing part including:
• a gas control element provided in said feeding system, the gas control element controlling the amount of gas dissolved in said liquid by using gas pressure acting on the gas control element;
• a pressure control tank functioning as a buffer for controlling the gas pressure acting on said gas control element;
• gas pressure changing part that changes gas pressure in said pressure control tank;
• gas pressure measuring part that measures gas pressure variation in said pressure control tank while said gas control element controls the amount of gas dissolved in said liquid fed through said feeding system, the gas pressure variation being caused based on the gas pressure change by said gas pressure changing part and the control of the amount of gas dissolved in said liquid by said gas control element;
• and
• gas control element exchange suggesting part that suggests a time of exchange of said gas control element has come when a gradient of the gas pressure variation in said pressure control tank obtained from a result of measurement of the gas pressure variation by said gas pressure measuring part becomes the same as or lower than a predetermined threshold.