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Instruments designed to measure and/or obtain preestablished concentrations of gases in liquids (e.g., gases in blood) by exposing samples to an ambient gas phase in such a way that each gas reaches equilibrium between the liquid and gaseous phase. These devices typically work using one of two techniques: (1) The thin-film technique use a humidified chamber kept at a constant temperature (usually 37 degrees C/98.6 degrees F), a glass or plastic rotary cup fitted in a shaft inside the chamber, a timer, and controls; the cup rotates in short bursts while flushed with the humidified gas for a given time causing the liquid (e.g., blood) in the cup to form thin layers on the inside of the cup; (2) The bubble technique uses a dedicated syringe that allows gas to be introduced into and humidified through the plunger, a gas line, and a thermostatically controlled heat block to keep the syringe at a constant temperature. A sample is loaded into the syringe, and gas flow is initiated from the gas line to the plunger; the gas bubbles through the blood in the syringe to the atmosphere. Laboratory tonometers are also used in research, to prepare blood samples for special tests that are not frequently performed in hospitals, and to prepare blood samples for quality control of devices (e.g., oxygen and carbon dioxide electrodes) used to measure blood gas samples.
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