Dissolved CO2 Instruments

Dissolved CO₂ instruments are analytical devices that measure the amount of carbon dioxide (CO₂) dissolved in a liquid sample such as water. They typically include a submerged probe that is covered by a thin organic membrane. When the probe is submerged in the liquid sample, carbon dioxide diffuses through the membrane at a rate proportional to the partial pressure. Increasing the partial pressure increases the diffusion amount. The liquid sample is considered to be saturated when the molecular activity of the carbon dioxide equals that of the liquid. Air contains only 0.035 % carbon dioxide by volume; however, CO₂ is nearly 30 times as soluble in water as oxygen. Carbon dioxide moves across the air-water interface according to the same physical process that affect the dissolving of oxygen. Both temperature and pressure affect the diffusion rate measured by dissolved CO₂ instruments. Accuracy and diffusion range are typically measured in parts per thousand or parts per million.

Dissolved CO₂ instruments vary in terms of user interface, features, and output options. Some devices provide analog or digital displays. Others include devices such as cathode-ray tubes (CRTs), liquid crystal displays (LCDs), or flat panel displays. User controls may consist of knobs or potentiometers mounted on a simple front panel. Digital front panels are programmable and work with a keypad. Some devices are battery-powered, self-calibrating, or equipped with self-test diagnostics. Others are triggered by events or include signal processing or filtering. Output options include analog voltage, analog current, analog frequency, and switch or alarm relays. Analog voltage outputs are a simple (usually linear) function of the measurement. Analog currents use feedback to provide an appropriate current regardless of variables such as noise and impedance. Analog frequencies use continuous physical variables such as voltage amplitude or frequency variations to transmit information.

Important specifications for dissolved CO₂ instruments include the maximum number of input channels, the maximum bandwidth, the resolution in bits, and the sampling frequency. Environmental considerations include the operating temperature, minimum shock rating, and maximum vibration rating. Dissolved CO₂ instruments that feature a computer interface are commonly available and may include non-volatile memory or removable hard drives. Communication protocols and bus types for dissolved CO₂ instruments include ARCNET, AS-I, Beckhoff I/O, CANbus, DeviceNet, Ethernet, Foundation Fieldbus, general-purpose interface bus (GPIB) or IEEE 488, InterBus, PROFIBUS, and SDS.

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