To generate a stable dew point, an air stream is bubbled through water reservoirs in two nickel-plated condensers to completely saturate the air with water vapor. The temperature of each condenser is precisely controlled to the target dew point by Peltier thermoelectric coolers. An internal radiator with a cooling fan dissipates heat from the coolers, providing a completely self-contained cooling system.
The condenser temperature is precisely measured by using a Platinum Resistance Temperature Detector (RTD). Temperature calibration accuracy is established through transfer calibrations using National Institute of Standards and Technology (NIST) recommended methods and NIST-traceable devices.
The water vapor stream exits the condensers via a port on the front panel (designed for 1/8 inches or 4 mm ID tubing) or can be split to a second port. An internal pump provides typical flow rates of 0 to 2.0 liters per minute.
NIST Calibration of the LI‑610 Portable Dew Point Generator
|NIST Dew Point (°C)||LI‑610 Pressure Corrected Dew Point (°C)||Error |
|LI‑610 Indicated Dew Point (°C)||Flow Rate |
Note: Common operating flow rates cause small over-pressures to develop in the LI‑610 condenser that exceed ambient pressure. The magnitudes of the over-pressure depend upon the flow rates used and the resistance to flow of any equipment or tubing attached to the LI‑610 output. Small over-pressures (e.g. 10 kPa or less) have no appreciable effect on the vapor pressure or dew point of air in the condenser, but the mixing ratio will be affected because the carrier gas will be compressed slightly. When the moistened air leaves the LI-610, it will expand to ambient pressure, causing the vapor pressure and dew point to drop below the condenser set point while the mixing ratio remains constant. The vapor pressure of the exit airstream can be estimated by multiplying the saturation vapor pressure at the LI‑610 setpoint temperature by the ratio P / (P + deltaP), where deltaP is the pressure difference between the condenser and ambient air pressure.
This correction derives from fundamental principles, not an instrumental error. The magnitude of the correction can be minimized by operating at low flow rates and by keeping the flow resistance of plumbing and equipment attached to the LI‑610 outlet to a minimum. The LI‑610 has a built-in water manometer that allows pressure measurements up to about 6 cm of water (0.6 kPa). This is sufficient for most calibrations at flow rates below about 0.5 liters per minute. A digital manometer can be used when operating conditions generate larger condenser pressures.