Al203 oxide sensors measure changes in partial water vapor pressure (PWVP). They follow complicated principles of physical chemistry. Henry’s Law defines the relationship between PWVP and water concentration in PPMW (μg /g).

Henry’s Law PPMW(μg /g)=PWVP * K

K is Henry’s constant. This constant is effected by sample matrix and temperature. COSA Xentaur has developed a sample system with an integral “Grab” sample to facilitate the determination of K in the “real” process. The sample system can then be used on a routine basis to validate K.

The procedure required to make a small number of empirical measurement is quite easy. By utilizing the “grab” sample, K is easily calculated. This is done at 2 critical concentrations. This data is then incorporated into a look-up table. The table is completed utilizing Henry’s Law theory. By using this approach PPMW (ug/g) measurements are possible directly from the sensor.

The HDT is a loop powered HART enabled dewpoint transmitter.

• Enclosure: Stainless Steel, IP66 NEMA 4X
• Dimensions & Weight: ~1.25”Dia. x ~5.68” long including sensor & connector; 0.5 lbs
• Pressure operating range:  Standard: 500 PSI (34 bar). Optional: 5,000 PSI (340 bar)
• Operating Temperature: 14°F to 158°F (-10°C to +70°C)
• Mechanical connection: 14 mm x 1.25 mm threads, and 3/4”-16 threads
• Electrical connections: Industrial Standard 9.4 mm, 4 pin connector. IP66 NEMA 4X
• Cable:  Two conductor cable. Min. #24AWG; for total cable length >5000 ft min. #20AWG (cable must be shielded to meet CE requirements)
• Power Requirements: 5 to 28 VDC, the instrument draws 4-20mA depending on measured dewpoint
• Input Resolution: 0.1°C dew point
• Indicators: None
• Engineering Units: °C(dp), PMVP(mb), PPMW(μg/g)
• Controls: HART interface, user’s selections are stored in EEPROM
• Outputs: Analog and digital outputs are available
  • A. 4-20mA drawn by the instrument from the power supply. The 4-20mA is linear to °C(dp), the range is programmable. Output resolution is 0.1°C(dp) or ~ 0.25uA whichever is greater.
  • B. The instrument can supply digital output by modulating the 4-20mA loop line. The interface is defined by HART. In the digital mode the HDT can be remotely operated and the dewpoint as well as temperature (and pressure if installed) can be read. In the digital mode multiple units can operate on the same loop cable as a multi-channel instrument. In this configuration each HDT draws only 4mA independent of the measured dewpoint
• Alarms: The 4-20mA signal may be used by an external device to operate relays. In addition, a digital output pin is provided which can be factory (or specially equipped customer) programmed to provide dewpoint alarm indications.
• Isolation: Sensor and case are referenced to the current loop negative side
• Warranty:  1 year

Specifications of HTF dew point sensor element XTR-LQ

• Type: Hyper-Thin-Film (HTF) high capacitance Al2O3
• Dewpoint Range XTR-LQ:  -80°C to 25°C
• Partial Water Vapor Pressure Range: 0.0005mb to 31.65 mb
• Capacitance: 5nF to 225nF
• Accuracy: ±2°C(±3.6°F)for -100°C to 0°C
• Dew Point: ±3°C(±5.5°F)for 0°C to + +20°C
• Dew Point Repeatability: ±0.9°F(±0.5°C)
• Temperature Range: +14°F to +158°F (-10°C to +70°C)
• Storage Temperature: -40°F to +176°F (-40°C to +80°C)
• Calibration Method: Multi-point calibration table with temperature compensation over the full range