file: asimet_module_specs.html
9 Apr 2010

ASIMET Module Specifications

ASIMET Barometric Pressure Module (BPR)

Specifications

Calibration Notes: Calibration is checked using a lab-standard electronic barometer and the bias is adjusted as required. Calibration constants are stored in the EEPROM. Usage Notes: A parallel-plate pressure port is used to reduce dynamic pressure errors (due to wind blowing over an exposed sensor port) to < 0.5 mb at 20 m/s. An internal baffle system impedes water and snow intrusion. The electronics housing is fouling and corrosion resistant (titanium) and immersion tolerant (o-ring seals).

 

ASIMET Relative Humidity & Air Temperature Module (HRH)



Specifications

Calibration Notes: Relative humidity is calibrated over a range of 25 to 95 % RH in a test chamber with stability of 0.1% RH. Temperature is calibrated over a range of 1 to 35 oC in a water bath with a resolution of 0.001 oC and an accuracy of 0.005 oC. Calibration constants are stored in the EEPROM.

Usage Notes: The sensor probe is protected by a Rotronic MF25 membrane filter and placed inside a modified R.M. Young multi-plate radiation shield for standard use. The electronics housing is fouling and corrosion resistant (titanium) and immersion tolerant (o-ring seals). Installation location should provide unobstructed air flow past the sensor and minimize heat-island effects. Motorized aspiration may be necessary in low winds.

 

ASIMET Precipitation Module (PRC)


Specifications

Calibration Notes: Calibration is done by adding successive, known volumes of water. Calibration constants are stored in the EEPROM. Usage Notes: The electronics housing is fouling and corrosion resistant (titanium) and immersion tolerant (o-ring seals). The sensor housing is susceptible to cracking if the column of water inside freezes. Rain rate must be computed by post-processing, taking into account the occurrence of siphoning intervals. Drainage time is approximately 20 sec. Catchment intake is susceptible to fouling by guano.

 

ASIMET Shortwave Radiation Module (SWR)

Specifications

Calibration Notes: The gain adjustment and calibration of the sensor electronics are made separately. The complete assembly is calibrated outside in sunlight against a traceable standard. Combined calibration constants are stored in the unit EEPROM. Usage Notes: The sensor is mounted on an aluminum base which provides a reference mass, and the base is mounted to a PVC endcap for thermal isolation from the housing. The sensor uses a temperature-compensated thermopile. Temperature Dependence is +/- 1% from -20 to +40 C. Linearity is +/- 0.5% from 0 to 1400 W/m^2. The electronics housing is fouling and corrosion resistant (titanium) and immersion tolerant (o-ring seals). Domes made of clear glass which is transparent to energy between 0.285 to 2.8µm. Domes are susceptible to fouling by salt particles, dust, and guano. The sensor must be mounted to avoid shadowing of the dome for all sun angles.

 

ASIMET Longwave Radiation Module (LWR)

Specifications

Calibration Notes: The laboratory electronics calibration procedure used at WHOI has been outlined by Payne and Anderson (1999). The complete module is then verified outdoors by comparison to a traceable standard. Calibration constants are stored in the unit EEPROM.

Usage Notes: The electronics housing is fouling and corrosion resistant (titanium) and immersion tolerant (o-ring seals). The sensor must be mounted to avoid shadowing of the dome for all sun angles. Isolation of long-wave radiation in daytime is accomplished by using a silicone domeand filter with a transmission range of approximately 3.5 to 50 µm. Domes are susceptible to fouling by salt particles, dust, and guano.

 

ASIMET Propeller and Vane Wind Module (WND)

Specifications:

Calibration Notes: The speed is checked at several standard values using an R.M.Young calibration unit. Operation of the vane encoder and compass are checked by rotating the module and vane slowly through 360 degrees with confirmation of indicated directions for at least two known directions in each quadrant. Vane alignment is checked and adjusted as required.

Sampling Notes: Scalar wind speed and direction are measured at 5 s intervals. Propeller pulses (3 pulses per revolution) are counted for 5 s while vane direction is sampled each second, and the compass is sampled in the middle of the 5 s interval. At the end of each interval, wind speed is calculated, vane direction is averaged and added to the compass direction to give wind direction, and the wind vector velocity is computed. At the end of 1 min the following variables are stored: the vector sum of 5 s velocities, the scalar averaged wind speed, the maximum of the (5 second) wind speeds, the last vane direction, and the last compass direction.

Usage Notes: The RM Young 5103 standard vane potentiometer is removed and the vane shaft extended down and coupled with an absolute angle encoder. A magnetometer compass is used to provide the north reference. Propeller bearings are replaced after 1 yr of field operations. The electronics housing is fouling and corrosion resistant (titanium) and immersion tolerant (o-ring seals). The sensor must be mounted to avoid flow disturbance created by the mounting platform.

 

ASIMET Sonic Wind Module (SWND)

Specifications:

Calibration Notes: TBD

Sampling Notes: Each measurement takes 25msec, so the base sample rate is 40Hz. There is overhead involved to output the data at the end of a sampling burst, resulting in 195 samples in a 5 s interval (39 Hz). Within each 5 s interval the compass is polled once, near the center of the interval. One minute averages are computed from 11 5-s intervals with 5 s of overhead for vector averaging. Proposed duty cycle reduction would be to produce 1 min averages from 8 s on and 7s off within 4 consecutive 15 sec intervals rather than “continuously” within 11 consecutive 5 sec intervals.

Usage Notes: Sonic probe elements are mechanically fragile during handling, but robust during deployment. The electronics housing is fouling and corrosion resistant (titanium) and immersion tolerant (o-ring seals). The sensor must be mounted to avoid flow disturbance created by the mounting platform.