CHAPTER 17.   OPERATION OF EQUIPMENT

SECTION 9.   HYGROTHERMOMETERS

17-76.   HYGROTHERMOMETER (HO-83, ASOS 1088, ETC.) - GENERAL

Unlike previous hygrothermometers that employ a lithium chloride dew cell to measure dew point, these hygrothermometers use a "chilled mirror" technique. The sensor, in the aspirator, contains a mirror and two temperature measuring beads. One bead, in the intake of the aspirator, measures the ambient temperature. Dew point temperature is determined by the mirror which is held at precisely the temperature at which a thin film of condensation is maintained on its surface. A thermal unit raises and lowers the temperature of this mirror as necessary to maintain the thickness of the condensation. The temperature of the mirror, measured by the other bead, is the dew point temperature.

17-77.   HYGROTHERMOMETER (HO-83) - OPERATIONS

Except for the possible cleaning of the mirror, if no electronic technician is available at the station, the HO-83 contains no operator serviceable parts. All instrument calibration and maintenance, preventive and corrective, shall be done by an electronic technician. However, the observer shall:

a.  Be familiar with switches and indicators present on the display.

b.  Make a daily check of the displays.

c.  Compare hygrothermometer against a standard as necessary but at least once a week.

17-78.   HYGROTHERMOMETER (HO-83) - INSTRUMENT ERROR INDICATIONS

Malfunction of the internal circuits of the HO-83 will, in most cases, be indicated by:

a.  The error light, which will blink if no signal is being received or errors are suspected or detected. The light will also blink for approximately 5 minutes when power is restored after being shut off. It takes about 5 minutes for the computer to receive enough data to compute the 5-minute averages of temperature and dew point.

b.  If no malfunction exists in the display panel (pressing the display button), all displays (temperature, dew point, maximum and minimum temperature) will read -188.8

17-79.   HYGROTHERMOMETER (HO-83) - QUALITY CONTROL

Equipment malfunctions will usually be indicated by the error light or an erroneous display when the test button is pressed, but equipment functioning properly can yield incorrect data if, for example, the mirrors are dirty or the intake tube of the aspirator is clogged. Although cleaning the mirrors and intake tube is usually done by the electronic technician, the observer is responsible for ensuring that the temperatures reported are valid by making periodic comparison checks.

17-80.   HYGROTHERMOMETER (HO-83, ASOS 1088, ETC.) - COMPARISON CHECKS (NA LAWRS)

Normal comparison checks consist of comparing the hygrothermometer's temperature and dew point with simultaneous values obtained from a properly exposed comparison standard--a sling or shelter mounted psychrometer. The comparison values may be observed at a site convenient to the office as long as the values obtained are representative of those at the hygrothermometer site. The observer shall make comparisons when no ice fog is present, and:

a.  Following preventive as well as corrective maintenance.

b.  At least once a week, as near 1200 LST on Monday as possible and more frequently when calibration is suspect or previous comparisons suggest the possibility of unstable or otherwise unsatisfactory performance. The frequency should be such as to keep, as far as practical, the performance within the limits shown below and in Figure 13-1, Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° F (e.g., HO-83, 1088, etc.), and in Figure 13-2, Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° C (e.g., HO-83, 1088, etc.).

(1)  For ambient temperatures, the difference between the values obtained from the station standard and the hygrothermometer shall not exceed 2°C. This limit is applicable to comparisons made with the station standard at either the hygrothermometer site or at the remote location convenient to the office.

(2)  Allowable dew point differences are based on the temperature/dew point spread as determined by the comparison standard (psychrometer). These are shown in Figure 13-1, Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° F (e.g., HO-83, 1088, etc.), and in Figure 13-2, Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° C (e.g., HO-83, 1088, etc.), and, like temperature, are applicable to comparisons at either comparison site.

(3)  Valid persistent temperature differences, if any, between the hygrothermometer site and a remote site may be applied as a correction to the comparison.

(4)  If limits are exceeded when the comparison is made at the location remote from the hygrothermometer site, the observer shall promptly make a comparison at the site of the hygrothermometer. If limits are still exceeded, the observer shall discontinue use of the system and notify the electronic technician for corrective action.

(5)  If limits are not exceeded but the trend of comparisons for the past three weeks show a relatively steady drift towards higher or lower temperature or dew-point values, such as from a dirty mirror, the frequency of comparisons should be increased.

(6)  A graph of comparison departures is required as an aid in evaluating system performance and to determine the frequency of comparisons, NOAA Form 59-6 or other convenient graph paper may be used to plot plus or minus departures above and below a horizontal zero reference line. From the graph the need for corrective maintenance or for more frequent comparisons should be readily apparent.