Instruments

This page describes how to use, where to find and how to maintain the various operational instruments at the Observatory. A large selection of inactive, historical instruments (not described here) are on display in the history room on the ground floor of the Observatory tower.

Temperature Wind Precipitation Pressure Sunshine, etc.
Psychrometer
 Wind Gust Chart Recorder Standard Eight-Inch Rain Gage Mercury Barometer Sunshine Recorder
Maximum & Minimum Thermometers Wind Speed Chart Recorder Weighing Eight-Inch Rain Gage Microbarograph Visibility
Hygrothermograph Aerovane Wind Recorder Ombroscope Four-day Barograph ASOS
Digital Thermometer (Nimbus) Maximum, Inc. Wind Recorder (Merlin) Digital Rain Gage Display (Nimbus) Digital Barometer (Nimbus)

Maximum, Inc. Gust Recorder (Max #1) Snow Board



Charts, etc.

Temperature Wind Precipitation Pressure Sunshine, etc.
Hygrothermograph Chart Wind Gust Chart Weighing Gage Chart Microbarograph Chart Sun Card

Wind Speed Chart Ombroscope Chart Four-Day Barograph Chart Dew Point Calculator

Aerovane Chart





Psychrometer
Psychrometer Use:
Dry and wet bulb temperature measurement; used to derive calculated dew point, relative humidity, and vapor pressure.

Location:
Enclosure Hazen shelter, hand-held, or sling psychrometer.  Enclosure shelter must be kept closed at all times except for a few minutes during readings.  Hand-held psychrometer can be placed in a shaded area on the iron fence around the Observatory or on the exterior of the shelter and left for several minutes to equilibrate.   A sling psychrometer (if available) can also be used in a shaded area near the enclosure shelter.

Procedure:
1) Completely saturate wick with clean water, turn on fan for ventilation, and allow wet bulb to reach a stable temperature.  This will take from one to ten minutes depending on conditions, but the shortest possible time should be allowed, as the wet bulb reading will be invalid if wick begins to dry.  In below freezing temperatures, note whether wick is wet or frozen.  If freezing occurs, wet bulb will require at least five minutes to stabilize.

2) Read the dry and wet bulb temperatures to the nearest tenth of a degree F from scale on glass, while viewing thermometer with eye at the level of the mercury. Readings to be recorded are the lowest temperatures noted during the observing period. Turn off fan and close shelter.

3) In the office, use the round psychrometric calculator to convert dry and wet bulb temperatures to dew point temperature and relative humidity. 

4) Vapor pressure is obtained from the calculated dew point temperature with a table in the office.

Maintenance:
Replace cotton wick around wet bulb twice each year.  Wick must fit tightly around entire bulb and be attached at top and bottom with thread.  Replace wick carefully, as the narrow section of the mercury thermometer is fragile.  Occasionally clean the glass surfaces as necessary.




Maximum & Minimum Thermometers
Thermometers Use:
Record official maximum and minimum temperatures for previous 24 hour period.

Location:
Enclosure Hazen shelter

Procedure:
1) Maximum thermometer (mercury) is held in horizontal position by metal latch at all times except when reading.

2) Read maximum thermometer by releasing latch and turning to a vertical position with the bulb at the bottom.  Read temperature to nearest tenth of a degree F from scale on glass, while viewing thermometer with eye at the level of the mercury.

3) Reset maximum thermometer by spinning rapidly several times.  Check that mercury level has been reset to the current temperature.

4) Shrinkage of the mercury column is accounted for by adding a correction to the maximum temperature.  The correction (in deg F) is the product of the total drop in temperature from the maximum to the morning minimum multiplied by 0.0138 (representing the amount of column shrinkage per degree F of temperature change). The corrected maximum temperature is derived with this formula:

     Tmax(corrected) = Tmax(obs) + 0.0138 * [Tmax(obs) – Tmin(obs)]. 

If the maximum temperature was observed shortly after it occurred, this observation is used instead of the corrected maximum.

5) Minimum thermometer (alcohol) is read in its latched horizontal position before turning on the ventilation fan (to avoid any vibration from the fan affecting the reading).   Read minimum temperature at the right-most end of the floating blue index inside the thermometer to the nearest tenth of a degree F from the scale on the glass.  

6) Reset minimum thermometer by releasing its latch and turning it upside down until the blue index drops to the top of the alcohol column.  Restore thermometer to a horizontal position and replace latch so that the bulb end is slightly higher than the top end.

Maintenance:
Little is required other than an occasional cleaning of the glass surfaces.




Hygrothermograph
Hygrothermograph Use:
Continuous chart recording of temperature and relative humidity for 24-hour period ending at 8 AM EST to derive hourly temperature and six-hourly relative humidity.

Location:
Enclosure Hazen shelter

Procedure:
1) To replace the chart, raise cover, lift pens from chart with lever, and remove drum from clock mechanism.  Replace chart and elastic bands, making sure elastic bands will not interfere with chart recording and that chart is properly positioned on drum (with paper touching the bottom of the cylinder).  

2) Wind clock when necessary (every few days), turning winding mechanism no more than seven or eight times.  Stop when spring begins to feel tight.

3) Replace drum on clock mechanism and make sure pens make contact with paper at the current time.  Adjust pens up or down as necessary, using dials at right to keep recorded values close to observed values.  Close cover.

4) Hourly temperatures are obtained from the chart as follows.  Write observed hourly temperatures from previous day and current day on chart. Note the time of the highest temperature on the chart.  Mark the chart with the previous day’s observed maximum temperature and the difference between the observed maximum and the chart maximum to the nearest tenth of a deg F.  Mark the morning minimum temperature and the difference from the chart minimum temperature.  At each hour for which no observations were taken derive a smoothly-varying correction between the previous day’s maximum and the morning minimum (to the nearest tenth of a deg F).  Add this correction to the chart temperature at each hour (to the nearest tenth deg F), and write the resulting temperature for each hour on the chart (rounded to the nearest deg F). 

5) Six hourly relative humidity is obtained from the chart as follows.  Write the observed relative humidity from the previous day (usually 10 AM and 1 PM) and the current day (7 AM). Calculate corrections between the observed RH and the chart RH to the nearest percent.  Apply a smoothly-varying correction for the intervening six-hourly intervals (7 PM and 1 AM).  Add the correction to the chart RH and write value on the chart to the nearest percent.

Maintenance:
This instrument requires periodic cleaning to remove dirt or pollen that collects on the surfaces. The hair element is very sensitive and should never be over-tightened or touched with fingers.  Relative humidity readings will be inaccurate if hairs begin to separate from the hair element, which should be replaced if such damage occurs.




Digital Thermometer (Nimbus)