HOMS COMPONENT

EVAPORATION PAN

1.   Purpose and objectives

     An instrument used to measure daily amount of water lost through evaporation.

2.   Description

     The instrument consists of the following items:

     i) Standard U.S. Class "A" evaporation pan with standard dimensions
        of 1207mm internal diameter by 254mm depth made of aluminium plate; 
    ii) Fixed point gauge;
   iii) Storage tank of water for replacing water lost from the pan by 
        evaporation;
    iv) Standard daily rain gauge (C27.1.02) or automatic rain gauge.

     The pan is installed on a wooden platform set on the ground with natural grass condition. The exposure of the pan should be free from obstructions that cast shadows over the pan. To measure the evaporation, a graduated measuring can having a surface area 1/100 that of the pan is used
to refill the pan up to the level of the point gauge in a stilling chamber. The amount of water added is equivalent to the evaporation measured for the day.

3.   Input

     Actual water added or removed from the pan.

4.   Output

     Daily evaporation measured by the appointed observer.

5.   Operational requirements and restrictions

     A standard manual rain gauge should be installed alongside the evaporation pan. This is to facilitate the calculation of daily evaporation on days when rainfall occurs during the day. For installation of rain gauge, please refer to HOMS component C27.1.02. The pan should be cleaned as frequently as necessary to keep it free from sediment, scum and oil films. Inspection for leaks should be carried out regularly as leaks will render the measurements of no use.

6.   Form of presentation

     Technical drawings for local manufacture are available.

7.   Operational experience

     It has been used satisfactorily as a standard instrument for daily evaporation measurements for a few years by the Drainage and Irrigation Department, Malaysia.

8.   Originator and technical support

     Drainage and Irrigation Department, Malaysia.

9.   Availability

     From the HOMS National Reference Centre for Malaysia.

10.  Conditions on use

     Technical drawings available free of charge. Purchase of the instrument, either complete or as individual items, from the local manufacturer in Malaysia can be arranged.

Last updated: 28 MAY 99)
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