Sanitary district chairmen

When I checked the history section of the website of Wang Nam Yen municipality in order to prepare the posting of the enlargement of the municipality, apart from the dates I already knew - created in 1980, updated in 1999, a third date in 1994 was listed. Roughly translated, the history reads as follows

The creation of sanitary district Wang Nam Yen was announced in the Royal Gazette on April 22 1980. On June 3, the municipality located in the minor district Wang Nam Yen got Pratip Chirawat (นายประทีป จิราวัฒน์), the district officer of Sa Kaeo district, as its first chairman. In 1994, the Ministry of Interior announced Wang Nam Yen as a sanitary district with adequate income to manage sanitation itself. Wanchai Narirak (นายวันชัย นารีรักษ์) was the first chairman who came into office by election.

So it was in the 1990s that sanitary district got electing chairmen (ประธานกรรมการสุขาภิบาล). I wasn't aware of this change before, as the sanitary district are rarely mentioned in the books and papers I have read so far. The history section of the website also mentions the Royal Gazette announcement in which this change was promulgated, so I was able to find a total of nine announcements, which each lists a varying number of sanitary districts which have reached adequate income - in Thai they are all titled ประกาศกระทรวงมหาดไทย เรื่อง รายชื่อสุขาภิบาลที่มีฐานะการคลังเพียงพอที่จะบริหารงานประจำของสุขาภิบาลได้. In detail the nine announcements are as follows:

• September 9 1987: 18 sanitary districts [Gazette]
• October 1 1988: 4 sanitary districts [Gazette]
• February 15 1990: 10 sanitary districts [Gazette]
• July 7 1992: 21 sanitary districts [Gazette]
• April 8 1993: 48 sanitary districts [Gazette]
• July 7 1994: 34 sanitary districts [Gazette]
• July 24 1995: 51 sanitary districts [Gazette]
• May 30 1996: 79 sanitary districts [Gazette]
• June 16 1997: 62 sanitary districts [Gazette]

If adding the numbers these are 327 sanitary district, however there were 980 of these entities - therefore most of them did not yet have elected chairmen when they were upgraded into subdistrict municipalities in 1999.