{"id":4511,"date":"2023-11-08T15:00:00","date_gmt":"2023-11-08T15:00:00","guid":{"rendered":"https:\/\/www.bcta.group\/attma\/?p=4511"},"modified":"2023-10-27T15:44:24","modified_gmt":"2023-10-27T14:44:24","slug":"understanding-air-density","status":"publish","type":"post","link":"https:\/\/www.bcta.group\/attma\/2023\/understanding-air-density\/","title":{"rendered":"Understanding Air Density"},"content":{"rendered":"\n
\"\"<\/a><\/figure>\n\n\n\n

Air density is calculated from your environmental readings and it applies several corrects to the flow calculated from your flow pressure readings.<\/p>\n\n\n\n

How is air density calculated?<\/h5>\n\n\n\n

It\u2019s best to simplify this by looking at how to calculate air density without factoring in that it may contain water.\u00a0 Air density for dry air is calculated as below:<\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n

So from this we can see that both the barometric pressure and temperature are being used in the calculation. Both and internal (\u03c1i<\/sub><\/em>) and external (\u03c1e<\/sub><\/em>) air density are calculated.<\/p>\n\n\n\n

Location<\/th>Temperature<\/th>Barometric Pressure<\/th>Air Density (Dry Air)<\/th><\/tr><\/thead>
Internal (\u03c1i<\/sub><\/em>)<\/strong><\/td>20\u00b0C<\/td>1000 hPa<\/td>1.1884<\/td><\/tr>
External (\u03c1e<\/sub><\/em>)<\/strong><\/td>10\u00b0C<\/td>1000 hPa<\/td>1.2303<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The equations for the internal and external air density are as shown below:<\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n
What happens when we consider water vapour?<\/h5>\n\n\n\n

Water as a gas is called water vapour, which occurs when water as a liquid evaporates.  Water vapour has less weight to it than both Nitrogen and Oxygen which together form 99% of air.  The difference in the molecular mass of water vapour (18.016 g\/mol) and dry air (28.966 g\/mol) is that water vapour has a molecular mass 37.802% less than dry air.  Therefore the more water vapour in air the lighter it is.<\/p>\n\n\n\n

This is why considering water vapour present in air applies a further adjustment to the air density calculation.\u00a0 This adjustment is applied to the absolute pressure by modifying the previous calculation as follows:<\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n

The equation to calculate the water vapour pressure is shown below.<\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n
\"\"<\/a><\/figure>\n\n\n\n

The other 3 numbers are constants and we also assume 50% relative humidity in the ATTMA test standards rather than measuring it.<\/p>\n\n\n\n

The equations for the internal and external air density are as shown below:<\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n

The comparison with and without the water vapour adjustment are shown below.<\/p>\n\n\n\n

Location<\/th>Air Density (Dry Air)<\/th>Air Density (Water Vapour)<\/th><\/tr><\/thead>
Internal (\u03c1i<\/sub><\/em>)<\/strong><\/td>1.1884<\/td>1.1823<\/td><\/tr>
External (\u03c1e<\/sub><\/em>)<\/strong><\/td>1.2303<\/td>1.2274<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Water vapour therefore makes a minor adjustment to the air density but as these calculations are handled in the background within your software there\u2019s no reason to not do these adjustments.<\/p>\n","protected":false},"excerpt":{"rendered":"

Air density is calculated from your environmental readings and it applies several corrects to the flow calculated from your flow<\/p>\n","protected":false},"author":3,"featured_media":4520,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[29],"tags":[],"class_list":["post-4511","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-news"],"publishpress_future_action":{"enabled":false,"date":"2026-05-07 07:39:13","action":"change-status","newStatus":"draft","terms":[],"taxonomy":"category","extraData":[]},"publishpress_future_workflow_manual_trigger":{"enabledWorkflows":[]},"_links":{"self":[{"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/posts\/4511","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/comments?post=4511"}],"version-history":[{"count":2,"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/posts\/4511\/revisions"}],"predecessor-version":[{"id":4521,"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/posts\/4511\/revisions\/4521"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/media\/4520"}],"wp:attachment":[{"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/media?parent=4511"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/categories?post=4511"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bcta.group\/attma\/wp-json\/wp\/v2\/tags?post=4511"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}