The wet bulb isolines are diagonal across the chart, as you see here, and there’s one place where the wet bulb and dry bulb temperatures are equal: at the saturation point.
ASHRAE PSYCHROMETRIC CHART SEA LEVEL SKIN
The New York Times had a really nice article relating wet bulb temperatures to climate change recently, and they defined wet bulb temperature as, “a measure of how well you can cool your skin by sweating.” When it’s hot outside, as it was in India this summer, that’s one of the primary ways we keep our bodies from overheating. Wet bulb temperature tells you how easy it is for water to evaporate. Take a look at a larger version of the ASHRAE psychrometric chart zoom in, you should be able to see the difference, especially if you look down at the bottom left. If you want to see the dry bulb isolines as vertical and parallel, that’s fine. There are still plenty of the old-style charts out there, but ASHRAE has settled on the Mollier version with humidity ratio on the vertical axis and specific enthalpy as the other coordinate. You can still understand the chart just fine without knowing about this detail. If that sounds like Greek to you, don’t worry about it.
One of the problems with this choice is that it makes finding specific enthalpies a bit difficult because the lines of constant enthalpy apply to saturation only, so then you have to put enthalpy deviation curves on the chart to make the adjustment for non-saturated conditions. Carrier originally used dry bulb temperature and humidity ratio as rectangular coordinates and used the various psychrometric relationships to place the rest of the quantities on the chart. That was Mollier’s big contribution to this chart. If you look carefully-very carefully-at the chart here, you can see something interesting about the dry bulb temperature isolines: They’re not all vertical and parallel to each other. When you draw lines through a chart along which the quantity under consideration remains the same, they’re called isolines. Dry bulb temperatureĭry bulb temperature is shown along the horizontal axis. Notice that the chart shown in this article says “SEA LEVEL” in big letters. When you change barometric pressure (or equivalently, altitude), you need to redraw the chart. In case you’re wondering why I said eight instead of nine, recall that barometric pressure is one of the nine I defined in part 2 of this series, and that variable determines the scale of the chart. OK, let’s jump in and place those eight psychrometric quantities on the chart and take a visual look this week. With all the computing power at our fingertips these days, we can get accurate results from smartphone apps and don’t need to try to pull precise numbers off of psych charts. In fact, the chart itself has shifted from being a great calculation tool to being more of an educational or illustrative tool.
(For more on the history and variety of psychrometric charts, see Don Gatley’s book, Understanding Psychrometrics. Aside from that, the other versions of the psychrometric chart out there are mostly specialty charts and curiosities.
Richard Mollier from Germany made an important contribution in 1923, as you’ll see in the next section. Willis Carrier was the first, with what he called the hygrometric chart in 1904.
ASHRAE PSYCHROMETRIC CHART SEA LEVEL HOW TO
On top of all that, having nine different variables means you’ve got a lot of options for how to show them in a chart.Ī lot of folks have been involved in the development of the psychrometric chart over the past century. It also can take a while to figure it all out. As you might expect, taking nine variables and putting them into one chart puts a lot of information at your fingertips. Now we’re going to delve into how we can combine those quantities and create the psychrometric chart. To this point, we’ve taken a look at what exactly psychrometrics is and defined the top nine psychrometric quantities.
0 kommentar(er)
