This month we are diving into some big-brain concepts regarding structural drying. I think it is safe to say there are a ton of restorers actively working in the industry who do not understand even the most basic concepts of vapor pressure and the crucial role it plays in drying. I could be wrong – but I doubt it – especially when it comes to the “old school” restorers. Each time I have taught WRT, students have a noticeable “AHA” moment when they learn how they can use vapor pressure differentials to formally evaluate the effectiveness of the drying environment – a way to measure drying performance. Before we dig into this, let’s figure out why this is so important. Let’s say you believe you have a drying chamber setup perfectly. On day-0 your target material is at 45% moisture content, and when you monitor progress on day-1, the moisture content has reduced to 36%. You’ve made progress – that’s good, but is it the best you could do? How do you know? For many years there was no way you could know – that’s changed my friends. Buckle up – here we go…
Many of you know Vapor Pressure appears at the far-right vertical column of the Psychrometric Chart and represents a physical measurement (volume) of water vapor in the air. We must first understand how to relate vapor pressure to a measurable amount of water vapor molecules in the air, and to do that, we must understand what vapor pressure means. The concept is pretty simple – water vapor molecules in a given space naturally repel each other, and that creates a measurable pressure. As the number of water vapor molecules increases, so does the pressure. Hence – higher vapor pressure equates to an increased volume of water vapor molecules.
The next concept we must understand is that vapor pressure, like all pressures, seeks equilibrium (high pressure goes to low pressure). Let’s simplify this by using the example of an open drying system (you know what that is, right? If not – you should). You have an isolated bedroom in a house where the carpet/pad is heavily saturated, causing significant naturally occurring evaporation, so the air in that room has a very high vapor pressure. Outside the air is very dry so it has a lower vapor pressure. If you open the windows in that room, the high vapor pressure air will quickly migrate to the outside until, eventually, their pressures will equalize.
These concepts were introduced in 1802 by John Dalton, an English chemist, physicist, and meteorologist, and we use the principles of his Law of Partial Pressures on every drying project.
- John Dalton was color blind (colour according to him) – I’m color blind!
- John Dalton was a Quaker – I was raised on Quaker oatmeal!
- John Dalton is like to Mac-Daddy of drying – Ok, he’s got one up on me there, but…
The similarities are uncanny, right? Anyhoo – let’s continue nerding out…
Like vapor pressure of the air, water, in liquid form, also has a measurable vapor pressure. Picture a pot of water on a stove. With the burner off, natural vapor pressure will cause the water to slowly evaporate. Add energy to the water by turning the stove burner on high, and that pressure will quickly increase – speeding the rate of evaporation.
Now let us take all these concepts and translate them to a structural drying project. Our goal is to create vapor pressure differentials – the difference between the vapor pressure of the water and the vapor pressure of the air. We understand high pressure seeks low pressure, so our objective is to increase the water vapor pressure and decrease the air vapor pressure. The greater the difference – the greater the rate of evaporation.
VPw = vapor pressure of the water remaining on wet materials. We increase this by adding energy (airflow, direct heat)
VPa = vapor pressure of the air. We decrease this by reducing the amount of water vapor molecules in the air (dehumidification)
The “so what” to all of this is we can now use the vapor pressure differential as a measurement of our drying performance. To do this, we must add another tool to our tool kit – an IR thermometer. Since we know water (liquid) vapor pressure is driven by temperature, we must know the temperature of the water remaining on wet materials to calculate its vapor pressure.
Clear as mud? Don’t worry. My good friend and one of the smartest restorers I know, Jeremy Reets, developed an APP (Reets DryCalc) which performs all the big brain calculations, based on three simple inputs (ST = surface temperature of target material, Air temp, and Relative Humidity), and displays the vapor pressure differential (referred to as Evaporation Potential – EP). This EP is essentially your score for how well you have setup your drying conditions. His research has determined an EP of 1.5 – 3.0 is optimum for conventional drying (drying using LGR type dehumidifiers).
For me as a restorer – this was a game changer. I could now tell if my daily progress from 45% moisture content to 36% moisture content was as good as I could do. If my EP was not at least 1.5, I did not create optimum conditions and likely could have had better progress. My focus then would be to increase the surface temperature (increase the vapor pressure of the water in the material) and continue to decrease the vapor pressure of the water in the air through dehumidification – it’s just that simple.
I’m sharing this information with you this month because understanding and employing these concepts on every structural drying project is important. It’s important for you as a professional restorer, and important for our industry so others know we are no longer just shooting from the hip. If you’re not employing these concepts, does it make you a terrible restorer? Yes, yes it does. Ok – I’m kidding, it does not, BUT – those that do are likely better restorers than those that don’t – let that sink in.
Until next month –
Nasty 7 out.