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Wet and Dry, a Little bit of Measuring...

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Argus

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There’s a lot of debate recently on this site (and others) around the subject of rust and its prevention on static hand tools – typically in unheated sheds and garages over the winter months.

This has led to the dreaded words “Dehumidification”, “Dew Point”, then “Psychrometrics”….. yes, “Psychr…..” put simply, it’s a fancy word for the science of the properties of air. It lends its name to a complex graph for dealing with the properties of air – its condition, or practically, “Air Conditioning”.

However, here’s an attempt to de-mystify all this by putting the complex bits we don’t need to one side and simplifying a few parts that we do need to the point where we can all deal with it without spending too much money or burning out too many brain cells.

Here goes. First, I’ll try to explain and isolate the problem from all the surrounding issues – then deal with how to come up with accurate measurements and interpret this to the point where people may devise their own solutions.

We all walk around in a gaseous soup that contains, among a lot of other things, particles of suspended water vapour. Although the extremes can be dangerous, as a species we can tolerate a wide range of temperatures and humidity – and they are linked together.

Put simply, there are two forms of humidity in air that can be measured: ‘Absolute Humidity’ – the amount of water vapour in a given volume of air by weight and ‘Relative Humidity’ – a variable amount of vapour expressed as a percentage of the potential containment at a given sensible temperature.

The former, Absolute Humidity (AH), can only vary if there is a fundamental change in the air’s properties, either through the addition of water – you boil a kettle and add steam – or you take some water out – it condenses on something. This is the basis of the whole Air Conditioning industry.

The latter, Relative Humidity (RH) will vary considerably; it is inversely linked to the sensible temperature of the air and as the temperature goes up, the RH goes down because higher air temperatures allow the air to contain a greater amount and the RH is a proportion of its containment. Conversely, if the air temperature goes down, the RH, which is inverse, goes up.

So, we know about humidity; how do we start to measure it?

If you’re still with me, and you want me to continue, let me know and I’ll show you how it can be done quite easily and most importantly, accurately with all the guess work taken out.

.
 

SammyQ

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Yes please. I welcome an objective, fundamental approach as opposed to all the 'froth' we read. There is nothing so irritating as half-digested, contextually inappropriate pseudoscience.
Sam
 

billw

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I'd love a really silly person's guide to all this. I store all my stuff in an unheated garage with little insulation and judging by the level of rust amongst the stuff that isn't mine I really need to understand how to cope with it.
 

Argus

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I began by outlining the absolute basics of humidity in air. It has its good points – we’d be in deep trouble without it but curse its effects on our tools when they begin to rust. I’m going to continue by describing how, with a little basic kit, we may begin to come up with a method of accurately measuring the size of what to all intents and purposes, invisible.

Call me old-fashioned, but I like maps.

Here in the UK we have a series of land maps – Ordinance Survey, or OS maps – that date back to a defence policy in the French wars 250 years ago. These maps contain a vast amount of information. With a knowledge of how to read them they can be as complex or as simple as you wish. Two co-ordinate readings making up two three-figure numbers will place you with a unique accuracy down to 100 metres anywhere in the UK. Unfortunately, now that we have Sat-Navs they assume a complexity they don’t deserve, but the information there will give your altitude, nearest road, phone box, pub, public toilet after the pub…… or the way home if you know how to read them.

In a moment I’ll explain how to get and read a ‘road map’ of the very basic conditions of air. As with the OS map above it will give you as much or even more information than you need, but you need just two co-ordinates to get onto it and travel anywhere from that point forward

Our intention firstly is:

  • To work out how to get the temperature co-ordinates – to measure wet and dry bulb temperatures.
  • Having got the temperatures, understand and apply them to the ‘RoadMap’ and find out how they relate to the main properties of air that we are concerned with – humidity and when we understand that, how to prevent it damaging tools.

If you search round the internet these days there are numerous meters, instruments, gadgets that will, allegedly, ‘read’ the ambient humidity. Take it from me, there is nothing more difficult to measure mechanically using little instruments with springs and bits of horse-hair than humidity.

What is even more difficult is to expect them to repeat their accuracy over time. Typically they’ll read a RH condition (They cannot see the Absolute Humidity) to within +/- 5%. That’s fine for an office wall, but no use for our purpose.

The other end of the scale we find very expensive electronic Hygrometers (that’s what they are called). Even these need calibration…. They may be better, but are again over the top for what we need.

The co-ordinates that we need consist of two simultaneous temperature readings. They are called ‘Wet-Bulb’ (WB) and ‘Dry-Bulb’ (DB) readings.

To be accurate, they must both be taken in very close proximity – side by side – in a rapidly moving sample of the air. Typically, this will consist of two identical glass thermometers, one of which has an absorbent lint sock around the sensing bulb, moistened with distilled water. Moving air is essential. They don’t provide reliable readings in still air.

As with everything else, this device has a fancy name. More than one, to be precise: ‘Whirling, Aspirated Hygrometer’ is one ‘Psychrometer’ is another….. those who use these things regularly call them simply a ‘Sling’. These two readings – the Wet-Bulb and Dry -Bulb temperatures will get us onto our ‘map’, They are the co-ordinates that will get us from point ‘A’ to point ‘B’.

You can now take a little time off to put those terms into the internet and come up with some google-pictures to illustrate what I mean. Take some time to read about Wet and dry bulb temperatures, too.

Personally, I prefer Rugby, but if you’re a football fan of a certain age, before we had vuvuzelas and before stuff was thrown at the goalie, you may remember fans went to games with a ‘Football-Rattle’, which was designed to whirl about and make as much racket as possible. (Cue a google search for images of Football-Rattles).
The thermometer Sling that I described just now resembles a football rattle except that it’s smaller and contains two thermometers, guarded inside the frame………. And it’s usually silent. No noise.

Here’s a picture of one of mine.

IMG_0633.JPG


You may see the two thermometers; it also comes with a slide-rule to calculate humidity, but we’ll do a bit better and put it onto a graph. On the side is a handle which allows it to be whirled at arm’s length. With the fabric sock moistened, the intention is to promote a simultaneous rapid air flow over both sensor bulbs. Both temperature reading will be different.

We may then apply both these temperature – our co-ordinates – to a chart or ‘road-map’ which will display all the physical qualities of the sampled air at that point. We can work out anything from this point forward.

Now, it’s possible to find these old-fashioned slings on the internet….. as wood workers we should be able to get a couple of thermometers, a bit of lint, some distilled water and make our own with some scrap wood.

If you’re still interested, next time I’ll walk you through the complicated bit – the psychrometric chart – or as I call it the road map to making sense of all this.
 

Argus

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This is the final part.........

Before we get into applying our temperatures into the Road-map or as I’ll now call it, the Psychrometric Chart (Psych-chart for short), it’s worth understanding what it is.

There are numerous versions available now, including interactive ones on the internet if you are so inclined.

The one I’ve selected is produced by ‘Carrier’, a well-known air-conditioning manufacturer, the original of which was based on the research of their founder, Willis H Carrier, and I’m pleased to acknowledge their expertise. It is in SI units, though if you prefer Imperial units, search for an American version produced by an organisation called ASHRAE. They will all be calibrated for use at sea level, because charts include data on air density which is altered by altitude.

Don’t get fazed by the amount of information it contains. This chart is capable of a huge array of calculations and projections concerning the properties of air, including temperature corrections for air density at higher temperatures, enthalpy (the energy contained in a volume of air) and a lot of other stuff. It is possible to plot and predict mechanical cooling and heating processes in detail on one of these.

However, for our purposes, looking after tools and the like, we are only going to look at what happens to air and the water vapour it contains across the range of temperatures we’ll encounter in our workshops and hopefully it’ll give you the ability to predict and deal with moisture-laden air when it approaches what is termed DEW-POINT, that is, the temperature where the relative humidity is at 100% and it cannot accept any more. This is where condensation occurs. Often where it is not wanted, like on your best planes. The chart will give you, for an input condition, the temperature at which your tools will attract condensation and resulting rust.

To get started, you’ll need to have the two temperatures that I mentioned, Dry Bulb and Wet Bulb. (DB/WB). The WB will always be lower than the DB. NEVER the other way round.

If you have in your possession, or have made, the sling device that I outlined, wet the wick with distilled water (tap-water will give an erroneous reading because of the chemicals is it contains). If you can’t get Distilled, melt a little ice from the back of the fridge. Keep the wick moist and give it a sling in the air, at about 60 – 80 RPM for about 30 seconds, or until you have a pair of stable readings. It’s crucial that you avoid any external heat that will corrupt your readings.

Note the temperatures and repeat if there is any doubt. Write them down like this: DB/WB.


Psych chart.jpg

This is a Psychrometric Chart.

The layout of the chart looks worse than it actually is for our purposes, because we are going to ignore most of it.

The bits we need are:

The Dry Bulb (DB) range - running horizontally along the bottom axis from minus 10 to 55 degrees: anything following a vertical line above a point on this axis is at the same temperature.

The Wet-Bulb (WB) range - situated diagonally to the right, along the ascending curve to the left, from minus 10 to 33 and beyond. (This scale coincides with the lines of constant enthalpy, but ignore those values). Any Value along one of these lines is at the same temperature.

In between is a series of curved lines showing a percentage; thesis are lines of Relative Humidity (RH). If your resulting condition is between any two, interpolate the value.

The right-hand vertical axis of the graph shows the moisture content by weight for a given quantity of air and follows a horizontal line. Any value along this line shares the same amount of moisture.

IGNORE ALL THE OTHER DIAGONAL LINES, SCALES AND CURVES.

Earlier, with a December morning outdoor ambient temperature of about 4 degrees Celsius, I took a couple of readings.

One in my indoor workshop, which although it’s heated, I keep around the frost-protection mark on the Rad-valves; the other in my enclosed, unheated garage. The former has all my tools, the latter is where I store timber and a band saw.

The readings were:

  • Workshop: 11 DB / 9 WB (11/9)
  • Garage: 5 DB / 4 WB (5/4)

We’ll use this as an example and input it into a Psych chart and see what we have, starting with the workshop. The unheated garage may follow, but the method is identical.

On the lower base axis, proceed from left to right. The units are in whole degrees and 0.2 divisions. Stop at 11 degrees.

At the same time, on the Wet Bulb or Saturated Temperature Scale on the left-hand curve – locate 9 degrees. Trace a line vertically up from 11 and diagonally downwards from the 9-degree point. Where these two meet are the conditions of the air in my workshop at that time.

It’s as simple as that. If you are doing this, don’t be worried about fractions of a degree or interpolating between given values.

On my chart, the meeting point, or Condition of the Air is almost exactly at 80% RH; going horizontally right, I can see from the vertical scale that the air contains 0.0065 (interpolated) kg of water vapour per kg of dry air. If I then proceed on this horizontal line to the extreme left of the chart – to the WB scale -I will see a temperature of 7 degrees which at this condition is 100% saturation in terms of humidity.

This is the often-mentioned DEWPOINT.

If any more water vapour is added to the air with no change of (Sensible) or DB temperature, the air cannot accept it and it will drop out. Conversely, any surface that achieves that temperature at those conditions, will attract condensation until the air reaches stability.

That is how we get rust on iron tools.

It is unimportant how much moisture the air contains or, within reason, what the temperatures are. If a surface is at the DEWPOINT for that condition it will get wet. It follows that if the surface is maintained just a little above that, no condensation occurs and the tool remains rust-free.

I hope that this has removed the mystery on this subject, simplified the main points and showed you how to measure it, predict its occurrence and come up with a plan to deal with it.
 
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Sgian Dubh

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The readings were:
  • Workshop: 11 DB / 9 DB (11/9)
  • Garage: 5 DB / 4 WB (5/4)
Clearly explained. Excellent information.

I think I may have spotted a small error as follows that you can probably edit to correct, see your original above:

  • Workshop: 11 DB / 9 WB (11/9) rather than 9 DB. Slainte.
 

Argus

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Thank you Richard, Well spotted and corrected now.
I did say that the WB was always lower than the DB, then went right ahead and achieved the impossible!
 

artie

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Thank you very much.
That is the best description I have come across, in words with few enough syllables for me to follow.
 

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