
<p><em>Photo by <a href="https://www.pexels.com/@filirovska">Julia Filirovska</a> on <a href="https://www.pexels.com/photo/wet-transparent-misted-window-with-question-mark-4913769/">Pexels</a>.</em></p>
<p><strong>Contents</strong></p>
<ul>
  <li><a href="#dew-point">Dew point</a></li>
  <li><a href="#absolute-vs-relative-humidity">Absolute vs. Relative humidity</a>
    <ul>
      <li><a href="#absolute-humidity">Absolute humidity</a></li>
      <li><a href="#relative-humidity">Relative humidity</a></li>
      <li><a href="#example-in-an-imaginary-space">Example in an imaginary space</a></li>
    </ul>
  </li>
  <li><a href="#the-psychrometric-chart-explained">The psychrometric chart explained</a>
    <ul>
      <li><a href="#dry-bulb-temperature">Dry bulb temperature</a></li>
      <li><a href="#wet-bulb-temperature">Wet bulb temperature</a></li>
      <li><a href="#relative-humidity-1">Relative humidity</a></li>
      <li><a href="#absolute-humidity-1">Absolute humidity</a></li>
      <li><a href="#dew-point-or-saturation-temperature">Dew point or saturation temperature</a></li>
      <li><a href="#a-quick-note-on-dew-point-vs-wet-bulb">A quick note on dew point vs. wet bulb</a></li>
      <li><a href="#the-psychrometric-chart-again-in-full-colour">The psychrometric chart again in full colour</a></li>
      <li><a href="#a-psychrometric-chart-and-dew-point-example">A psychrometric chart and dew point example</a></li>
      <li><a href="#what-can-the-psychrometric-chart-do-for-you">What can the psychrometric chart do for you?</a></li>
    </ul>
  </li>
  <li><a href="#lets-go-back-to-dew-point">Let's go back to dew point</a>
    <ul>
      <li><a href="#what-does-this-mean">What does this mean?</a></li>
    </ul>
  </li>
  <li><a href="#useful-links">Useful links</a></li>
</ul>
<h2 id="dew-point">Dew point</h2>
<p>What is the dew point, in simple terms? When you look at your environmental monitoring charts, you are used to seeing the temperature and relative humidity lines. However, there is also another line. The dew point line, and this line can be a bit trickier to interpret.</p>
<p>What does it tell us? I have found several explanations of dew point, so I will attempt to sum it up here as clearly as possible:</p>
<ul>
  <li>Dew point is read as a temperature. You will see it in °F or °C.</li>
  <li>It represents the absolute moisture content of the air.</li>
  <li>Dew point is the temperature at which Relative Humidity (RH) in your space is at 100% so that the moisture will start condensing out of the air.</li>
  <li>This means that the dew point is the temperature at which dew will begin forming in your space.</li>
  <li>If your dew point temperature is close to your dry air temperature, your RH is high.</li>
  <li>If your dew point is well below the dry air temperature, your RH is low.</li>
  <li>In other words, it is not something you want to be too close to your dry air temperature if you want to keep your things dry.</li>
</ul>
<p>
  <img src="/DewPoint-Graph.jpg" alt="A regular environmental monitoring graph"><br><em>This is a space without any HVAC control. There is no artificial humidifying or dehumidifying or heating or cooling. If you look closely, you will notice that the dew point line is basically the same as the temperature but a few degrees below it.</em>
</p>
<h2 id="absolute-vs-relative-humidity">Absolute vs. Relative humidity</h2>
<p>Before going a bit more into dew point, it's important we understand two concepts clearly. You will find much better, more scientific definitions in physics sites, but the following are supposed to be straightforward and clear.</p>
<h3 id="absolute-humidity">Absolute humidity</h3>
<ul>
  <li>Assuming constant atmospheric pressure, absolute humidity is the actual amount of water (in weight) that can be "held" by air at a particular temperature. It's actually more about vapour pressure equilibrium, but let's not overcomplicate for now and use the word "hold".</li>
  <li>It is expressed as weight of water per weight or volume of air. So it can be in grams of water per kilograms of dry air. Or grams of water per cubic metre of air. Or similar measurements using feet and pounds.</li>
</ul>
<h3 id="relative-humidity">Relative humidity</h3>
<ul>
  <li>Relative humidity is that amount of water expressed as a percentage of the total amount of water that can be "held" by air at that same particular temperature and pressure.</li>
  <li>It is expressed as a percentage.</li>
</ul>
<h3 id="example-in-an-imaginary-space">Example in an imaginary space</h3>
<table>
  <thead>
    <tr>
      <th>Absolute humidity in g/m3</th>
      <th>Temperature in °C</th>
      <th>Relative humidity %</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>5</td>
      <td>10</td>
      <td>53.2</td>
    </tr>
    <tr>
      <td>5</td>
      <td>20</td>
      <td>28.9</td>
    </tr>
    <tr>
      <td>5</td>
      <td>30</td>
      <td>16.5</td>
    </tr>
  </tbody>
</table>
<p><em>Numbers calculated using <a href="http://www.michell.com/calculator/">Michell instruments calculator</a></em></p>
<p>If you look at the above example, you can see that in an imaginary space that holds 5g of water per cubic metre of air, the relative humidity will vary depending on the temperature even though the amount of water in the space has not changed.</p>
<p>This helps explain how, if the only thing you can control is temperature, you can lower it to increase humidity, or increase it to lower humidity. There is an inverse relationship.</p>
<h2 id="the-psychrometric-chart-explained">The psychrometric chart explained</h2>
<p>The psychrometric chart is a really intimidating looking graph with a ton of lines going in 6 different directions. For the sake of clarity and because conservators don't really need to use them, I have not drawn in the enthalpy or volume lines. Let's leave those to the engineers and HVAC specialists. Don't worry about the actual numbers in the following charts. These are not strictly accurate charts, but are only being used diagramatically.</p>
<p>
  <img src="/Psychrometric-Black.png" alt="Simplified psychrometric chart"><br><em>Look at all those crazy lines. Notice that there are temperatures, absolute humidity and relative humidity in there. But which is which and going where and what is this and what do we do with it?</em>
</p>
<p>Before we begin parsing this out, the answer to your question is <strong>YES</strong>.<br>What is your question? Probably</p>
<blockquote>
  <p>Isn't there an online calculator for this horrible thing instead? <strong>YES</strong></p>
</blockquote>
<p>You can use the Mitchell humidity calculator that I linked above with the table, or you can use the <a href="https://www.imagepermanenceinstitute.org/">Image Permanence Institute</a> Dew Point Calculator, which I link below in the links. That said, you might not always have access to the internet (Crazy! I know!), so why not understand the vintage way of going about it?</p>
<p>Let's separate things line by line...</p>
<h3 id="dry-bulb-temperature">Dry bulb temperature</h3>
<p>
  <img src="/Psychrometric-Dry.png" alt="Dry bulb temperatures in orange"><br><em>The dry bulb temperature is the fancy way of saying what we normal people mean by temperature. It is the x-axis at the bottom of the graph and will be seen going upwards. You could also say it is read from left to right in the sense that the lines are vertical but going left to right.</em>
</p>
<h3 id="wet-bulb-temperature">Wet bulb temperature</h3>
<p>
  <img src="/Psychrometric-Wet.png" alt="Wet bulb temperatures in green"><br><em>The wet bulb temperature is traditionally measured by using a thermometer with a water-soaked cloth around it. The temperature you get will be affected by the evaporation of water from the cloth. If you have <strong>100% relative humidity</strong> (blue), the wet bulb temperature (green) will be the same as the dry bulb temperature (orange) because they will both be essentally surrounded by water (Notice the 15°C I have marked for you in light yellow). To get a wet bulb temperature reading, either your manual thermohygrometer has to be digitally setup to tell you the wet bulb temperature, or you will have to use one of these old-style <a href="https://www.preservationequipment.com/Catalogue/Instruments/Hygrometers/Whirling-Hygrometer">whirling hygrometers</a> that have two thermometers in them. These readings have diagonal (green) lines that cross towards the bottom right and use the (blue) 100% RH line as the base axis.</em>
</p>
<h3 id="relative-humidity">Relative humidity</h3>
<p>
  <img src="/Psychrometric-RH.png" alt="Relative humidity in blue"><br><em>The relative humidity in a psychrometric chart can be found as curling curves that get wider as they go from the bottom left towards the top right of the chart. It's very easy to tell it apart from the other readings because it's a percentage number.</em>
</p>
<h3 id="absolute-humidity">Absolute humidity</h3>
<p>
  <img src="/Psychrometric-AH.png" alt="Absolute humidity in pink"><br><em>Absolute humidity will be running as the y-axis on the right of the chart and will have to be read using the pink numbers on the right axis. Absolute humidity rfers to the amount of water being "held" in your air.</em>
</p>
<h3 id="dew-point-or-saturation-temperature">Dew point or saturation temperature</h3>
<p>
  <img src="/Psychrometric-Dewpoint.png" alt="Dew point in yellow"><br><em>The dew point will be read by moving horizontally (yellow) using the same lines as if you were reading absolute humidity (pink), but <strong>the number readings will use the (blue) 100% RH curve line as the base axis</strong>, just like with the wet bulb temperature (green). <strong>This does not mean that wet bulb temperature and dew point (also called saturation temperature) are the same number (they are not), they are just sharing the same 100% RH curve for readings.</strong> Go back to the wet bulb graph above and compare it with this image if you need to. The wet bulb (green) temperature is read <strong>diagonally</strong> while the dew point (yellow) temperature is read <strong>horizontally</strong>.</em>
</p>
<h3 id="a-quick-note-on-dew-point-vs-wet-bulb">A quick note on dew point vs. wet bulb</h3>
<p>Just in case all this talking about absolute humidity, relative humidity, dew points, wet and dry bulbs confused you. Here is a quick table to explain the difference between the wet bulb and the dew point.</p>
<table>
  <thead>
    <tr>
      <th></th>
      <th>Dew point temperature</th>
      <th>Wet bulb temperature</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Definition</td>
      <td>Temperature at which Relative Humidity (RH) in your space is at 100% so that the moisture will start condensing out of the air.</td>
      <td>Temperature you get from a moistened thermometer bulb exposed to air flow. Affected by the evaporation of the water.</td>
    </tr>
    <tr>
      <td>At high RH</td>
      <td>Close to dry bulb temperature</td>
      <td>At 100% RH, it equals dry bulb temperature. Notice that at 100% in the graph below, the blue and green lines meet.</td>
    </tr>
    <tr>
      <td>At low RH</td>
      <td>Below dry bulb temperature. At 10%RH, your dew point will be around 5°C. Read along the 100% blue line at the blue 10% level.</td>
      <td>Low. See graph below. At 10%RH your green wet bulb will be about 20°C while your orange dry will be around 43°C.</td>
    </tr>
  </tbody>
</table>
<h3 id="the-psychrometric-chart-again-in-full-colour">The psychrometric chart again in full colour</h3>
<p>
  <img src="/Psychrometric-Colours.png" alt="Absolute humidity in pink"><br><em>Now that you have seen all the lines coloured in one by one, this psychedelic graph should be less shocking. Hopefully, it is also much less intimidating than the original one where all the lines were black.</em>
</p>
<h3 id="a-psychrometric-chart-and-dew-point-example">A psychrometric chart and dew point example</h3>
<p>Why did we go through all that? The point of the psychrometric chart is that you should be able to get any of the variables as long as you have at least two other variables that you can cross-reference in the graph.</p>
<p>
  <img src="/Psychrometric-Example.png" alt="An example using the psychrometric chart"><br><em>Let's take a look at this example. Imagine you measure a space with (orange) 40°C and (blue) 54% relative humidity. Unpleasant, I know. But just look at the numbers. Using the chart, you can find the place where the orange 40°C dry bulb and the blue 54% RH intersect (red dot). This will tell you that your space has (pink) 25g of water per kg of dry air. If you move your eyes <strong>horizontally</strong> to the left and read along the 100% RH line, this means your dew point is ~28°C (yellow). Should you require it, look at the <strong>diagonal</strong> green line, and you will tell that your (gren) wet bulb temperature is 31°C.</em>
</p>
<p>
  <img src="/Psychrometric-Example2.png" alt="An example using the psychrometric chart"><br><em>Now imagine that you don't have any dehumidifiers or humidifiers, but you manage to lower the temperature to 35°C. If you look at your graph now, your (green) wet bulb temperature has moved down to 30°C. You should still have 25g of water per kg of dry air, but now you have about 70% RH (blue). Notice your dew point has not moved because the amount of water (pink) has not changed and this number is read <strong>horizontally</strong>. It is still at ~28°C (yellow).</em>
</p>
<h3 id="what-can-the-psychrometric-chart-do-for-you">What can the psychrometric chart do for you?</h3>
<p>Once you become comfortable reading a psychrometric chart and following the lines in the correct directions, the chart is a powerful tool. It means that once you know a couple of variables, you can calculate the others. This, in turn, means that if you want to change variables, you know what other variables you need to move in what direction in order to get what you want.</p>
<p>You should be able to calculate how many grams of water you need to remove from a space, and therefore how many hours you should work your dehumidifier. Or, on the contrary, if you want a certain RH%, you can decide whether you should turn on the AC or the heating to get it.</p>
<p>Feel free to play with <a href="https://www.weather.gov/epz/wxcalc_dewpoint">this other calculator</a> and then compare those numbers using this chart. Make up some exercises for yourself and see if you get it right using the charts. Remember my graphs are not precise, so look for rough number similarities to know you are correct. <strong>Tip on how to use this calculator:</strong> Use 1013.25 millibars for your pressure to keep it standard. Plug in your test numbers on the left and click the Convert button to see your results for wet bulb temperature and RH on the right.</p>
<h2 id="lets-go-back-to-dew-point">Let's go back to dew point</h2>
<p>Remember how I pointed out in the example above that even though you reduced temperature and the RH% dropped accordingly, the theoretical dew point didn't change? This is because the amount of water in the air didn't change. It means that while you were messing around in that imaginary room, nothing else happened to mess with your numbers. This imaginary room is not a real-life situation. In real-life situations, other things happen in your room. For example, leaks, spills, system changes, windows or doors opening, etc.</p>
<h3 id="what-does-this-mean">What does this mean?</h3>
<p>This means that when you have a supposedly closed space, your dew points should remain relatively stable and should reflect your temperature readings just like the ones in the graph at the beginning of the article.</p>
<p>However, if you notice a sudden spike or drop which differs completely from the temperature line, then something has happened. Being able to identify this will help you investigate and decide if everything is still okay.</p>
<p><strong>A sudden change in your dew point will indicate a sudden change in the absolute humidity of the air in your space.</strong></p>
<p>Chances are this change can be easily explained.</p>
<ul>
  <li>Maybe it means someone opened a window and it was really damp/dry outside compared to your room.</li>
  <li>Maybe you got a leaking pipe that is adding water to your room.</li>
  <li>Maybe someone turned on/off a de/humidifier.</li>
  <li>Maybe Cinderella came in and washed all your floors.</li>
</ul>
<p>I hope this explanation of dew point, humidities and psychrometric charts helps you out wherever you are. I'm not a physicist or HVAC specialist, so do let me know if I didn't explain something quite right, and I will make sure to fix it.</p>
<h2 id="useful-links">Useful links</h2>
<ul>
  <li><a href="http://www.dpcalc.org/">Dew Point Calculator by the Image Permanence Institute</a></li>
  <li><a href="https://s3.cad.rit.edu/ipi-assets/publications/sustainable_preservation_practices.pdf">Image Permanence Institute Guide to Sustainable Preservation Practices for manageing storage environments</a>. See chapter 5 for dew point.</li>
  <li><a href="https://www.gasparetto.it/en/psychrometric-chart-dew-point">Gasparetto Engineering</a></li>
  <li><a href="https://demonstrations.wolfram.com/ReadingAPsychrometricChart/">WOLFRAM Demonstrations Project</a></li>
</ul><br><br><br><br>
<p>If you liked this post, you can follow me on <a href="https://twitter.com/ConservaLlama">Twitter</a>, check out my newest <a href="https://www.instagram.com/a.isa140/">Instagram experiments</a> or connect with me on <a href="https://www.linkedin.com/in/angelicaisaadaniya/">Linkedin</a>. You can also sign up for the newsletter below to get my latest articles straight in your inbox.</p>


A colour coded explanation on how to read psychrometric charts and understand dew point.

Understanding psychrometric charts and dew points


<h2 id="what-is-this">What is this?</h2>
<p>When I first started working, I didn't have dataloggers or any way of monitoring some of the spaces where we were storing collections. I also don't have an HVAC system or any sort of automatic or even semi-automatic way of controlling the environment in spaces. All control requires a person turning something on or off, opening or closing a door, etc.</p>
<p>I needed to make decisions about placement of shelves, furniture, collections and dehumidifiers, but I didn't really know what was going on in these spaces. A datalogger could tell me general information, but I needed detailed knowledge about different areas within the same space, so I came up with a simple environmental monitoring solution that can be applied when you have no budget at all.</p>
<h2 id="why-this-model-works-for-me">Why this model works for me</h2>
<p>I have run this model 3 times. Once in 2016, and twice in 2017 - in the summer and in the fall. I believe I should probably run it a few more times in different weather conditions to see how things change over the changing seasons in the year. It will not look the same throughout. However, knowing how the humidity looks in my spaces has helped me decide on where to place my dehumidifiers and what to do about opening and closing doors at different times of the year.</p>
<p>Keep in mind that this is a basic modeling tool I came up with. It may not necessarily work for you. It might not be worth doing if you have more sophisticated systems for monitoring and controlling the environment. However, if you ever find yourself in a situation similar to mine, let me know if you tried it out and if it worked for you!</p>
<h2 id="what-you-need">What you need</h2>
<ul>
  <li>A small step ladder (to take the higher readings)</li>
  <li>A handheld thermohygrometer</li>
  <li>A bit of time to take your measurements. It does take a while if you do a lot of spaces.</li>
  <li>Microsoft Excel</li>
  <li>This <a href="https://drive.google.com/file/d/1oM3SRanaAIO0iop56KxlkZ5wvfK4aOtI/view?usp=sharing">template</a> if you don't feel like making your own from scratch. You can mess with it however you wish for your own personal use, but please download it first. Don't change the original.</li>
</ul>
<p>Obviously, this method will be hindered by unmoveable furniture in your way depending on how your spaces are set out, but you can probably figure out if this is still possible working around the obstacles.</p>
<h2 id="the-basic-concept">The basic concept</h2>
<p>The idea behind this model is extremely simple.</p>
<p>
  <img src="/HRmodeling-example.png" alt="Square room divided into a 3x3 grid"><br><em>Figure our your space and divide it into a grid. The more measurements you take, the more detailed your final model will be. Be aware that less than 9 grids will not give very satisfactory results. I know from personal experience that just four or five points (corners and centre) does not work at all.</em>
</p>
<ol>
  <li>Map out your space. To make it easier, think of it as a square. Naturally, if your space is rectangular, use a rectangle and more grids. Know the location of your doors or other openings such as ventilation systems or windows to help you with later interpretation.</li>
  <li>Take temperature and relative humidity measurements at 3 different heights (ground level, 1m/3ft and 2m/6ft off the ground) at each different grid of your space. If you have really high ceilings, you might have to take more measurements heightwise. Make sure you step away from your sensors while the measurement is being made so your breathing and body temperature do not affect it and that you have a standardised way of taking the measurements. For example, waiting x number of seconds before reading or resetting the equipment in between reads. <strong>Note: I have mapped temperatures too but the variations were so small, they didn't really give a useful model for me, so unless you're particularly concerned about temperature, it might not be worth doing a model for that.</strong></li>
  <li>Turn the information you get into a <strong>3D surface chart</strong> in Excel.</li>
</ol>
<h2 id="making-your-3-d-surface-charts-in-excel">Making your 3D surface charts in Excel</h2>
<p>
  <img src="/HRmodeling-3Dsurfacechart.jpg" alt="Screenshot of 3D surface charts in Excel"><br><em>Screenshot of 3D surface charts in Excel. This is the one you will have to use, although you can play around with the contour or 2D versions if you prefer.</em>
</p>
<h3 id="1">1</h3>
<p>Copy over your data into an Excel table that replicates the room grid you made. Make a table for each height level. I am using metres.</p>
<p>
  <img src="/HRmodeling-Exceltables.jpg" alt="Excel tables"><br><em>Excel tables replicating the spaces measured. Note that I have coloured in the wettest and driest numbers for my own benefit.</em>
</p>
<p>I am using the cardinal points to name the different areas in the room. I have also pointed out where the door is. You can basically equate these tables to the square of the grid you made.</p>
<p><strong>Important:</strong> Please note that when the graph gets drawn, <em>the column names at the top of your tables will actually show up at the bottom of your graph as the X-axis</em>, so don't put the names of the top locations on those columns. This is why I have left them empty. If you don't think it will confuse you, you can put the names of the bottom locations of your room in the names of the columns.</p>
<h3 id="2">2</h3>
<p>Organise your graphs so that you can see the highest at the top and the ground level at the bottom.</p>
<p>
  <img src="/HRmodeling-Models.jpg" alt="Screenshot of 3D surface chart results in Excel"><br><em>3D surface chart results in Excel. Make sure that you colour code your graph consistently and that you are using the same axis formatting so that you can compare across graphs.</em>
</p>
<h3 id="3">3</h3>
<p>Interpret your data and make your decisions.<br>Some of the things you can see in the above results for an imaginary space are:</p>
<ul>
  <li>In the winter, humidity seems to be coming in through the door and is highest at ground level.</li>
  <li>In the summer, for some reason, the NW corner of this room is quite humid, particularly at ground level.</li>
</ul>
<p>Decisions you can make based on this:</p>
<ul>
  <li>Close the door in the winter</li>
  <li>Place dehumidifiers in the NW corner in the summer and near the door in the winter. Try to find out why that corner is more humid than the rest of the room, so you can take action.</li>
  <li>Avoid placing humidity-sensitive objects close to the ground.</li>
</ul>
<h2 id="pros-and-cons">Pros and Cons</h2>
<ul>
  <li>These are not fool-proof graphs, nor are they comprehensive. It is best if you do the exercise several times at different times of the year to refine your results.</li>
  <li>Obviously, you are missing out on areas where you didn't take measurements.</li>
  <li>The graph does not cover the whole space seamlessly.</li>
</ul>
<h3 id="however">However</h3>
<ul>
  <li>The graphs can still give you a rough visual guide for what's going on in a space regarding relative humidity and will enable you to rethink your decisions coming from a place of at least some knowledge rather than just educated guesses.</li>
</ul>
<p>If you liked this post, you can follow me on <a href="https://twitter.com/ConservaLlama">Twitter</a> where I'll be posting more content on conservation tidbits and how to improve your collections at home.</p>


A simple environmental monitoring solution using Excel.

Excel for Environmental Monitoring: Modeling Humidity


<p><em>Photo by Monstera on <a href="https://www.pexels.com/photo/illustration-of-woman-analyzing-financial-line-graphic-6289065/">Pexels</a>.</em></p>
<h2 id="why-we-send-our-sensors-to-be-calibrated">Why we send our sensors to be calibrated</h2>
<p>All of us who work in museums or heritage and run environmental monitoring programmes know the answer to this.</p>
<p>It's pretty basic. If your datalogger sensors are not working properly, they could really affect your decision-making process for those areas where you keep important, sensitive objects.</p>
<p>So we know it's important to have the sensors working properly.</p>
<p>The thing is, I only recently discovered that the extent of my knowledge on this was limiting the results I was getting. The following is information that lab analysts and engineers may consider very common and obvious, but it's something that I had never seen before and was certainly not included in my (otherwise excellent) training in conservation. Are you in the same boat?</p>
<p><strong>What I was told:</strong></p>
<blockquote>
  <p>Just send your loggers off to calibration service every now and then to fix them up.</p>
</blockquote>
<p>That sounded straightforward enough. However, here are the things I <em>didn't</em> know and nobody ever told me...</p>
<ul>
  <li>Time in between calibrations should not necessarily be standard. Just because you're used to sending your loggers of every 'x' years, doesn't mean that's the correct time for your loggers.</li>
  <li>If you're not applying the results that come back in the calibration report, there's a chance you might as well not have done anything.</li>
  <li>For the most important bit, I will let <a href="https://en.wikipedia.org/wiki/Calibration">Wikipedia</a> give you the following because it's the clearest way of saying it:</li>
</ul>
<blockquote>
  <p>In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Strictly speaking, the term "calibration" means just the act of comparison and <strong>does not include any subsequent adjustment.</strong></p>
</blockquote>
<p>Did you know that when you send your sensors off to a "calibration" service, that does not necessarily mean they're fixing them? I certainly didn't!</p>
<p><strong>First important conclusion:</strong> Make sure you know what service you are requesting and whether your service includes actual sensor <strong>adjustment</strong>. If your service does not include adjustment, then I'm afraid you can't just put your reports away and forget about it. Turns out I had sorely misunderstood the meaning of the technical term "calibration". What about you?</p>
<p>In this post, I will only talk about temperature and humidity for the sake of brevity and simplicity, but you can probably extrapolate the information to all other sensors. <strong>Please note that the following is normally applied to laboratory or commercial or auditing analytical equipment that requires high accuracy for various commercial and legal reasons.</strong> If you're just monitoring environments, you will have a bit more leeway regarding acceptable errors in your numbers.</p>
<h2 id="what-information-do-calibration-certificates-give-you">What information do calibration certificates give you?</h2>
<p>First of all, if you are in charge of getting these services, remember that it should be conducted by a provider accredited under the <a href="https://www.iso.org/ISO-IEC-17025-testing-and-calibration-laboratories.html">ISO/IEC Standard 17025</a>.</p>
<p>There are a number of different types of information which your calibration certificate should contain. If you want to look at those details, check out this <a href="https://www.omega.com/en-us/resources/calibration-certificates#:~:text=A%20calibration%20certificate%20is%20a,measurement%20accuracy%20of%20the%20device.">page</a> by the Omega company in the United States.</p>
<h2 id="ok-i-sent-it-off-and-the-numbers-look-small-so-its-fine-can-i-put-it-away-and-forget-about-it">Ok, I sent it off and the numbers look small, so it's... fine? Can I put it away and forget about it?</h2>
<p>I certainly did for a while! But the answer is maybe. Let's do a mini-checklist.</p>
<ol>
  <li>My service included adjustment. ---> Brilliant, you can put it away, but you should still look. If the numbers were big, you will want to keep an eye on it over time.</li>
  <li>My service did not include adjustment ---> Do not just put it away without looking. You will need to adjust your results.</li>
</ol>
<h2 id="sigh-what-am-i-supposed-to-be-looking-for">Sigh. What am I supposed to be looking for?</h2>
<h3 id="1-the-errors-of-your-sensors">1. The errors of your sensors</h3>
<p>It is normal to do 3-point calibrations for three different temperatures and three different humidities. They will show a positive or negative number based on how they differ from the standard used.</p>
<p>If your sensors have not been adjusted in the service, you will have to add or subtract these numbers from all your readings in order to get the accurate measurements. This will be important if you have very sensitive spaces and/or if the numbers obtained will make a significant difference in your readings.</p>
<p>
  <img src="/Calibration-Error.jpg" alt="Adjusting readings with calibration information"><br><em>Adjusting readings with calibration information.</em>
</p>
<h3 id="2-the-deviation-of-the-measurements">2. The deviation of the measurements</h3>
<p>These will also affect the accuracy of your measurements. The number provided alone might be within your acceptable ranges, but it might push you out of them if you take the deviations into account. The following is a table with arbitrary numbers just to show you a visual example of how deviations can mess with your data.</p>
<p>I have coloured the dots blue where all is in order, red where they are out of range, and orange where they are within range on their own but out of range if the deviation is considered.</p>
<p>
  <img src="/Calibration-Deviation.jpg" alt="Deviations change your expected measurements."><br><em>Deviations may be enough to cause doubt about whether your numbers are within range or not. If you were to look at these numbers without deviations, you'd be within range 70% of the time (7 out of 10 measurements). With deviations, this changes to 30% ok with 40% being potentially not, and 30% out of range. Not quite the same, is it!</em>
</p>
<h3 id="3-the-period-of-use-for-your-results">3. The period of use for your results</h3>
<p>Your results will be applicable until the next time you run the calibration. This time can vary between a number of months or a few (1-3) years. This means that you will need to apply the adjustments in your calibration certificate until the next time you run the service.</p>
<h2 id="i-get-it-i-probably-should-look-at-these-things-what-else-am-i-missing">I get it. I probably should look at these things. What else am I missing?</h2>
<p>Remember I mentioned that time in between measurements should not necessarily be standard? I will add to this another question. How do you know when it's time to replace your sensors entirely? The answer, as I have come to understand it, is bullseye charts.</p>
<p>
  <img src="/Calibration-BullseyeCharts.jpg" alt="How bullseye charts look."><br><em>As the name implies, bullseye charts look like... well... bullseyes. These have been made in Excel. As you map your calibration results on, you will start to see where the dots place themselves in the bullseye, and this will help you decide what to do next.</em>
</p>
<p>You can make your own decision regarding where to draw the circular lines in your bullseye charts, but I like 5°C and 5% because they are nice, round numbers and 5% humidity is the lowest acceptable variation within a day as presented in this <a href="https://www.icom-cc-publications-online.org/1199/Predicting-risks-to-artefacts-from-indoor-climates">paper</a> by Boris Pretzel.</p>
<p>You are welcome to download my Excel template <a href="https://drive.google.com/file/d/13Gg-1GPtjafEsZSnSMu1q7BFKr1M0I6x/view?usp=sharing">here</a>. Keep in mind that the circles have already been drawn for you in the BullseyeCircles tab and that you should only mess with it if you want to change their sizes.</p>
<h2 id="how-to-add-info-to-the-bullseye-charts">How to add info to the bullseye charts</h2>
<p>I think the image above and Excel provided are pretty clear, and I'm sure you can reverse engineer the information based on the provided Excel, but here are a couple pointers:</p>
<ul>
  <li>Make a row for every calibration point run. In these made-up examples, the four sensors shown have been run through three points for three consecutive years. The points used will depend on the standards used by the service provider you hire. You can ask them what they are, and they might even tell you what points they have so you can choose which ones you want. Try to pick some that your logger normally encounters when in service.</li>
  <li>Make a bullseye chart per sensor over the years or over the calibration services. Don't put different sensors on the same graph.</li>
  <li>You can keep adding information to the tables below if you have more results and just add the selected cells necessary to the graphs as you go.</li>
</ul>
<h2 id="how-to-interpret-the-bullseye-charts">How to interpret the bullseye charts</h2>
<p>As shown in the image above, there are four potential results. You should be looking for <strong>reliability</strong>. Here is a brief breakdown of the four types shown above:</p>
<ol>
  <li>Valid and reliable (top left): Results are great. Your sensors are working well and giving accurate results. You can trust them. You can check how often they get sent out for calibration and if the results keep coming back the same, you can lengthen the time in-between services.</li>
  <li>Not valid but reliable (top right): Results are fine. Your sensors are working well, but they're a bit off. This can easily be corrected by applying the error adjustments given by your calibration certificate. This will bring the results back into the middle for valid results.</li>
  <li>Valid but unreliable (bottom left): Your sensors seem okay, but the dots are a little too spread out for comfort. Consider reducing the in-between time for calibration services and maybe find a service that includes adjustments if you haven't already.</li>
  <li>Not valid and not reliable (bottom right): Your sensors are all over the place and should not be trusted. Check with a provider if they can be adjusted at all or just consider replacing them altogether.</li>
</ol>
<h2 id="benefits-of-knowing-all-this-stuff">Benefits of knowing all this stuff</h2>
<p>Yes, it's not all just hassle - although I will admit it feels like a big hassle.</p>
<p>Guess what? If you're low on budget, running these graphs might help you reduce the number of calibration services you do. If you notice that, over time, your sensors are just fine, then maybe you don't need to run the service as often as you thought. On the contrary, if you notice your sensors are all over the place, maybe you should get new sensors instead of using up your precious budget on calibrating equipment that should be let go.</p>
<p>If you liked this post, you can follow me on <a href="https://twitter.com/ConservaLlama">Twitter</a> where I'll be posting more content on conservation tidbits and how to improve your collections at home.</p>


It turns out calibrating sensors is not something you can necessarily do yourself. Find out what the real meaning behind your sensor calibration certificates.

Understanding sensor calibration


<p>Articles on the topic of environmental monitoring for collections.</p>


Articles on the topic of environmental monitoring for collections.

Environmental monitoring

Related Articles

Understanding psychrometric charts and dew points

Excel for Environmental Monitoring: Modeling Humidity

Understanding sensor calibration

Subscribe to my conservation newsletter