I know what you are thinking after reading our first two parts of this series: What dataloggers should I buy to measure my climate? There are loads out there and some are pretty expensive!

Here’s the thing: What makes dataloggers expensive is accuracy, especially when it comes to measuring relative humidity. You can buy pretty cheap models, but what makes them cheap is usually cheap sensors. Now, you probably wonder how bad it is to have a cheap sensor. As nearly always, it’s a “depends”. Depending on what? Depending on how stable your “room climate” (although we already saw that there is no such thing) is and how accurate you need your measurement to be.

One thing you have to know is that sensors have a range they operate in, humidity sensors usually and logically between 0 and 100% relative humidity. Within this range they might vary in accuracy. Humidity sensors are typically most accurate in the middle range around 50% rH and become less accurate at the edges under 10% or over 90% rH, they also might become less accurate in very cold or very hot surroundings. Usually, you find these error ranges as graphs in the datasheet of each sensor or device.

That’s pretty theoretically, so I made a test to show you what this can mean (in retrospective I should have used a more diverse set of colors for the diagram, my apologies for that). I put in the same spot three sensors:

1. A pretty cheap DHT22/AM2302 ¹ which is a very common temp/rH sensor in the maker/microcontroller scene and which you will find frequently in DIY devices. It is given with a normal accuracy of +/- 2% RH and +/- 5% RH maximum discrepancy (in which range it reaches this maximum discrepancy isn’t given).
   This is the light green line.
2. A mid-priced generic Chinese version of a Sensirion SHT31 ² which you will also find in more refined DIY projects. It is also given with a +/- 2% RH accuracy.
   This is the dark blue line.
3. A high-priced original Sensirion SHT35 ³ which you will find in professional devices. It is given with an accuracy of +/- 1.5 % RH in a range between 0 and 80% relative humidity, which means it becomes less accurate above these values.
   This is the green line.
4. In addition, you will find a blue line: This is the linear correction of the DHT22 sensor which is based on the measurements I did with this sensor compared to an Assmann psychrometer. I found the DHT22 was 31 percentage points off the “real” relative humidity I found with the Assmann at 55%. So, I subtracted 31 from the value the DHT22 measured. ⁴

So, what does the graph show us and what can we learn from it?

All sensors are pretty much the same when it comes to temperature. This tells us that if we only want to measure temperature, we can go with a cheap device.

When it comes to humidity, things become really interesting:

1. The original DHT22 readings show a much too high humidity, something we already expected because of our measurements with the psychrometer. Naturally, as the sensor can only read 100% max, the lines are cut at 99.9%. As our correction of the sensor is mainly a linear subtraction, it transfers the straight line to 68.9% for our corrected values in these cases (blue line).
2. The expensive original SHT35 and the cheaper generic SHT31 are not really far apart. When humidity crawls towards the 70% and over it, it seems that the cheaper one (dark blue line) measures a little less humidity than the expensive one (green line). ⁵
3. What is really funny is that the cheap sensor seems to have a tendency to over-dramatize events. As we take a closer look at a drop (and later rise) in humidity on July 27 the DHT22 shows a dramatical decrease from 100% in the morning down to 49.7% at 3 p.m. Its corrected version sees an equally dramatic event from 68.9% to 18.7%. As we take a look at the other two sensors nothing this dramatic happens. The most expensive one sees a drop from 66.3% to 40.7%. Still a mild catastrophe if this were a gallery or storage room (it wasn’t), but a huge difference to have a 25% drop than a 50% drop.


A close look at July 27

Especially the last point tells us a lot about cheap and expensive dataloggers. Not because cheap devices necessarily contain an old, beat up DHT22, but because it shows us the general problem with sensors: they are not necessarily acting linear. They’ll need adjusting.

Now, every adjusting is an expensive step. Devices are adjusted to reference points which means the manufacturer does more or less what I did with the DHT22: they measure it against a calibrated source (i.e. a salt solution) and then adjust the output accordingly.

If it is a cheap device it might be only adjusted to one reference point, resulting in what we see in the DHT22. Just because it was 31 percentage points off at 55%, this isn’t necessarily true for the whole range. Instead, it is very unlikely for a sensor to react linear throughout the whole spectrum. It will react differently for different humidity ranges. This is why expensive devices are measured to more reference points and calibrated accordingly, resulting in much more accurate readings throughout the whole range.

I assume that the generic SHT31 was also only tested against one reference point and that the difference we see in the readings against the original Sensirion when the humidity rises is already a sign of it, but as it is still within the possible error range, I can’t prove that.

For me personally, I wouldn’t use DHT22 anymore, for obvious reasons. I can live with the generic SHT31 for cases where I need to get an idea of a setting and in less problematic areas. I’d always go with the high-priced original parts if I have to depend on the readings for loans or critical storage environments.

So, more generally speaking: can I cut costs by buying a cheap device? Yes, if you just have to measure temperature. And yes, if the only thing you need is a rough idea of what happens in regards of rising and dropping or the humidity, but not detailed values. You have to be aware of the fact that it might show a more dramatic drop than what actually happened as well as it is thinkable that it shows you a less dramatic change than what actually happened.
You might want to turn to high quality products when it comes to your more critical applications and it’s always a good idea to take a critical look at the datasheets to know what you are buying.

May your storages and galleries always have a nice and stable climate!

Angela

A common misconception is that sensors know the room temperature and relative humidity. They don’t. There is no weather fire dwarf who walks to the artifacts and says “Well, here we got 51% relative humidity”. A sensor measures what the temperature and humidity is right where it is. If it sits 2 meters above the ground, it may measure a beautiful 51% and 21 °C (70 °F) while your artifacts are overheating at 28 °C (82 °F) and dry out at 34% behind the glass in your display case with a badly done light installation. You might panic glancing at the 32 °C (90 °F) in your gallery while in fact it’s just that direct sunlight falling on your sensor. Or, for that matter, it’s placed over one of those heaters and gets roasted.

One of the difficulties you face when you decide on where to put a datalogger is that only in very rare cases there is something like a “room climate”. In most rooms, there are several climates. There is that outside wall that is always a little colder than the rest of the room. There is that wall that has bright sunlight from 11 a.m. to 2 p.m. in winter and 9 a.m. to 4 p.m. in summer. There is the space at the entrance which always gets a flush of cold and damp air as soon as someone opens the door. Then, there are your display cases.

All things try to reach equilibrium, which means that they strive to reach a common room climate, but while doing so, they create different and sometimes problematic room climates. The warmer air will inevitably go to the colder areas, but when reaching there, the once nice 51 % at 21 °C (70 °F) might become 68% in that 16 °C (61 °F) corner of two outside walls. ¹

To get a good idea of what happens where in your room or gallery, you might wish to place several sensors. Life and the museum sector being what it is, you probably don’t have money to do that. What you can do, however, is measure at different areas, moving your datalogger to get an idea what happens where. Then, for the permanent location of your datalogger, choose the place that captures the climate your artifacts experience best.

Next up, we want to take a look at the difference between cheap and pricey sensors, which is – besides other things like WiFi functionality – what determines the price of a datalogger.

Stay tuned for part 3 of this article.

When I think about my early childhood, there is one picture that comes up from time to time: It’s my grandfather standing before the so called “weather station”, a wooden panel with three brass gauges: a thermometer, a hygrometer and a barometer. Then my grandfather gently knocked against the glass lid of the barometer with his knuckle and checked the barometer again. As a child I was convinced he woke up the little worker inside, who in return checked if the weather would become fair, rainy or if it was changing. This made perfect sense to me, because I knew there was a special force responsible for the weather: the weather fire dwarfs. After the evening news the news anchor would say “And now the weather fire dwarfs for tomorrow”. It was much later that I asked my mom and she explained to me that it wasn’t the “Wetterfeuerzwerge” (weather fire dwarfs), but the “Wettervorhersage” (weather forecast). Until then, I was convinced that being a weather fire dwarf was just a job like being a firefighter, a baker or a teacher. They looked out for the weather and made assumptions on how it was going to be. It also made perfect sense that this job was done by dwarfs, given the small instruments they needed to operate in.

You might wonder where I’m heading to with those seemingly random childhood memories? Now, my grandfather certainly did neither wake up the little weather fire dwarf, nor did he perform some other kind of magic. He just made sure the barometer, which had a small metal box called an aneroid for measuring the pressure inside, was showing the tendency. When you tap them, the pointer will move to the current pressure, giving you an indication if the air pressure is rising or falling. He also knew that the little markers that read “very stormy”, “rain”, “change”, “fair” and “very dry” were not to be take literally. In winter “fair” weather often meant it became very cold. He not only looked at the barometer – he also knew how to interpret what he saw.

Fast forward to 2018 I struggle with explaining the use and the accuracy of sensors to colleagues. I can’t fight the feeling that digital has somehow taken a toll on our imagination and our expectations towards instruments. It’s especially true for hygrometers, as humidity is far more complicated to measure exactly than temperature ¹. A digital hygrometer usually provides you with two digits behind the comma. So instead of reading a little above 50% on the analog hygrometer it provides you with a straight 51.23 % relative humidity. That’s astounding. Sadly, it doesn’t mean what people think it means. Because it seems to provide such an exact figure, people assume it’s more accurate than the analog reading of grandpa’s old weather station. But it isn’t necessarily so. If it’s a very good digital humidity sensor with a 1.5% accuracy the reading will mean it’s somewhat 50 to 52ish. If it is a more common sensor with a given 5% accuracy it tells you that it can be anything along the lines of 46 to 56% ²

But why are they giving two digits behind the comma anyway, is this just a fancy feature? Yes and no. For that, we have to understand the difference between accuracy and resolution, both specifications you will find when you are buying a datalogger or sensor. What we just talked about is the accuracy. The resolution is often higher than the accuracy. Your humidity sensor might have an accuracy of +/- 2% RH, but a resolution of 0.1. This seems like a contradiction at first, but it isn’t. Let us imagine a little weather fire dwarf who is able to feel how wet it is. He says: “It’s 52%”. Now, while we saw that this can mean anything between 50% and 54%, if we send him out again, he will be able to tell us if it is becoming wetter or dryer than before. And he will be able to tell us more detailed than he can do it with the general humidity. He is able to say: “It’s getting damper, now it’s 52.1%.” So, while the digits behind the comma mean nothing in terms of general humidity, they can help us understand where the climate of a room is heading to and how severe the changes are. If you measure every 5 minutes and get a reading of 52.1%, 52.3%, 52.2%, 52.1%, 52.2% and 52.1% over half an hour it will tell you something else than a series that reads 52.0%, 52.2%, 52.3%, 52.4%, 52.6%, 52.8%. While both series can still mean it’s 2% more or less humid, the tendency of the second series shows that something is going on which makes the room damper. Think of the resolution as a measurement that helps you to see change, just like grandpa tapping on that old barometer.

Next up, we will ask the question if sensors really read the temperature and humidity of a room.
Here comes the 2nd part…

Angela

Growing up in the middle of Europe my earliest Easter memories are joyfully searching for Easter Eggs in the garden. Many years later a recurring task in my professional life is searching for things that are not where the database says they should be. For this Easter I thought I’d write about the differences:

Searching for objects is not joyful

While as a child the search for the Easter Eggs is full of joy, the search for an object is all but that. You search for an object because it is needed, often needed fast, for a research request, loan or exhibition. There is time pressure and if you don’t find it, it has consequences. It can mean a whole lot of work for other people, like curators having to search for alternative objects that convey the same message as the original object. It can mean that whole parts of an exhibition must be changed because they were designed around this special object. It can also mean that a researcher can’t answer the question s/he is working on.

Objects are not hidden on purpose

Other than Easter Eggs no one hides objects on purpose. While a small portion is really stolen, most are lost because people miss to record or communicate location changes (we already looked at this in Failures in Figures). The reasons are multifold: thoughtlessness, laziness, arrogance (“That’s not my job”), confidence in the own abilities to remember everything, the belief that “I just take it out now and put it back in immediately”. No traits of the Easter Bunny but of many museum professionals.
Recently I discovered an additional reason why object locations are not updated: magical thinking. The belief just because there is Wi-Fi in the storage and the objects are barcoded a magical superpower knows exactly where each object is. Sorry folks, that’s not how it works.

Looking in all the right places

While searching for Easter Eggs is often unsystematic or just starting at point X and ending at point Y of the garden, searching for objects requires a different approach. If your storage has 3,000 square meters and much more shelf space, you can’t just search everything. And you can’t just go into the storage looking for the object. Instead, the first thing you do is a hands-off approach. You think about what most likely happened to the object.

The starting point is the last entry in the location field. When did it get there, and can you imagine that it was used somewhere else in the meanwhile? A good database comes not only with a location entry but also with a field stating the date the location change was made, along with a history of former location changes. It also has entries of whatever happened to the object and when – was it on loan, did it need conservation, was it photographed, was it cited… All these can provide you with ideas where to start your search. Sometimes a picture was taken after the last date of location change – chances are the object is still in the hands of the photographer. Sometimes the object was on display after the last date of location change – chances are the object is still in the boxes that were packed at the deinstallation of this exhibition. Other objects that were in the same show case at exhibition X went to storage location Y – chances are your object is also in storage location Y.
You are making a list of possible people to call and places to search before you actually start searching.

A mindful use of energy

As a child on Easter morning it’s great to run into the garden full of energy and search for those eggs. As a museum professional with a tight schedule and a lot of tasks on your plate you have to be more mindful about how you will approach the search. You have to weigh the time you can invest in the search against the likeliness of finding the object.
If the object can be retrieved with just a few phone calls, everything is fine. If the object is needed next week and the last storage location is a place that doesn’t exist anymore (i.e. because you disassembled the shelves or moved into another storage location 10 years ago) it’s probably better to inform the researcher or curator immediately and ask her or him to look for an alternative, if possible. If there is a high likeliness of the object being in a pile of boxes that don’t have proper location entries, it’s probably best to work through this pile updating all locations – you will save yourself a lot of time for future requests.

Make sure you know that you don’t know

Remember as a child searching the same place twice because you forgot whether you have looked under that tree? You should make sure this doesn’t happen to you when you search for an object. The most important thing is to mark an object as “location unknown” as soon as you discover it isn’t where it is supposed to be. That way everyone knows the object is not accessible at the moment and can already think about alternatives. It also helps you to keep an eye on all the objects with “location unknown”: Is their number in the database decreasing, chances are you are doing a good job as collections manager. If the number increases, chances are there are problems in your collections management and logistic workflow and you might want to take a closer look to find the underlying issues.
That you should tick off the things you already did to find the object on your list should go without saying.

I hope the only thing you have to search for now are real Easter Eggs.
Happy Easter, all!
Angela

Documenting our documentation is new for us all. Let’s start simple!

First you need to decide if you make a documentation of your whole museum or if you split it into different collections. That depends how large your museum is and if different people are responsible for different collections.

Overview of the collection

1. Elevator Pitch: Imagine a very important person is visiting your collection. They know nothing about it. You have 30 seconds on the elevator to tell them the most important facts.
   Describe your collection in three sentences. (Normal sentences, not scientific-paper-five-comma-sentences). Important are : Content of the collection, size, significance.

   On your way to the storage there are a few polite questions. Now you have a bit more time to answer, but stay focussed: Give an overview, do not go into details of collection concepts or the donor’s biography. That will come later.

2. Collection in context: Is your collection part of a larger museum collection? Is there any way to distinguish your collection from the others? (separate storage, different inventory number, classification,..)
3. Significance of the collection: How important is your collection in comparison to the other collections in your museum/to similar collections in other museums? What are the main differences?

Maria Scherrers

Maria Scherrers is museum specialist with degrees from HTW Berlin and the University of Leicester. She spent most of her working life so far in company museums and collections. She is fascinated by the way our everyday life is changed by brands and how that influences our cultural history and what we will be collecting in the future. In the mean time she is a consultant for company that wish to build and use historic product collections.
She spends her little free time on her family and on politics.
www.historicalassetmanagement.de

Collection work is more than just storage and object information in a database. Sometimes we only realize how much more there is when important people leave or when we have to painfully puzzle together different information to answer simple questions.

Just imagine 20/50/80 years ago one of your predecessors had documented what they did, why they did it and how they did it. Wouldn’t it be wonderful to understand where this object numbering system comes from, who meticulously documented that part of the collection on index cards or where that part of the collection was stored over the years.

Over the next weeks I want to post some questions you might want to answer for yourself. Think of what all the people coming after you might want to know about your collection work.

These topics will be part of the questionnaire:

1. Overview

2. History of the Collection

3. Collecting and Collection Criteria

4. Documentation and Documentation Criteria

5. Digitisation and Object Photography

6. How the Collection is used

7. Storage and Conservation

8. Plans and Future Developments

Stay tuned!

Maria Scherrers

Maria Scherrers is museum specialist with degrees from HTW Berlin and the University of Leicester. She spent most of her working life so far in company museums and collections. She is fascinated by the way our everyday life is changed by brands and how that influences our cultural history and what we will be collecting in the future. In the mean time she is a consultant for company that wish to build and use historic product collections.
She spends her little free time on her family and on politics.
www.historicalassetmanagement.de

About a year and a half ago, my editor at Rowman & Littlefield asked if I was interested in preparing a new edition of my book, Things Great and Small: Collections Management Policies. The first edition had been published in 2006 which meant that it was long overdue for an update, so of course I said yes (I wanted to call the second edition Things Greater and Smaller, but, alas, that didn’t happen).

As I started working on the project, I realized just how much some aspects of the museum field had changed since 2006, particularly the amount of information and how we access it. As I reviewed several score of new books, journal articles, and listserv discussions, I was struck by how many more resources are now available than when I wrote the first edition. I also thought back to when I started my museum career―admittedly this was back in the Pleistocene, also known as the 1970s―at that time, the museum literature was very sparse and there were no internet resources (because there was no internet). The only way to consult with one’s colleagues was to call them on the telephone (back then you actually paid for each long-distance call) or wait until you bumped into them at some meeting or other. The rich variety of print publications and web resources that are readily accessible now and our ability to instantly consult with a brain trust of seasoned museum professionals via cell phone, email, or web-based discussion groups has changed the field considerably, and much for the better.

When I mentioned that I was preparing a new edition of the book, people would usually ask what I was planning to change. Indeed, the changes are many, beginning with a rewrite of most of the text and the addition of several new text boxes, and a few photographs have been added to the revised edition. There is updated and expanded coverage of deaccessioning and intellectual property rights, and a new section on digital collections. The laws and legislation appendix has been revised, and the bibliography expanded. The second edition text reflects changes in our thinking about standards for collections care and storage environments. Some information has been added about collection management policies for zoological parks and botanical gardens, and about the curation of culturally sensitive collections. One of the most noticeable changes in the new edition is that the sample policies from real institutions have been replaced with model policies from fictitious institutions. Because the sample policies in the first edition all came from real museums, they were limited in their applicability to other situations and in any case, most had become dated. Although I warned readers of the first edition not to copy policies from other museums but to write their own, most people preparing policies need something to start with, so the new edition offers models to provide users a way to get started.

This new edition incorporates much that I have learned about how the book is used by its readers. This primarily came from feedback from presenting workshops, teaching classes and webinars, and from people who took the time to let me know what they thought about it. It is somewhat daunting, but always instructive, to assign your own book to a class and then watch how they interpret what you have said.

Although collections management policies are very important to museums, even I have to admit that policy preparation can be, for want of a better word, boring. To help out with that aspect of policy preparation I have developed a board game to go along with the new edition of Things Great and Small. I call the game Monopolicies (you can guess what board game inspired it) and after some beta testing, it is nearly ready for a public launch. The idea is that instead of those dry and interminable discussions with colleagues about collections management policies for your institution the staff can, instead, have their discussions in a more relaxed atmosphere while having fun playing a game. Monopolicies will be made available in early spring as a free download—watch for it! (Update 2018/02/04: It’s available for download here: http://www.museumstudy.com/courses/course-list/monopolicy/ )

Things Great and Small is available now from the publisher (https://rowman.com/ISBN/9781933253039/Things-Great-and-Small-Collections-Management-Policies) and other booksellers.

John E. Simmons
Museologica
and
Adjunct Curator of Collections, Earth and Mineral Sciences Museum & Art Gallery, Penn State University