Jean and Lou both have other things they have to do :-(. Next week, we’ll give a better idea of what we’re thinking about for the March and April sessions (2nd and 4th Tuesdays).
One of the things that went less well on Shrove Tuesday was our test of heat stratification. The balloons worked flawlessly, even the supplier lift table – but the boilers didn’t. Christ Church has a two boiler system, and one of the boilers overheated at some time during the wee hours of the morning. That means the building probably wasn’t fully warm when the tests were done. For this building, we aren’t even sure what “fully warm” means. The room thermostat is badly sited, so setting it to 16C means the space could be anywhere from 14C to 21C depending on conditions. We actually only know that from when a frost stat went horribly wrong. In practice, we guess how long to set the heating based on the weather, and try to stop it before the room stat kicks in.
We also knew we wanted to cut the heat coming out of the radiators a bit before the test to reduce their influence and keep the balloons from drifting around. The system is partially gravity fed with pump assistance, but there’s no pump overrun, so it takes an hour for the air to stop heating after the boilers stop. Jean misjudged how long it would take to tie the balloons, so the boilers actually stopped two hours ahead. Plus it took 40 minutes to take all the measurements. So the big unknown is how long the heat stays in the space and how that influenced the readings. We’re working on parts of that, but the answer has to be “not long”.
Even so, we did learn from the exercise. There was only about a 2C difference between ground level and half way up the roof space. Meanwhile, the anecdata is that the very top of the space was warmer, but some micros were tied lower than others and out of the warm pocket. In addition, when we turned the heating back on, one of the balloonists reported an immediate rise in the high level temperature; that certainly doesn’t happen at low level.
This is important enough to the church that we want to do it again. Jean will be asking permission to take the balloons out on Thursday the 5th of March in the evening. The plan is to stick up clusters at the top of the nave in two places (2/5 and 4/5 of the way down the aisle), with the Jeenodes tied as high as possible – and then leave them there for the full heating session so that we get longitudinal data. She’ll probably have to sit there with her sarnies but you can always bring her cups of tea! With any luck, there will be less wind stripping heat out of the building as well. Here’s hoping for calm weather and smooth sailing.
A good time was had by all this last Tuesday when HeatHack took over Christ Church Morningside for the day. We were there as part of innovative learning week with Edinburgh University. Our aim was to map air currents and temperature stratification in the church. We went armed with helium tanks, balloons, a blimp, fishing floats and pancakes. (The pancakes weren’t for scientific purposes, they were because it was pancake day and hungry scientists need to eat!)
Here are some photos from the day, and watch out for a video coming soon!
We’re still actively discussing helium usage, drop consequences, and the design of the balloon clusters.
Martin has made three rival blimp envelopes from mylar and evostick – two two-piece flat ones of different shapes, and the beautiful one with six gores. It’s not a very easy process, and they all leak, but his very first one leaks the least. He estimates the volume of it to be an enormous 760 litres. That’s well more than enough lift, but if we fill it up all the way, we have to think about whether we have enough helium for the balloons without going to the university tank.
Qualatex says an 11 inch balloon is capable lifting 10g beyond its own weight. That’s enough for the 8g sensor package on the Jeenode Micro and the fishing line, with a gram to spare. We don’t have Qualatex balloons, so maybe ours are heavier, and we’re not experts at filling them, plus our helium is 5% oxygen. That means in practice, one of our test 12 inch balloons lifted just under 8g. Meanwhile, we don’t want to mount our sensors on single balloons in case they pop. Even though we have hard hats, it’s sensible to avoid testing whether they work! That’s fine because we can use our fishing balloons for a bit of added lift, and as a brake – if the 12 inch balloon pops, the fishing balloons will slow down the descent.
Let’s assume we use 10 inch fishing balloons because they’re easy to buy. The smaller, the better, I think, because less lift means it’s easier to see how they drift with the air currents. If there are two, and the 12 inch balloon pops while the Jeenode is at the maximum height of 14m, it will hit the ground after a leisurely 11 seconds. It will still be going 2.5 m/s, but that’s a lot better than the full 16 m/s! One 10 inch balloon gives 3 seconds for descent and a terminal velocity of 9 m/s. That’s a bit fast, but we are ignoring the drag on the balloon. It’s probably pretty substantial – as long as both balloons don’t pop at once.
One of the discussions we expect to have is whether we should attach two 10 inch helium balloons as brakes, but here’s a different option – fill one of them with air. We should have enough lift to tow that up along with the Jeenode. After all, it’s only a balloon. The advantage of air is it reduces the chances of that balloon popping, because it should hang differently from the other balloons and therefore in different atmospheric conditions. Most notably, if the others stray too close to the lights, this one will still be cool because it will be below of the beam.
Or maybe we should forget additional balloons altogether and go for parachutes. Plenty to talk about, anyway.
Our supplier says the tank on site fills “up to 250 balloons”. Presumably that’s at least 8 inch, since otherwise the customers might complain! That means after testing and the blimp we’ll have at least enough helium for 20 12 inch balloons, or 6 12 inch and 24 10 inch. Or fewer, and a top up for the blimp. Or, if the supplier is generous, enough to fly another day. It’s a pity there’s no gauge on the tank!
There are many people in the community interested in our Shrove Tuesday event who can’t come for the whole day. Here’s our schedule for the day, which we hope will be of some help. Last year’s event didn’t entirely run to schedule, but we will do our best!
Please register following the instructions on the event page especially if you want fed.
10:00 – arrival at Christ Church Morningside. As people straggle in, we’ll be teaching them to tie blood knots and having them assemble their fishing balloons for the first exercise.
At 10:10, Jean will give a crash introduction to church heating – those who need to know more can take away a copy of Bordass and Bemrose. She’ll also sketch what we’ll be doing for the day and introduce our low-tech 3D modelling technique.
At 10:30, Dimitri will demonstrate the correct use of a hot wire anemometer, and Jean will explain how to take qualitative air speed readings using the fishing balloons. This part may take some experimentation! Then we’ll be splitting into zones and marking air speeds and directions on the 3D model. There may be the chance for people who are brave to assist in taking readings in our interesting, but scary, organ chamber. We think we’ll see a classic air movement pattern in the nave – that’s the main body of the church – but what happens in the fancy area up the front?
At 12:00, we’ll move into one of the two halls – either visible through the plate glass window in the modern building out the back, or through a door under the church at the very back end. Tim will explain the electronics that we’ll be using for the temperature sensing. For those most interested, there will be plenty of opportunities for pick his brain over lunch! Lunch is also the opportunity to use a sling psychrometer, understand what thermal imaging cameras do, and assemble a sensing balloon cluster for the next exercise. We also expect to meet the blimp. We’re still not guaranteeing it will actually fly, but we know many people are interested in talking to Martin about the challenges behind this complex construction.
By 14:00, we’ll be back in the main space and taking temperature measurements. This will start with Jean and Dimitri demonstrating the correct health and safety procedure. After a knot inspection, we’ll take and mark the measurements at a range of heights using our 3D model. We’ve left half an hour from 15:30 for trying the blimp, or possibly a competition in the main space involving a different kind of fishing equipment. It’s possible we’ll try to fly the blimp early if it’s filled so that we don’t leak too much of the helium. We’ll try to tweet it a little ahead if anyone is hoping to catch it.
We’ve told observers and friends to arrive at 16:00 for 16:30 if they want pancakes. We’ll bring the model and balloon clusters to the hall under the church, but some people may want to demonstrate some of the things we’ve learned to use! If the competition doesn’t happen upstairs, we have a simpler version of it in mind for during pancakes. The day officially closes at 17:00 but some of the organizers are sure to be there a bit longer – as long as we remember that the next group comes in at 18:00.
Here’s the envelope in all its six gore glory. It’s being tested for air tightness. That thing in the background that looks a bit like a bed headboard is actually the jig used to create the shape. Getting this right is a serious business! Martin, our aeronautics specialist, says the mylar is very finicky about the adhesive and it would be very easy to ruin the whole thing on the last join.
We’ve gone back and forth about estimating procedures, but we make the blimp volume out to be 510 liters. We’ll have two sources of gas on the day – one, pure helium, will be a van drive away, and the other, on-site is 95% helium, 5% oxygen. Approximating air as 100% nitrogen, and starting with the molecular weights (since this is, after all, a science project), we get a maximum liftable payload of 546g for the helium, and 514g for the convenient stuff. Of course, losses, air movement, air pressure at height, and everything else operate against getting the maximum lift. From reading around, it seems common to leave around 100g headroom, and maybe more depending on the conditions.
And here are the possible payloads: the minimum for a short run of temperature sensing, the medium for longer, and the big payload for the all-singing, all-dancing blimp experience.
|Weight||Minimum Weight||Basci / Large Battery||Camera / Separate Power supply|
|Pi Model A||31||y|
|SD Card, Wires, tethers, etc||5||y|
As you can see, the minimum payload is a fairly safe bet. The maximum is a bit on the heavy side. The gondola is a fair whack because it’s designed for ease of use, stability, and safety. Parts of it are also the easiest thing to sacrifice, although we hate to mar its good looks. We’d also consider wresting some of the heavier connectors off a Pi, since we’re not really planning to hoick a monitor all the way up to the ceiling and play videos from some kind of overhead display. So the first goal on the day is testing and making any necessary modifications. As I’ve been told by many people so far, helium is monatomic and leaks very readily! After that, flight; air current assessment; temperature measurement; and happy snaps in that order. We’ll actually be delighted if we get as far as flight. Martin has hours of flight as a licensed model plane operator, but this is his, and our, first blimp. The idea of a blimp has been surprisingly popular in the church community, especially among priests. That means that even if we only get as far as proof of concept, it’s likely to get more outings.
We’re still working out how to show the blimp on the day because it’s only possible for a couple of people to work on it at a time. We think those who are particularly interested can go chum Martin for a bit as he leads the testing. We expect an explanation of the principles and updates on progress during breaks. We’ve scheduled time in the main space to demonstrate flying late in the day before we break for pancakes, if he thinks it’s ready, but we can adjust the schedule on the fly to suit.
Our main focus for Shrove Tuesday will be taking high level readings using balloons and the blimp – quantitative for temperature, and qualitative for air speed and direction. However, we do have some additional equipment that will be using to augment particularly the lower part of the 3D model we build. You can find descriptions and pictures of them in use at last year’s event.
- hot wire anemometers for quantitative measurement of air speed. They’re hand-held, so we can use them wherever we can reach! Our intrepid organist intends to take one as close to the edge of the organ chamber as he can get. That’s the highest accessible point in the space, and a very tight fit that involves squeezing past pipes and knowing which bits of the organ are designed to take weight. The wands are designed to take measurements inside ducts. If we have time, we’ll do the church a good turn and find out whether the fan system in their modern building needs something to block air egress when the fans aren’t in use. That’s something the church’s energy consultant is wondering and couldn’t tell from the access he had.
- hand-held remote temperature sensors. The biggest issue here is that they work up to 12m away, but the highest part of the roof is 14.78m. Luckily, this year, we also have a thermal imaging camera. That’s basically a large matrix of remote temperature sensors that produces pretty pictures as well as giving the underlying data.
- sling psychrometers. They take a bit of time to use, so I think this year we’ll use more than one!
- smoke sticks. We discovered last year that they’re unlikely to set off smoke detectors in a space this big, but that’s especially true for this site – in the worship space, there aren’t any! We might discover that they’re useful for diagnosing the airflow in the chancel. Again, we can do the church a good turn if we have time, and just check the wall vents aren’t blocked anywhere.
Where we can’t easily put things on the 3D model, we can record short videos explaining what we’ve seen. By the time we’re finished, we should know quite a bit about the building.
If everything goes well, on Shrove Tuesday, the HeatHack blimp will be making its maiden voyage. Martin has constructed a very graceful gondola out of balsa wood for the propulsion unit and our instrumentation. He’s hanging it under a classic envelope of mylar (space blanket) stuck together with Evostick. The payload will be more or less the same as on the balloon clusters. Now that Tim is familiar with the Jeenode Micro, we’re thinking we might stick them on the balloons – to reduce the drop risk and the size of the cluster – with a normal Jeenode on the blimp, since there the weight should be less critical. On Tuesday we expect to hard-wire the sensors to the Micros, but use pluggable connectors on the heavier kit just to increase the flexibility for their reuse. We don’t know where the blimp can go that the balloons can’t and vice versa, but well, “innovative” and “learning” – the clue’s in the name.
Since we’re taking a different perspective on the church building by measuring it, we fancy visuals to match. If all goes very, very well, we may send up an alternative payload on a second run: a Pi model A with a wireless connection, temperature sensor (of course!), and low light camera that can stream video back to a laptop. The envelope will hide the ceiling and and there are some obstructions that may block the rose windows, but we think we should at least be able to look at some of the whopping great subsidence cracks, quick before the workmen pin the back wall back on.
At last year’s Innovative Learning Week event, we attempted to assess the thermal comfort of St John’s Princes Street by taking many of the same kinds of readings we’ll be taking on Shrove Tuesday : air and surface temperatures, relative humidity and air speed. There was one thing at the event that really let us down – our method for recording data on worksheets. The worksheets were based on faces of the building, but they were hard to understand and read. This time around getting a sense of the whole space at once is even more important because we want to be able to understand the air currents in 3D, so we’ll be constructing a 3D model.
Obviously, this is a church space, not a computer lab, so we can forget all having access to, say, some kind of shared Sketchup. That means we’ll be going Old School to create a 3D grid in the rough shape of the building to hold our measurements. We thought about Meccano, but it’s pretty expensive and time-consuming to build up. Toothpicks and gumdrops work for representing networks and molecules, but we need to be able to reach right into the model to record readings taken in the middle of the space. That leaves us with kebab skewers and marshmallows, or maybe Turkish Delight. Then quantified readings can be written on scraps of paper and speared onto the marshmallows using cocktail sticks. High level air speed readings will necessarily be qualitative – we’ll have to agree what counts as slow/medium/fast, but the orientation of the stick will be helpful for indicating the direction.
We think we’ll need a big cardboard box in about the right shape to give some rigidity to the model, with the whole thing maybe in 1:10 scale. Some kind of transparent rigid plastic would be even better, but even though it’s cheap for secondary glazing, it’s probably a bit above our budget here. To save time, Dimitri intends to build the basic shape ahead of time. I think he’s looking forward to pre-testing the strength and checking the advantages and disadvantages of different foodstuffs! In a pinch, we can build up the top layers as the last stragglers arrive, particularly if that helps it fit in a taxi on the way to the event.
On Shrove Tuesday, we’ll be mounting pre-assembled and programmed Arduino-style boards equipped with radios and temperature sensors on balloon clusters, and then working in teams to construct a 3D model of the worship space at Christ Church Morningside. So as part of the preparations, we’ve been discussing how many balloons we’ll need. The answer appears to be “probably more than you think”, but for some surprising reasons.
First off, we can’t afford to lose any balloons in the space, so we need them to be on tethers. We’ll be taking a lot of extra precautions when we’re measuring the chancel (that’s the fancy area front of house where the priests and choir hang out) because there the risk is more than aesthetic if the air currents take balloons towards the organ chamber. We’ll be testing this – very carefully – with and without the organ blower on. The tethers are monofilament nylon fishing line, .25mm diameter. Since the height of the church is something like 14 meters, and nylon is 1.15g per cm3, the tethers will weigh .6-.8 g. It depends on whether we take them to the floor or leave them hand-height/dangling where the air currents are low – we’re going to test, and affix something visible but light to the end when they’re safe not to hold, because we can’t create any trip hazards. So let’s call that 1g. Not much, but not nothing.
Second, balloons themselves weigh something. Working backwards from the data sheets of a major supplier, and the densities of helium and air at room temperature and standard pressure, I get
|declared volume in litres||lifting capacity of that much pure helium (g)
||declared lift in g||therefore, the balloon must weigh at most (grams)|
|18 inch “foil”
|11 inch latex||0.015||15.36||10||5.36|
There has been much discussion of these data sheets from different angles, not least that most “balloon gas” isn’t pure at all. We’re lucky enough to have access to a supply of pure helium (thanks, University of Edinburgh School of Informatics!) that will cover most if not all of our needs. The data sheets say the declared lift is conservative, but does that mean they could lift more, or they could lift less? An engineer might think the former, but as a human factors person, I think it could be the latter. One of the reason for publishing such tables is a California law requiring weights so that balloons don’t end up as pollution, so it might be more about how much weight you need to tie on to not be responsible for the results. Between that and the fact that it’s hard to fill balloons completely accurately, we’ll be testing this ahead – but for now, let’s take the declared lift as reasonable.
As you can see, foil balloons are pretty useless for lift. On the other hand, they’re much less prone to popping than latex. We’re still thinking about what microcontrollers we want to use for the exercise, but if they’re the heavier ones we have, the payload will end up being around 25g. 3 11″ latex balloons should work, but if they all popped at once, anyone underneath would have less than 2s to get out of the way or be hit with an object going at nearly 17 m/s. Single foil balloons only shift that slightly, but the more you have, the harder it is to take temperature readings. Meanwhile, we can tie the tether through the microcontroller so it can’t come off, but what about the join at the balloons? So that’s the current behind-the-scenes discussion – about likely failure scenarios, hard hats, safety glasses, warning signals, and landing pads. We don’t intend to have any fall off – we’re consulting an angler and planning a knot inspection before anything goes up – but safety above all else.