Post by Ron Walker on Apr 20, 2022 19:41:58 GMT -7
Posted by: Ray Worthy Nov 15 2008, 10:34 AM
Note.
Unless something unforeseen crops up, I have finished the series of articles dealing with aspects of home made projectors, and I shall turn my attention to the building of inflatable domes. I began with an inflatable dome because I had no choice really. It was needed as an adjunct to my astronomy class and there was no space in the school, which could be dedicated to a permanent dome.
My First Dome. Part One.
In the early nineteen eighties, we, who lived on the east coast of England suffered a spring and early summer, when for a period of about six weeks, a flat layer of slate gray stratus cloud covered the sky. Imagine going six weeks of not seeing the Sun. This may have been depressing in the normal way, but when one was a student who needed to run some continuous astronomical observations for a practical examination, then it became a calamity.
I determined that, by the following year, I would have my own planetarium. At the time, there was no thought of running a business. The planetarium was simply an education tool par excellence. Once I could see that the projector was going to work, I turned my attention to the construction of a dome. I decided on one, which could be inflated and put away in storage, as the occasion demanded.
I had seen an example of the "Starlab" from Learning Technologies Inc. when my local astronomical society brought it over from Northern Ireland for a week and, although I was won over by the idea of such a dome, I had certain reservations about its design.
In that period, the current ladies’ fashion was the mini-skirt. More than once, I saw a young lady teacher refuse point blank to get down on her hands and knees to crawl through the entrance tunnel of the Starlab.
Also, when we tried to get some folks of a more advanced age to view the starry skies within, they found the tunnel too daunting. Then there was a cold snap and the air temperature dropped quite markedly. The inflation air, coming in from its tunnel spread on to the audience and I noticed an open space appearing as the audience shuffled away from the incoming air. At least a third of the floor space was unused.
I wanted a design by which the audience could enter with some dignity and the design should also possess some means by which the incoming air should not cause discomfort. I was puzzled also by the fact that the operator had to crawl outside of the dome to alter the airflow. The inflation fan made a noise like an airplane propeller and sometimes the lecture was delivered in a shout.
Quite by chance, I discovered the advantages of the tangential rotating cylinder fan. In the USA they are some times called "Squirrel Cage " blowers. Visiting a supplier, I found there was a sale on and I picked a fan up for half price. These were fans intended for the ventilation of factories, but the advantages for my purpose soon became evident. The fan could be continuously controlled remotely by a rheostat on a cable. That handset could be taken inside of the dome and fixed into the control console. No more crawling through the tunnel to turn it up or down. Naturally a plug and socket arrangement would be employed for dismantling and moving purposes.
Possessing this fan control allowed me to solve the "Entrance with dignity" problem. Entrance was by way of a simple zip in one of the panels. In the period when the door was being used and air was escaping, the fan pressure would be turned up to maximum and when the door was closed for the programs, the fan was turned as low as possible commensurate with the dome remaining inflated. At this point, I made a great discovery. At this low speed, the fan made no noise at all and the lesson could be delivered in a whisper if necessary. Another advantage was that quiet music could be used in programs.
The door was a vertical zip measuring, 1.7 meters long. Inside, the zip was protected by a long flap. This flap could be closed against the inside fabric by means of a Velcro strip. Later, when I found that a heavy-footed boy, eager to see the stars, tore the base of the zip. After that, I designed the bottom of the zip in an upside down "T" configuration. This gave some room for clumsy footed youths to enter without causing damage.
The problem with the incoming air, I solved by simply slowing the speed down by delivering it through an expanding inlet. The air rushing out from the tangential fan the air spread out to enter the wall of the dome across ninety degrees of its circle. The effect was that one could sit quite close to the incoming air duct and feel no cold draught.
The design of the dome was one thing but the great question for me at that time was what fabric was I going to use. I recall entering a school hall towards the end of a session run by a colleague from the Astronomical Society, when I saw a young lad leaving through the tunnel a few minutes earlier than the rest of the class. Think he was unobserved, he took up station alongside the tunnel. He had seen the possibility of a good practical joke. Most people who used the tunnel, brushed against the top and their position could be easily seen from outside. As his friends came through each of them received a bonk on the head to loud noises of complaint. By the time the teacher came out, all was quiet and the pupils demure. I could see why, although the Starlab was less than a year old, the roof of the tunnel was covered in crosses of a sort of band-aid.
What I wanted was a fabric more durable than that. I make no excuses for what, given the twenty twenty vision of hindsight, I made some terrible blunders. I simply record what decisions I made at the time. The robust material chosen was PVC. There was a tarpaulin factory a five-minute drive from my school, which I could reach in the lunch hour, and maybe that helped my decision. At the beginning of this adventure, I assumed that the manager of this factory would be competent so I listened to his advice. I required a white interior for the screen, but I need the dome to be opaque. The manager assured me that if I chose white PVC, he could render the dome impervious to light. I had provided him with the mathematical notes on how to generate a dome, but soon found that I had to spend a lot of time in the factory showing the entire workforce how to mark out the panels.
The trouble was that I knew the factory workers by name. I had taught them. The factory management seemed to have a policy of giving jobs to kids from the lowest streams. Some of them were incapable of writing their names, never mind understanding what a cosine was. However, we all muddled through and I had to go into the factory at weekends when I was available and the dome was finished. As a dome, it was all I wished for then, but when it came to rendering the dome opaque that was a different matter. I was given a black paint, which had to be sprayed on. This was how the tarpaulins were painted black. It soon became obvious that the black paint could not keep light out and the smell became all pervading.
I hired a commercial garage in which to inflate the dome for the spraying and was compelled by the odor to leave it inflated overnight to dry off. In the school the only place large enough for the inflated dome was the hall and I began finding some boys attending who were decidedly not the quality of my astronomical students. Unfortunately my beautiful dome had acquired the reputation of being a glue sniffers paradise. The dome needed to be erected before it could be aired off and the only place I had was the school hall. I became very unpopular with the caretaker as the whole building began to reek of the solvent. After a month or so the smell went away, but I could not use the dome in a lighted room. In the end, I made an outer cover of black agricultural polythene. After that, the inside of the dome was as dark as a coalmine.
There was another "sophistication" in the design, which I have not mentioned. I had the idea of having school children sitting on the floor with the horizon down at their eye level, but adults should be on chairs and the horizon adjusted to their eye level. This was achieved by means of two sets of brass rings, with one set a meter above the other. A drawstring was threaded through them. When the string was pulled, the horizon would be at the lower level and when the string was relaxed, then the horizon would be at the upper level. In practice, this arrangement lasted about two months before it was abandoned. The idea was clumsy and was always too much trouble. I always needed the assistance of some members of my Astronomy Club. I was never short of volunteers.
Ray
Note.
Unless something unforeseen crops up, I have finished the series of articles dealing with aspects of home made projectors, and I shall turn my attention to the building of inflatable domes. I began with an inflatable dome because I had no choice really. It was needed as an adjunct to my astronomy class and there was no space in the school, which could be dedicated to a permanent dome.
My First Dome. Part One.
In the early nineteen eighties, we, who lived on the east coast of England suffered a spring and early summer, when for a period of about six weeks, a flat layer of slate gray stratus cloud covered the sky. Imagine going six weeks of not seeing the Sun. This may have been depressing in the normal way, but when one was a student who needed to run some continuous astronomical observations for a practical examination, then it became a calamity.
I determined that, by the following year, I would have my own planetarium. At the time, there was no thought of running a business. The planetarium was simply an education tool par excellence. Once I could see that the projector was going to work, I turned my attention to the construction of a dome. I decided on one, which could be inflated and put away in storage, as the occasion demanded.
I had seen an example of the "Starlab" from Learning Technologies Inc. when my local astronomical society brought it over from Northern Ireland for a week and, although I was won over by the idea of such a dome, I had certain reservations about its design.
In that period, the current ladies’ fashion was the mini-skirt. More than once, I saw a young lady teacher refuse point blank to get down on her hands and knees to crawl through the entrance tunnel of the Starlab.
Also, when we tried to get some folks of a more advanced age to view the starry skies within, they found the tunnel too daunting. Then there was a cold snap and the air temperature dropped quite markedly. The inflation air, coming in from its tunnel spread on to the audience and I noticed an open space appearing as the audience shuffled away from the incoming air. At least a third of the floor space was unused.
I wanted a design by which the audience could enter with some dignity and the design should also possess some means by which the incoming air should not cause discomfort. I was puzzled also by the fact that the operator had to crawl outside of the dome to alter the airflow. The inflation fan made a noise like an airplane propeller and sometimes the lecture was delivered in a shout.
Quite by chance, I discovered the advantages of the tangential rotating cylinder fan. In the USA they are some times called "Squirrel Cage " blowers. Visiting a supplier, I found there was a sale on and I picked a fan up for half price. These were fans intended for the ventilation of factories, but the advantages for my purpose soon became evident. The fan could be continuously controlled remotely by a rheostat on a cable. That handset could be taken inside of the dome and fixed into the control console. No more crawling through the tunnel to turn it up or down. Naturally a plug and socket arrangement would be employed for dismantling and moving purposes.
Possessing this fan control allowed me to solve the "Entrance with dignity" problem. Entrance was by way of a simple zip in one of the panels. In the period when the door was being used and air was escaping, the fan pressure would be turned up to maximum and when the door was closed for the programs, the fan was turned as low as possible commensurate with the dome remaining inflated. At this point, I made a great discovery. At this low speed, the fan made no noise at all and the lesson could be delivered in a whisper if necessary. Another advantage was that quiet music could be used in programs.
The door was a vertical zip measuring, 1.7 meters long. Inside, the zip was protected by a long flap. This flap could be closed against the inside fabric by means of a Velcro strip. Later, when I found that a heavy-footed boy, eager to see the stars, tore the base of the zip. After that, I designed the bottom of the zip in an upside down "T" configuration. This gave some room for clumsy footed youths to enter without causing damage.
The problem with the incoming air, I solved by simply slowing the speed down by delivering it through an expanding inlet. The air rushing out from the tangential fan the air spread out to enter the wall of the dome across ninety degrees of its circle. The effect was that one could sit quite close to the incoming air duct and feel no cold draught.
The design of the dome was one thing but the great question for me at that time was what fabric was I going to use. I recall entering a school hall towards the end of a session run by a colleague from the Astronomical Society, when I saw a young lad leaving through the tunnel a few minutes earlier than the rest of the class. Think he was unobserved, he took up station alongside the tunnel. He had seen the possibility of a good practical joke. Most people who used the tunnel, brushed against the top and their position could be easily seen from outside. As his friends came through each of them received a bonk on the head to loud noises of complaint. By the time the teacher came out, all was quiet and the pupils demure. I could see why, although the Starlab was less than a year old, the roof of the tunnel was covered in crosses of a sort of band-aid.
What I wanted was a fabric more durable than that. I make no excuses for what, given the twenty twenty vision of hindsight, I made some terrible blunders. I simply record what decisions I made at the time. The robust material chosen was PVC. There was a tarpaulin factory a five-minute drive from my school, which I could reach in the lunch hour, and maybe that helped my decision. At the beginning of this adventure, I assumed that the manager of this factory would be competent so I listened to his advice. I required a white interior for the screen, but I need the dome to be opaque. The manager assured me that if I chose white PVC, he could render the dome impervious to light. I had provided him with the mathematical notes on how to generate a dome, but soon found that I had to spend a lot of time in the factory showing the entire workforce how to mark out the panels.
The trouble was that I knew the factory workers by name. I had taught them. The factory management seemed to have a policy of giving jobs to kids from the lowest streams. Some of them were incapable of writing their names, never mind understanding what a cosine was. However, we all muddled through and I had to go into the factory at weekends when I was available and the dome was finished. As a dome, it was all I wished for then, but when it came to rendering the dome opaque that was a different matter. I was given a black paint, which had to be sprayed on. This was how the tarpaulins were painted black. It soon became obvious that the black paint could not keep light out and the smell became all pervading.
I hired a commercial garage in which to inflate the dome for the spraying and was compelled by the odor to leave it inflated overnight to dry off. In the school the only place large enough for the inflated dome was the hall and I began finding some boys attending who were decidedly not the quality of my astronomical students. Unfortunately my beautiful dome had acquired the reputation of being a glue sniffers paradise. The dome needed to be erected before it could be aired off and the only place I had was the school hall. I became very unpopular with the caretaker as the whole building began to reek of the solvent. After a month or so the smell went away, but I could not use the dome in a lighted room. In the end, I made an outer cover of black agricultural polythene. After that, the inside of the dome was as dark as a coalmine.
There was another "sophistication" in the design, which I have not mentioned. I had the idea of having school children sitting on the floor with the horizon down at their eye level, but adults should be on chairs and the horizon adjusted to their eye level. This was achieved by means of two sets of brass rings, with one set a meter above the other. A drawstring was threaded through them. When the string was pulled, the horizon would be at the lower level and when the string was relaxed, then the horizon would be at the upper level. In practice, this arrangement lasted about two months before it was abandoned. The idea was clumsy and was always too much trouble. I always needed the assistance of some members of my Astronomy Club. I was never short of volunteers.
Ray