Gas station without pumps

2014 September 11

JanSport warranty works

Filed under: Uncategorized — gasstationwithoutpumps @ 18:35
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As I reported in Testing JanSport warranty, I sent my worn-out JanSport backpack in to their warranty service a month ago, as it was no longer usable (hole in the leather bottom, main zipper fails frequently, shoulder straps fraying where they join the body of the pack) to try out the JanSport warranty:

JanSport engineers quality, durable, and reliable products. So, if your pack ever breaks down, simply return it to our warranty center. We’ll fix it or if we can’t we’ll replace it or refund it. We stand by our packs for a lifetime and since we’ve been making packs since 1967, that’s a guarantee you can stand by.  [http://www.jansport.com/shop/en/jansport-us/content/warranty]

Shipping the pack to San Leandro, CA only cost me $5.32, which seemed like a better deal than $55 for a new backpack.  The replacement pack came today (as I expected, they figured it would be cheaper to replace rather than repair the pack), a month later than when I sent the old one in.

The new pack is a different color (they no longer make a brown pack), and they have gone cheap on some aspects of the design—the old pack had an organizer that held 4 or more pens and pencils, plus a checkbook or two.  The new one barely holds 3 pencils and credit card, and the material for the organizer seems flimsy.  They’ve added a new feature—a laptop sleeve that adds weight to the pack while not adding anything I find useful, just making the main compartment more cluttered. I’ll use this one for a while, but I’ll probably be looking for a pack that has a decent organizer, since that was the feature that drew me to the old JanSport pack, and they have abandoned the one feature that made it stand out from a bunch of similar packs.

So the new pack is not as useful for me as the old one was when it was new, but better than the worn-out pack.  At least the zipper works and the shoulder straps aren’t in danger of falling off. The JanSport warranty works, if you don’t mind that they no longer make their sturdy pack with a decent organizer.

Thermal models for power resistor with heatsink

Last night I fit a simple thermal model to temperature measurements of some power resistors: T(t) = PD+A+(T_{0}-PD-A)e^{-t/(DM)}, where P is the power in watts, D is thermal resistance in °K/W, M is thermal mass in J/°K, A is the ambient temperature in °C, and T0 is the initial temperature.

I ran into problems with the 1.8Ω 50W THS501R8J resistor, because it heated up very fast and I could only get a few measurements when delivering power, before I had to turn it off.  I proposed adding a heatsink, a 6″×12″ sheet of aluminum 0.063″ thick, to increase the thermal mass M and decrease the thermal resistance D.  I estimated that the thermal mass should increase by the heat capacity of that much aluminum (74.33 cm3 at 2.422 J/°K/cm3, giving 180 J/°K), but I did not have a good way to estimate the change in thermal resistance.

The 6"×12" plate is much larger than the power resistor, which is bolted in the center with M3 screws (American 6-32 screws are a little too big for the holes in the resistor).

The 6″×12″ plate is much larger than the power resistor, which is bolted in the center with M3 screws (American 6-32 screws are a little too big for the holes in the resistor). I used a thin layer of white thermal grease to get better thermal conduction between the resistor case and the aluminum plate.

I do not expect the simple thermal model to work well, because it assumes that you have an isothermal object—all the aluminum at the same temperature.  But a large flat plate is going to have significant thermal spreading resistance, so that the resistor in the center is hotter than the edges of the plate.

With a heatsink the time constant DM is about 260s, only a little faster than the 347s without the heatsink, but the thermal resistance is much lower, so the maximum temperature (PD+A) is  much lower.

With a heatsink the time constant DM is about 260s, only a little faster than the 347s without the heatsink, but the thermal resistance is much lower, so the maximum temperature (PD+A) is much lower.

As expected, the fit is not great. When cooling off, the initial temperature of the resistor is higher than of the surrounding plate, so the initial cooling at the resistor is faster than the eventual cooling, when resistor and the plate are closer in temperature, because heat is being transferred to the plate as well as to the air. The increase in thermal mass  (about 100 J/°K) was less than my crude estimate based on the heat capacity of the added aluminum (180 J/°K)—this is probably also due to the thermal spreading resistance and the non-uniform temperature of the heatsink.

resistance rated power heatsink? test power M [J/°K] D [°K/W] DM [s] T [°C]
10.10Ω 100W No 8.288W 101.7 6.38 649 75.7
8.21Ω 50W No 10.169W 32 11.58 371 143.7
1.81Ω 50W No 43.174W 31.9 10.87 347 495.8
1.81Ω 50W Yes 43.174W 131.7 1.97 260 110.3

Note: the asymptotic temperature T in the table above is with the 9V power supply I have, which does not have quite constant voltage over the range of powers tested. With a 12v supply, temperatures would be much higher: D V^2/R +A .  The asymptotic temperature is also the maximum when the resistor is sitting in still air that is unconfined.  A fan would reduce thermal resistance and make the asymptotic temperature lower, but confining the resistor in a box (like in the incubator design) would make the “ambient” temperature not be constant—the relevant thermal resistance is how slowly the air in the box loses heat, which for the thick-walled styrofoam boxes we’ll use is a very high thermal resistance.  Without a feedback loop and PWM to keep the power down, even the 10Ω resistor would get very hot in a styrofoam box.

I should probably test the 10Ω 100W resistor on the heatsink also, to see if that reduces the time constant DM.  I expect the thermal mass to go up by something between 100 and 180 J/°K, but the thermal resistance to drop to around 1–1.5 °K/W, getting DM in the ballpark of 300s.  I don’t think I’ll do that today, though, as making measurements every 20 seconds for 2000 seconds is tedious and leads to cramping in the hand that aims the IR thermometer and keeps the trigger pulled.

Which raises a pedagogical question: Should I have students do the measurements?  Should I show them how to make a recording thermometer with a thermistor first? They’ll need to figure out how to use a thermistor for measuring air temperature anyway.

The thermistors I have at home (NTCLE100E3103JB0) only go up to 125°C, and I’d want them to have one that goes to at least 175°C for this lab, which means using something like NTCLG100E2103JB (10kΩ, ±5%, ±1.3% on B-value, -40°C to 200°C), which is only 35¢ in 10s, so still cheap. I should get myself some of these higher temperature thermistors and test out the recording . (Or the tighter tolerance NTCLE203E3103SB0, which only goes up to 150°C, or the wider temperature range 135-103LAF-J01, which goes to 300°C.)

How will I attach the thermistor to the resistor for temperature measurement? tape?  (I have to be sure not to short out the thermistor leads on the aluminum case of the resistor.)

Air temperature sensing poses less of a mounting challenge, but the thermal delays will be quite large—I have to look at how difficult it will be to tune a PID or PI controller with large delays—we really don’t want huge overshoot.  If the students have multiple temperature measurements (resistor temperature and air temperature, for example), they may need a more complicated control loop than a simple 1-variable PID controller.  How much can we simplify this?  (Perhaps a PI or PID controller based on the air temperature, with over-temperature shutdown on the resistor temperature?  Then tuning the PID controller with the constraint that the gain be kept low enough to keep the over-temperature shutdown from kicking in?)

Thermal models for power resistors

Filed under: freshman design seminar — gasstationwithoutpumps @ 06:46
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I recently bought some power resistors, to use as dummy loads for testing PWM circuits and to use as heating elements in an “incubator” design for the freshman design seminar.  I bought 3 resistors: 10Ω 100W HSC10010RJ,  8.2Ω 50W THS508R2J, and 1.8Ω 50W THS501R8J.

I want to make simple models for the thermal behavior of these resistors when they are not mounted on a heatsink, but are just sitting on a low-thermal-conductance surface.  The simple model will have two parameters: a thermal mass M (in joules/°C) and a thermal resistance D (in °C/W).  If we just had the thermal mass, we would have \frac{dT}{dt} = \frac{dE}{dt}/M = P/M, where E is the thermal energy, and P is the power delivered to the resistor, and the temperature would increase linearly: T(t) = T_{0} + Pt/M. But as the temperature increases above the ambient temperature, the resistor loses energy at a rate proportional to the temperature difference from ambient: \frac{dT}{dt} = (P - (T(t)-A)/D)/M.  We can rewrite this as a standard first-order differential equation: \frac{dT}{dt} + \frac{T(t)}{DM} = \frac{PD+A}{DM}, which has the solution T(t) = PD+A+(T_{0}-PD-A)e^{-t/(DM)}.  Note that \lim_{t\rightarrow\infty}T(t)= PD+A, independent of the thermal mass, and the cool down with P=0 is dependent only on the initial temperature, the ambient temperature, and the product DM, not on D and M separately.

To find the parameters for each resistor, I connected each to my 9V 6A power supply, and measured the temperature at regular intervals with an IR thermometer.  For the 50W resistors, I blackened the bodies of the resistors with a felt-tip pen to make the IR thermometer more accurate—I had not done that with the 100W resistor, but it took so long to make the measurements on that resistor that I did not want to go back and remeasure it.  It had a colored finish and may have been closer to being a blackbody radiator than the 50W resistors, so the errors may not be too large.The errors due to not holding the gun in a perfectly fixed position probably contribute more error.

The fits are not too bad—this simple model seems to represent the thermal behavior of the resistors fairly well.

The 100W resistor, as expected, has a very high thermal mass and fairly low thermal resistance.  With a low power input (8% of rated power), the equilibrium surface temperature is still quite low.

The 100W resistor, as expected, has a very high thermal mass and fairly low thermal resistance. It heats up and cools down slowly. With a low power input (8% of rated power), the equilibrium surface temperature is still quite low, only about 76°C—well below the 240°C melting temperature of styrofoam. Even with a 12V supply the temperature would only get up to (12V*12V/10Ω)*6.38°C/W + 25°C=117°C.

The 8.2Ω 50W resistor has a lower thermal mass but a higher thermal resistance than the 100W resistor.  It heats up much faster, and cools down somewhat faster than the 100W resistor.  It is being run at about 20% of the rated power, and it is supposed to be able to be run at up to 40% of rated power (20W) without a heat sink.

The 8.2Ω 50W resistor has a lower thermal mass but a higher thermal resistance than the 100W resistor. It heats up much faster, and cools down somewhat faster than the 100W resistor. It is being run at about 20% of the rated power, and it is supposed to be able to be run at up to 40% of rated power (20W) without a heat sink.

The 1.8Ω 50W resistor has similar thermal characteristics to the 8.2Ω 50W resistor (is is the same package in the same series), but because the power is much higher 86% of rated power, it heats up very fast and would exceed the temperature specs for the resistor if left on for more than a couple of minutes.

The 1.8Ω 50W resistor has similar thermal characteristics to the 8.2Ω 50W resistor (it is the same package in the same series), but because the power is much higher 86% of rated power, it heats up very fast and would exceed the temperature specs for the resistor if left on for more than a couple of minutes.

Adding a large heatsink would increase the thermal mass and decrease the thermal resistance of any of the resistors. If I want to use the 1.8Ω resistor, I will definitely need a heatsink! I can run the 8.2Ω resistor without a heatsink at 9V, but at 12V it would get up to 230°C, too close to the melting point of styrofoam. The 10Ω 100W resistor could be used safely even at 12V. I’ll try adding a 6″×12″ sheet of 0.063″ thick aluminum.  According to the Wikipedia article on heat capacity, the specific heat capacity of aluminum is about 2.422 J/°K/cm3, so the sheet should add a thermal mass of about 180 J/°C, but computing the thermal resistance is complicated, so I’ll just measure the temperature rise and fit the model.  Even if the heat dissipation were not increased (very unlikely), the greater thermal mass and resulting 7× slower response will make measurements easier and less likely to result in overheating the 1.8Ω resistor.

I’ve now tested that my power supply is capable of delivering 8.84V/1.81Ω = 4.88A. I still need to put the 1.8Ω and 8.2Ω resistors in parallel and see if I can get 6A from the power supply. The output impedance of the power supply seems to be about 78mΩ, given how much voltage drop there is with increasing current. Most of that may be the wiring from the power supplies to the resistor, as the power supply senses the voltage as it leaves the power supply, before the IR drop of the wiring.

2014 September 10

edX finally finds the right market

Filed under: freshman design seminar,home school — gasstationwithoutpumps @ 10:02
Tags: , , ,

I have long been of the opinion that MOOCs are pretty useless for college students but are good for home-schooled students and high school students who don’t have access to higher-level courses in their local schools.  It seems that edX has finally realized that this is an important market with their High School Initiative:

Colleges and universities find that many students could benefit from taking a few extra courses to help close the readiness gap between high school and college. To address this need, edX has launched a high school initiative–an initial collection of 26 new online courses, including Advanced Placement* (AP*) courses and high school level courses in a wide variety of subject areas.

Completion rates will still be low, as a lot of people will sign up on a whim and then not follow through, or will sign up for more than they can handle and be forced to drop some.

AP courses will probably be the most attractive courses, as students can validate their learning with the AP exam, which is widely accepted by colleges as proof of higher-level course work (unlike the “Verified Certificate of Achievement” that edX sells).

The hardest courses to do well on line will be the lab science courses.  Simulated labs are no substitute for real-world labs, as no simulation captures all the phenomena of the real world and few come close to developing lab skills.

There are on-line science courses with real lab components. For example, my son took AP chem through ChemAdvantage.com, which had some rather cleverly designed labs that could be done at home with minimal equipment. Despite the cleverness of the lab design, the lab skills practiced were not exactly the same as would have been practiced in a more traditional lab setting.  And the lab kit was not cheap, costing as much as a community college chem lab course would have (if my son had been able to get into the over-subscribed chem lab course).

I don’t know whether edX has gotten their AP science courses audited by College Board (if not, they’ll probably be forced to remove the AP designation), but the AP audit requires lab time for the AP science courses, and I don’t know which of many mechanism edX is using to provide the required lab content.  Other online AP courses either devise home labs (requiring the purchase of a lab kit) or do weekend or week-long lab intensives in various parts of the country.  These lab intensives can be quite good (if done in college labs with real equipment) or ludicrously overpriced time wasters (if done in hotel ballrooms with crummier equipment and less time than the home lab kits).

The edX AP Physics course, created by Boston University, says “The course covers all of the material for the test, supported by videos, simulations, and online labs.”  So it seems that they have no real labs in AP Physics, but only simulations.  While simulation is a wonderful thing, it does not develop much in the way of real-world lab skills.  I note that in the freshman design course I taught last year, often the only experience that students had had in building anything had been in their high school’s AP Physics courses.  That hands-on experience is very important for developing engineers.

So the edX courses will be valuable for students who have no other access to AP-level material (which is a lot—fewer than 5% of US high schools offer AP Computer Science, for example), but students will still usually be better off finding a community college course or other way to real lab experience for the AP science lab courses.

I wish edX great success on this endeavor, since I have seen first-hand the need for reasonable quality, affordable courses for advanced high school students, which many local high schools cannot provide, because they do not have enough students ready for the course in one place to justify creating and offering the courses.  It is a much more worthy market than trying to compete with brick-and-mortar colleges, which was the initial goal of Coursera, Udacity, and edX.  Udacity has already abandoned that goal in favor of corporate training (again, a reasonable market).  It is good to see the edX is moving in a reasonable direction also.  When will Coursera realize that their original “disruptive” dream was a pipe dream (probably as soon as they’ve burned all the venture capital)?

2014 September 9

Why are students going to for-profit colleges?

Filed under: Uncategorized — gasstationwithoutpumps @ 18:03
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Faculty in public colleges are often mystified why students would choose to take out enormous loans to attend for-profit colleges whose degrees are mostly not respected either by industry or other colleges. In Confessions of a Community College Dean: Corinthian Learner “Dean Dad” explains why for-profit colleges managed to attract students, despite the low quality and ripoff financing:

Put simply, for-profits rushed in to fill the void left by the publics.  Decades of relative neglect of public higher education, combined with a certain (ahem) narcissism within the sector itself, left entire populations underserved.  Perhaps for impure reasons, for-profits figured out how to reach students nobody else bothered to reach.  They pioneered evening, weekend, and online delivery.  They built schedules around student needs.  They focused on a few distinct majors that both students and employers could understand.  And for a while, in some sectors, some of them got decent results.  In the late 90’s, you could do a lot worse than graduating with a degree in CIS.

For-profits filled a void. If you want to prevent the next catastrophe, tend to the void.

That means consciously and aggressively using the public sector—both community colleges and four-year regional campuses—as hedges against future disaster. It means making a dramatic and sustained turn away from the long-term trend of austerity for the publics and an open spigot for for-profits. When you include the cost of bailouts, the “efficient” for-profits wind up inflicting a far greater fiscal burden on the public than more generously funded publics would have. That’s even more true when you factor in student loan debt from students who never graduated, or who graduated but never earned salaries commensurate with their debts.

If dampening demand isn’t really an option, and diverting demand to the private sector leads to financial catastrophe, maybe…stay with me…we could fund the public sector well enough to meet the demand itself. Keep student cost down, get quality up, and learn some valuable lessons from for-profits about meeting students where they actually are. Prevent the next wave of for-profit megagrowth by choking off its air supply.

That means getting away from flat or declining operating budgets supplemented by “targeted” grants that fade away in three years, and instead pouring a fraction of what a for-profit bailout would cost into the public sector. When I was at DeVry, I didn’t see fear of the government or fear of lawyers. I saw fear of the nearby community college. There was a reason for that.

As long as for-profits are considered in isolation, we’ll continue to miss the point. Yes, close loopholes, prosecute liars, and enforce regulations, but those amount to fighting the last war. If you want to prevent the next bailout instead of the last one, you have to address the demand side. Give community and state colleges the resources—and, yes, the flexibility—to flood the zone. It’ll cost some money upfront, but it’s cheaper, more humane, and far more productive than bailouts and legal fees after the next collapse. We don’t have a great record of learning from catastrophes, but this one should be easy.

His post was made in response to the failure of the Corinthian Colleges chain, where the investors had siphoned off all they could get away with and the federal government was not allowing them to be the recipients of student loan money any more.  Other for-profit chains (Anthem Education, for example) are also on the ropes, now that the Federal spigot of infinite loans is being turned back a tiny bit. No attempt is expected to claw back the money from the “investors” who ripped off the students tricking them into taking out enormous loans for a fake education.

I think that his message is indeed correct. It is a lot cheaper to fund public universities well than to clean up the mess left by a completely unregulated for-profit education “industry”.  Students should not need to take out enormous loans to get an education, particularly not the inferior one that the for-profits provided.  Society is better served by having an educated populace that is not hugely in debt.

 

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