31 Mar 2011, 10:46pm
fixing things:
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  • A rare dodgy piece of German engineering in this propagator heat mat

    Germany has got itself a reputation for top quality engineering, so I was somewhat surprised to come across a pretty poor example of German engineering in terms of the Bio-Green Sahara heating mat. The mat itself is fine, and continues to do sterling service. It is foil with heating wire run through it, the aim is to stick this under seeds being propagated.

    It addresses the fundamental problem that the UK is too far north for most of our vegetables, which were used to the more balmy climates of lower latitudes. So the seeds don’t really get the feeling that it is time to grow until too late in the year, when the temperature reaches what they expected in the Spring, but we get in Summer.

    The heat mat comes with a thermostat with a remote probe

    which DGF reasonably assumed to be connected with a wire. I had noticed that this damn thing was adding a pretty outrageous extra load to our daily power usage – it is a 65W heater, so if it were on continuously, then it would draw the same amount as the fridge, namely 24×65/1000=1.5kWh.

    It was time to break this out again this year, and I took a look at it. The probe reminded me of the sort of thing used on a gas valve to stop the main gas valve opening until the pilot light is lit. These are often thermopiles driving solenoids these days but in the past they were a copper tube with a volatile fluid in it which vaporised on heating to increase the pressure. A capillary tube takes this to the gas valve and opens the valve under pressure.

    True enough, this appeared to be the case here, and the device was associated with a disturbing smell of chloroform, which is presumably the active ingredient.

    Now in a gas cooker you don’t expect to move the sensor, so having a rigid copper capillary tube is okay. But a heating mat that is described by some retailers as

    These all new aluminium encapsulated versatile heatmats easily roll up when not in use.

    should not be supplied with a device that is designed to be deployed to a fixed installation. Flex that sensor tube too often and the bugger will crack, releasing the chemical into the atmosphere so that the thermostat will never turn off. The manual really ought to tell you that this is extremely delicate and should not be flexed repeatedly.

    The next thing that is painfully wrong is that they tell you to whack the sensor into the soil. Stands to reason, right, that’s what you are trying to control? Not so fast – there is a problem in that the delay between the heat getting to the sensor means there is a large overshoot, as the sensor tells the heater “turn it up, turn it up, turn it UP WHOA THERE turn it DOWN you’ve gone far too much turn it DOWN”.

    For the mathematically inclined this is a control system and the lag mucks about with the poles on your Bode plot. I think that’s what I recall from uni. As an engineer it was a lot easier, the mantra was always get your sensor right next to the heater. Which is counter-intuitive because you are measuring not at the point of delivery, but it gets the delay down. You will get a static error due to the thermal resistance from the heater to the soil, but that’s better than ending up with large temperature swings. Industrial control systems use proportional control and may add rate-of-change and integrating loops to go for greater accuracy but these are seeds, they just want to feel they’ve been shifted southwards about 20 degrees of latitude.

    The instructions should also have included the practical stuff to make an efficient plant mat, as well as how to avoid knackering the device. The heat only needs to go upto the seeds, rather than downwards, so the heat mat should be placed on an insulating substrate, otherwise energy will be used worthlessly in heating the potting bench. Celotex is probably ideal, but I used a couple of sheets of expanded polystyrene foam covered with aluminium foil. You obviously want to consider what happens under fault conditions with the heater on permanently and dimension accordingly 😉 The heat mat then sandwiches the heating cable in two sheets of thick aluminium foil, spreading the heat better.

    For those looking to do this on a budget, this can be made using standard heating cable such as used for keeping pipes frost-free, with thick aluminium foil either side. If you are doing that using mains power, you should know the difference between class I and class II insulation and how that pertains to your construction. For UK / Northern Europe you’re looking at about a maximum power input of 150W per square metre, the thermostat will kick that back as required.

    The thermostat probe then needs to be placed above the foil, rather than in the soil, and the whole lot covered in a thin layer of builder’s sand, on which the plastic modular trays with the seeds are placed. Since I am using this in a conservatory and don’t want the floor covered in sand I constructed a tray from some plywood and bits of pallets to contain the sand. The disadvantage of using wood from pallets is all the pieces are different widths which makes you look a rotten carpenter if you don’t have access to power tools to trim them to size, or the patience to do it with a jack plane. You can’t argue with the price, however!

    tray with heater mat and sand

    I was still left with a defective thermostat, so I replaced this sucker with a Dallas DS1820 digital temperature sensor and a 16F628 PIC microcontroller to drive a triac controlling the mains powered heating mat. It ended up looking more like a piece of laboratory equipment than a cuddly Bio-Green growing device, but is a lot more accurate. That system was originally part of a project to propagate sweet potatoes but the development time was a couple of weeks too long so I missed the start time and the tubers rotted 🙁 My device had a second sensor and serial output because those sweet potatoes are finicky, this was something that originally grew in Mexico and South America. You are seriously taking the mickey trying to persuade them to think about growing in a chilly British March…

    Temperature controller. It doesn't have the friendly curves of the Bio-Green devices so it looks like a piece of lab equipment but it is far more accurate.

    For a propagator I don’t need 0.5C accuracy, so if we end up needing more of this sort of thing I may just use an analogue system using thermistors. I also had a Sankey propagator base I used to use for tomatoes, this is thermally balanced and could do with temperature control to save power and get more reliable, it can easily drift up beyond 30°C, which isn’t that great and makes the contents a sod to keep watered.

    To save anyone who may come across this having to look the germination temperatures up, here are the values I swiped from a Plant Propagation lecture by the Organic Growers Alliance:

    28°C Cucumber

    25°C Aubergine, Pepper, Tomato

    15°C Celery, Celeriac, Calabrese, Early Cabbbage, Brussels Sprouts

    12°C Sweet Corn

    10°C All others

    All a fair amount of rant from a poor piece of engineering, though it says something for German engineering in general that the odd dodgy one stands out so much. Bio Green do make the electronic version of the device I constructed for about £50.

     

    31 Mar 2011, 6:42pm
    peak oil personal finance shares:
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  • A New Financial Year looming, plus the Sound of Thunder in the Distance

    April 6 is the new financial year in the UK for some curious reason, as it seems the fiscal year ends on the more rational March 31. I’ve maxed my ISA for now, so the change of year means I get to have another bash at building tax-free assets that won’t be counted as income in future. It’s also a chance to have a general reshuffle. Every so often I have to get to lift the drains up and hose out the accumulated fat and grease of the finances to see if it accords with my values and beliefs. That’s not the same as getting the right answer, because my crystal ball is as cloudy as anyone else’s. but at least it will be my own mistakes 😉

    I don’t bugger about with formal rebalancing of my ISA, because I’m just not that kind of guy, and also because I am still in full ISA purchase mode what I do this year could shift my asset allocation by about 40%. So I rebalance by going to buy what I don’t already have a lot of. Which probably means mining, pharma, some REITs and some financials, but I have to research this.

    What I need is a jolly good stock market crash this year, so I can buy cheaper. There are distant sounds of thunder – some of the eurozone rumblings and of course all that oil war adventure is probably good for some of this. Some part of me suspects that this distant sound of thunder is the beginning of the end, as Peak Oil starts to overcome industrial civilisation as we have currently set it up. Although I wasn’t economically active in 1973 I was sentient, and we’ve been here before, so I may get my jolly good stock market crash this year, possibly on the popular revolution in Saudi Arabia. The challenge, of course, will be whether it (the stock market, rather than Saudi…) gets up off its knees afterwards as it did then. An awful lot of companies’ business cases would look a lot different with oil at $250 a barrel, and not many of them would look better.

    I wasn’t used to how National Savings Index linked certificates worked last year. In particular I didn’t realise that these were desert blooms, only available for a short time after the Spring rains. My plan was to buy £500 of these each month to give me a steady index-linked income boost in three year’s time (now two years). That doesn’t fit with the seasonal availability, so I only got £2000 into that before they were summarily canned. Which sort of put the kibosh on that bright idea 🙁

    However, NS&I may still serve me well. I have an emergency fund of about £7500 in a two year’s back to back Nat West Cash ISA, which, all credit to them,  has actually continued to provide a3%-ish interest rate. Now on reflection, there is a lot to be said for shifting this to NS&I certificates, because you can

    • get the money out at short notice, although at an interest penalty
    • but it’s a little bit of bother, so you have to think about it
    • and thr RPI indexing means an emergency fund of £10k today will be able to fix the same amount of emergency in five years hence
    • oh yes, and did I say it’s tax-free, so what the heck is the point of sterilising some of my ISA allowance looking after cash?

    Which beats the cash ISA option, which dies a little bit by about 2% a year. There’s also an opportunity here – I believe I can transfer the Cash ISA into my shares ISA ands still load up with this year’s ISA allowance, ie I could get £17700 into my shares ISA earning an income for me rather than £10200 into it this year. Of course the downside of that is I have to save the £7500 to go into NS&I in the next month, plus save up £10200 over this next year plus increase my pre-tax savings in AVCs to keep that greedy tyke Osborne out of my pay packet.

    That’s a very serious big ask and I may not make it, though my outgoings and non-financial investment costs have dropped. But it’s a potential opportunity.

    As to the asset allocation, for myISA I have shifted it to this

    March 11 ISA asset allocation

    which, compared with December has changed to more accurately reflect my views on what an ISA should do for me. Which is buy me an income that isn’t considered an income for tax purposes. I’ve dropped all holdings of precious metals in my ISA because I have come to the conclusion that an ISA is not the place for precious metals. This isn’t because I have decided holding precious metals isn’t for me, I simply need to get my policy on that right, so at the moment I am exposed to currency debasement big time, apart from my non-financial investments.

    Overall financial asset allocation

    Overall my total share allocation including pension AVCs and stuff outside my ISA is more balanced. The obvious holes as mining stocks, AsiaPac and the US, all of which confuse me.

    The US is home to an enterepreneurial bunch of go-getting people who aren’t known for taking no as an answer, and I am sure this will work to their advantage in future. However, they have some deep systemic problems arising from being a reserve currency, which has permitted some extreme excesses which are denied everyone else. I’d prefer not to be caught in the crossfire of unwinding those debts. The US is also hellishly exposed to Peak Oil, in a way which is so much more extreme than anywhere else.I can imagine a Europe without liquid transport fuels. I struggle to imagine a US without gasoline, with perhaps the exception of New York and some of the East Coast. And the low taxation of oil makes the US economy far more sensitive to increasing crude oil costs.

    I am sure Americans may be resourceful enough to sort it, but they really do have to get off their butts and engage, simply repeating that “the American Way of Life is Not Negotiable” is not what I would consider a rational response. I am not sure that the military option is such a great answer either. There seems to be some doubt about whether it increased oil production in Iraq  some say yes, but it is not a universally held view. So at the moment I don’t do America, other than as part of my FTSE100 holdings.

    Mining, yes, shame that is riding high at the moment 🙁 A missed opportunity from last year.

    AsiaPac – with the current state of the pound that all looks jolly expensive. I don’t do China, because I don’t understand it, and the demographics suck. I could combine mining with an Aussie tracker ETF, since mining seems to be a lot of what Australia is about.

    I am tempted by India, which has strong demographics and a go-getting entrepreneurial class, though some very serious strategic problems. It is hard to gauge performance where the currency has such a shocking inflation rate of around 10%. db-xtrackers do a GPB denominated ETF but this is a synthetic tracker using derivatives and swaps, which introduces a lot of hidden extra counterparty risks.

    Fortunately there’s no great hurry, apart from targeting that NS&I investment in April, and I have a war chest saved which can sort that for me, I will hit NS&I up to aim to hold a total RPI-indexed emergency fund of about 10k, so I can think about how to tackle the ISA over time.

     
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