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Forum Index : Solar : Deciding if I should build this solar heater
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| rogerdw Guru  Joined: 22/10/2019 Location: AustraliaPosts: 852 |
Not sure what it is, but fairly shallow. Certainly nothing like 35 degrees. I was surprised to see you screw them straight down on the roof with no rails ... certainly saves on a lot of hardware. Do you have any trouble keeping the cables from being squashed? I was surprised at that as well. Was it difficult to avoid damage to the lower ones as you dragged them across? I envisiage bringing the panels up on the other side of the roof and across from there, so I shouldn't have to worry so much ... I hope.  I decided to buy a datalogging multimeter as well, so once that gets here I can experiment a little. Will be interesting to compare light levels etc with the final output temperature Yes, in the back of my mind I think I'll have to buy a heap of seals ... and then see what sort of tank I can use to experiment with. They are something like $99 for 30 on ebay from a Victorian company ... unless I can find them on alibaba perhaps. Something else I heard today from Marcel, a guy in the US who has been experimenting with this stuff ... he said he's not interested in heat storage now (rocks, wax) ... he thinks the hot air should be pumped into a small dedicated room housing a heat pump water heater ... to store warm water for later use. Had certainly never thought of that, although it adds the cost of a heat pump ... so may as well heat water directly with the tubes I suppose. Anyway, always good to hear of other angles. His youtube videos show a spiral barrier wound around the air tubes down the centre of the evac tubes, to make the air spend more time gathering heat as it winds its way out. That is also something I need to experiment with ... but I'll need to go back to single tubes and run comparisons side by side. I need to find two identical fans to run at the same time. Me too. Sounds a bit like a light diode. Cheers, Roger |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
I have heard a lot of concerns from people about not using rails but I have yet to see ( or hear) a practical or significant reason why it is detrimental. I believe is some parts of the world that is how it is done which also makes me wonder If using them is just more of a profitability angle for an industry that it for the most part, Rotten to the core. The main argument is ventilation under the panels but I give that pretty much zero Credibility. One only has to put a panel out on a sunny WINTERS day, Zap it with a thermo gun and see that the thing is going to be hotter than it's rated output temp anyway. Might be OK if you are in snow but if not.... Panel is already off it's curve and the temperature is not going to be any different with ventilation on a summers day that's for sure. I haven't had any problems with Crushed cables. While the frame of the panels does sit on them going across the roof Corrugations, it's a flat not a sharp section and not like the things have 1000 Kg of force on them. I try to position the connectors so they are not lying in the ridges but across them and out of any water coming under the panels. It wasn't difficult to avoid, I simply haven't had any at all. Sometimes you will get a mark across the panels which is not a scratch, I believe it may be some coating or lacquer from the frame coming off. I notice it does come off anyway. Unlike the panic merchants bedwetter at the other place would have you terrified into believing, panels are pretty dam tough and they get all sorts of grease and film and deposits on them over time. Bit like road grime on a car or if you ever leave a car out in the open you see the same settled crap. The bedwetters go on like you have to handle the thing like you would a 500 Yo piece of ancient parchment and any mark or blemish will render the things useless. Just not true. Lets face it, the people in the industry are not highly skilled technicians with the finesse of a brain surgeon. They are backpackers and Labourers with from what I have seen first hand, little training and knowledge. Many just show up to wherever the labour agency points them. I have seen promotional pieces for companies where they show the installers walking on panels!! I'd never do that but clearly some do. Where I can I do the rows in a stepped fashion and pull the panels over the tin roof but that's not easy on the steep roof at the ends so they go over the other panels carefully as I can trying to keep them flat rather than lifted with a small High pressure point and all been good with no problems at all. On the back where the verandah is I have to bring them up on the flat section them carry them across and up the roof. It's harder work and more dangerous to me and the panels than pulling them more or less up into place. If your roof area is less steep, probably not so much of a concern. Unlike me, You'll probably be sane and only do it once so whatever works easiest is best! $3 ea isn't bad. They are pretty big pieces and I can't think of anything else that would have anything that size for that price. Rubber and neoprene etc is always pretty exy, even plain O rings. ... he thinks the hot air should be pumped into a small dedicated room housing a heat pump water heater ... to store warm water for later use. Had certainly never thought of that, I have with an oil burner but only for stopping the my old AC unit Icing up in very low night temps. Putting it next to it so it can draw the arm air off to get the temp up above freezing and put the thing back on it's curve as an average with the surrounding air. Thinking it through, I spose one could use the heat pump to concentrate the low value energy into a higher value energy and therefore get more storage in a smaller area / container. It would only be worth while in extreme conditions that I can see like in sub Zero. Certainly at temps above 10C ( and probably half of that) the pump will do just fine on working as normal pulling the latent heat out of the surrounding atmosphere. That's what they do. I myself can't see why you would want to go to the trouble and expense of then having the tubes. On the 3rd hand, the tubes will do decent hot water storage all on their own without the HP. Of course that's only coming from someone as he says has a lot of opinions but never done anything. There's clearly a lot I have done even with other things that makes me question his conclusions with great scepticism. I think he is completely on the wrong track with his spiral Idea in the tubes wanting to make the air travel the greatest distance. The surface area of the collector tubes is Fixed. Making the air travel further will not pick up any more heat than is there. At best all he is doing IS disturbing the boundary layer of gas clinging to the sides of the tubes as I have mentioned before and increasing the residence time. Learned about that playing with cars doing ground effect mods and also with things like cooling of brakes etc. and playing with model aeroplanes and jet engines. Boundary layer is similar to the Coanda effect. His spiral effect is simply slowing the air down allowing a greater heat transfer through longer residence time. Nothing to do with making it travel further as he asserts. Was doing the same thing with car radiators 30 Odd years ago tilting them to increase air residence time and putting restrictors in the thermo housings to slow the water flow which allowed the water to absorb more heat and reduced the drag of the water pump. His outcome is correct, his theory behind it is not. I also find flaws in his heat pump/ dedicated room idea which seems from what he is saying, is an opinion he has and something yet done. Problem I see with that is 2 Fold. Or 3...? Firstly the dedicated basement presumably has limited airflow mainly being the heated air in his scenario. This removes the latent energy in the surrounding air which would normally be passing through the machine when installed outside as they are designed. These things are simply heat concentrators taking the very low value energy out of the air and putting it into the water at a higher temp. They exhaust cold air so pretty much straight away in a basement or dedicated room as he describes it, the thing would be fighting with itself unless the cold exhaust gas was vented out which he makes no mention of and seems to overlook... having an opinion but never done it ... :0) Or much with AC either it seems. he mentions the heat in the basement but over looks 2 things. Firstly that heat energy presumably locked up in the walls and whatever else would be quickly used up in the air and then would need to be radiated from those surfaces and thermal mass back into the air which with a presumably low temp differential will be a slow process. That all happening, the only incoming source of heat would be that being supplied from the tubes. We have well established my knowledge of tubes is only slightly above my non existent experience, BUT, I would still be willing to say with my knowledge and experience of AC which a heat pump is, That the air flow and temp coming out of an array of tubes unless was unusually large, would struggle to keep up with and have the energy input to keep up with what a Heat pump could use to the max of it's capability. As a total and utter guess, I'd say you would want something like a 6" pi[pe running real good airflow at something above 30-40o if that was the only heat source as would be in the gentlemans description..... and again, it's going to be facing dilution problems if it's not carefully directed in and out of the machine and the room. Not meaning to one up the guy or be a smart arse but I can see holes in this theroy you can drive a truck though. Now, I'll certainly give that all these are correctable and are in no way deal breakers but then we come back to the fact where you and I are, you could just stick the heat pump out in the yard like normal, run it through the day and the thing is going to do just fine on it's own the efficiency improvement is going to be bugger all and in no way worth setting up the tubes to begin with. He mentions the efficiency increase with the temp rise of the air but I get the impression he is naieve to the airflow required to make that efficiency happen... Which again without having specificly looked up water heaters, I have some reservations about. I have an idea that these too use scroll compressors in an inverter setup and can therefore ramp up and down as needed. If that's right then there would be little benefit to the hotter air because the speed of the compressor will Adjust and If I'm wrong, then unless the airflow and temp supplies all the latent energy the machine is capable of handling, then the thing will be quite inefficient because it's like revving a car in 3rd gear on the highway when it could be in 6th. -IF- the heat pump is located in near zero conditions, yes, this would be a help I think. They have a curve but from memory, most of them are still doing ok down to 3 or 5 but may be lower as well. In your setup,you would be far better off pumping the heat from the tubes directly in the house and just having a heat pump outside, on the sunny side of the house and running it through the day and be done with it. Of course there is still the matter of spending $3K+ to heat your house and water when you would have plenty of space for panels that for similar cost could heat, cool and power everything as well including your far cheaper resistance heater for whatever your FIT is worth. Ah, yes, you have it right, he does not. He thinks it's up to a longer travel distance which is incorrect. Surface area is constant, residence time is the factor in play here. Of course the face palm with his residence time theory is he could just run his fans slower in the first place without all the stuffing around with soldering wires to tubes etc!! Sorry, again don't mean to be a bitch but the guy fails to impress me on this one. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
The way I see this, heat input to the tube is pretty constant under whatever the solar conditions are prevailing at the time. The tube temperature will keep rising until the heat flowing out is equal the heat input, and an equilibrium heat balance is achieved. That may be with very hot tubes with low airflow, or barely warm tubes with massive airflow. Once the whole mess has reached a stable operating point the heat coming out will pretty much always equal the solar heat going in. And there is not a whole lot that can be done to change that. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Well said. The input is going to be whatever it is. The output is going to be that providing the airflow is enough to take all the energy away. Just like with the Rogers setup now. The guy mentioned temperature out of the tubes but I didn't see him mention airflow, only an emphasis on not restricting it. |
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| rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852 |
I shall respond to the other posts after tea and a movie. Family responsibilities. In the mean time, todays results were pretty impressive. Not a record for maximum peak output ... but a lot of hours of heat just pumping into the house. Even saw my wife with her jumper off. Peak output reached 1,240.75 watts 6hrs 42 mins at 30 degrees and above 4hrs 59 mins at 35 degrees and above 2hrs 11 mins at 40 degrees and above ... and a max of 41.9 degrees  Cheers, Roger |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
That is a very encouraging result, considering its still a scaled down version. It would be interesting to try driving a dc powered air blower direct from a solar panel. Cheers, Tony. |
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| rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852 |
Fair enough, though I'm not sure I could do that. My OCD would kick in and I'd have to have them all up unsquashed and tidy and definitely up away from any water. Yeah, until I try and do something like that it always seems like it's fraught with difficulties, so glad to hear it wasn't an issue. Haha, yeah ... that's why I don't bother going over there very often ... everything is an issue and is howled down. If we listened to them we'd never do anything ... not even get out of bed! Yes, that makes sense to me, and glad there's been no issues. I wish. Hopefully I will go in with eyes wide open and make the right choices from the start ... otherwise it'll be the same probably. I figure if I have too much trouble, we have a few acres here, so I could mount them on the ground if I really had to. When you put it like that you're right. I'm also looking at the cost of one of their complete hot water systems and they are very cheap ... bit like your comments about the spare parts for your diesel heater. Yes, I can see potential there, though to add in an expensive bit of equipment and a lot of complication in a heat pump water heater, you'd really want to see a massive gain. Having said that, I was looking at the specs on some Quantum heat pumps and they list the Hot Water Production Rate in Litres per Hour ... plus the Coefficient of Performance (COP) ... and they list those figures for different ambient temperature ranges. At 35 degrees their 270 litre model can produce 177litres/hr COP = 5.75 At 20 degrees ... 75 litres/hr ... COP = 4.53 At 0 degrees ... 33 litres/hr ... COP = 2.75 There's no point them quoting higher ambient temperatures than 35 ... but if the machine could handle it and you could reliably provide 45 or 50 degrees for a couple hours a day ... you could heat the whole tank and have an even better COP. Yes, I certainly agree .. at least at this stage and unless there's some massive, currently hidden benefit. ... and then there's that. And that may actually have something to do with it. I don't really know what to think there. In all the reading I had done and ideas you guys have presented ... in the back of my mind, I really did feel I need to at least do some experiments with the airflow within the tubes and see what sort of differences occur. One thing that I thought I should try is drilling some holes in the centre tube ... so that most of the air still goes to the bottom and then up the outer section ... but some air will squirt out small holes at right angles in variouus places on the tube which DO break up the boundary layer and help mix it into the main stream coming up from the bottom. Various things like that. I imagine it would simply be vented to outside, unless there was a need for cool air somewhere else. That may be the case, but depends on how much hot water is required. eg the Quantum 340 litre heat pump can produce 199 litres per hour at 35 degrees ... so in less than 2 hours it should be able to heat the whole tank (depending on starting temp) ... ... and if they would run ok at 50 degrees, could be much quicker. I have no idea if they really would, but there must be a million possibilities. For sure, that is where I'm at too at the moment ... and my next move (after the Warpinverter) would be to preheat the water going to my instantaneous water heater and reduce our gas use by a half or more perhaps. Unless I can be convinced otherwise, a heat pump is a long way down my list of requirements ... though the reason I mentioned Quantum heat pumps is because there was a second hand 340 litre one for sale on marketplace for $150. Came out of some sporting clubrooms that were being upgraded. For that price I might play with one ... but that's still years away. Cheers, Roger |
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| rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852 |
I forgot to mention that this morning before 9am, I reversed the fan on the unit where it was mounted. I simply removed the duct tape, rotated the fan and taped it back in. What that does is to suck air through the unit instead of blow it through. And it sucks air from what was the outlet ... down around the outer part of the evacuated tubes and back up the centre tubes ... a complete reversal from before. I don't imagine it made any difference ... but because tomorrow is supposed to have similar conditions to today (1 degree higher), I plan to put it back to 'normal' before I fire it up tomorrow. Reason I tried this is because I came across the advice a couple times in airheating circles, that you must draw the air through and not blow it through. Of course no explanation was given ... but were very adamant. Other reason was because Marcel's setup operated by blowing air down the outside of the inner tube and back up the centre of the centre tube ... opposite to how I had set up mine. And that is quite likely exactly how it does work. I know that if there's not a lot of sun, the O/P temp drops ... and if I reduce the airspeed, the temp will increase ... or at least drop slower. I also know that if the fan is off and the sun is shining ... the tubes definitely store heat. Many of my charts show this, in that I have mainly been firing it up at 9am just for consistancy, even though often there is no decent heat gain until 10:30 ... ... but some days when there's been sunshine before ... when the fan is turned on, there's a massive amount of heat pouring out for a while until it is depleted. That can be a real trap for testing ... because it's only after constant airflow for some time that an equilibrium is reached ... and from then on the output is in relationship to the current sun level. Mmmm ... and that is exactly what you said above. Yes, certainly encouraging, though we're unlikely to get lots of days like this ... except for tomorrow maybe ... one degree hotter and sunny. One thing that I noticed on todays chart is that between 13:30 and 15:00 the temp in the sun stayed virtually level ... yet the output went on a steady decline from 41.9 - 37.8 I can see a dc blower doing the job. It probably still needs some smarts to decide when it should be allowed to run and when to cut out etc ... but the speed could simply be left to the output of a well matched solar panel. I should be able to log the output power of a panel when my new multimeter gets here, so that will give us a lot more idea. Edited 2021-05-22 01:22 by rogerdw Cheers, Roger |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Your heat input definitely seems to track measured solar pretty well. A solar panel powered air blower should at least adjust the airflow up and down in the right directions, and start and stop appropriately. I have never seen this done before, but my gut intuition is that it will be dead simple and work very well. Sometimes the simplest solutions are the best. Smarts would be very easy to add, the main hurdle being control of the motor power. If its a dc motor its pretty straightforward. A 230v induction motor will require phase control, from a mains isolated dc control voltage. I would expect a commercial power module to do that might cost around $250. Preheating the water going into a normal gas domestic hot water heater will work very well for you, its a very well trodden path. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Having seen how the Fans on my inverters track the output from panels, I'd say there would only be 2 Controls, if any at all, you would need. One would be voltage/ amperage limiting on the top end. You would want the thing to start up when the sun came out as it will start barely Turning but not be blowing too hard in the middle of the day. Someone here or elsewhere made a comment about the start up burning out the motor with it sitting there without enough current to spin. What can I say? Had 3 fans hooked up like this for Literally years with no problems. 2 have been running like this probably 9 months or more of the year for 4 years. ( I disconnect them in winter in the shed because it's cold when they blow and don't need them) and the other on the inverters for at least 2 Years. If they stop now I'll say I did well with them anyway. You may want to limit the top end as the panel may cause the fan to blow too hard but then again, we come back to the same amount of energy at a lower temp and Higher volume and if the thing is flat out, the production from the Tubes probably is too and their output. It may be easier to actually limit the intake size. If you restrict the amount of air the fan can draw in at max speed to the temp you want, then everything below that should be fine and relatively proportional. Might want to adjust it once a month or so as seasons Change but other than that, should work fine. The other thing you -may- want is a thermostat so the air in the tubes has to be a certain min temp before the fan runs. My thought is with the efficiency of the tubes and that of a panel being so much lower, the tubes will have some heat time the fan slowly spools anyway so I don't see that being a problem. Set the panel and the fan direct and see what happens and monitor your temps and then judge if there is anything you need to do. Maybe a lot easier just to restrict the intake if you need rather than try to control the blower. Best, cheapest, most readily available DC fan I can see for this is a cabin blower fan from a car but have a look on fleabay etc and see what is available inevitably from Chiiinayah. A lot of the cabin Blower assembly's now have stepper motors on the housings to direct the air to the vents, your feet, windscreen etc. Might be possible to use these for dumping air if you need to according to the air temp from the tubes. Personally I see it as being pretty Linear with as I said, maybe limiting the top end voltage as it's going to be over from most panels anyway. Hasn't worried my car fans though in all these years of having them howling through summer. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
The car heater blowers usually have several switched speed settings, and that is just a selection of switch selected series resistors. With a bit of experimentation with the the size of 12v solar panel, and resistor, I am sure the system could be "tuned" to work pretty well overall. Fan loading increases cubed law with rpm, so it takes almost zero power to get the blades slowly turning initially, so I cannot see that being a problem. I am sure if you visit your local automotive graveyard, and ask for a cheap blower and you don't care what it came out of, you will get either something very common that he has a lot of, or something so odd and rare he would never otherwise sell. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
The only trouble with blowers is they are pains to get out as you have to take all the plastic ware out of the dash and find all the hidden Screws. The motors come out easily enough usually and that's what most wreckers take out because anything where the housing is damaged is a write off anyway. Only other reason would be to get the heater core and they generally don't fail too often now. Because they are time and effort to get out, they may charge a bit for the fan and housing but as you say, If it's been there while, they might take $20 to get something and be rid of it. Also if just the blower motor and fan was purchased, wouldn't be that hard to make up a housing for it and depending on what one got, with the weird and difficult shapes of most housings, might be easier to modify build one to order than modify and existing one. VW Kombi blowers are a good straightforward Blower and housing but probably getting very rare and exy now. the price those old things go for is insane even unrestored. A housing with the motor and the heater core would be a great way to store heat in water. Could be set with the dampers to have some air going to the house and some through the core to heat water. Problem is the water would be heated very hot so would need a lot of water to store a little energy. If the water was heated directly by the tubes then a heater core would be a great way to recover the heat from the water. They will transfer a LOT of heat. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
One advantage of heating water with hot air is it pretty much solves the frost freezing problem. A car heat exchanger would be buried inside a duct or a box and probably would not freeze on a frosty morning, especially if the housing was insulated. Bare black tubes fully exposed to the night sky probably would freeze, and if filled directly with water would undoubtedly crack. Cheers, Tony. |
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| rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852 |
Yes, I'm warming to the idea of a DC motor directly driven via pv panel. Certainly worth a try. The exhaust fan I am currently using apparently has a speed control that can be added, but how expensive it is and how it is controlled I'm not sure. It quite likely would be ok, but need to make sure there's no airflow if the oulet temperature gets down to the indoor temperature or below. No point cooling the place down after heating it up. Haha, yeah I saw that thread. Some people have no clue. Exactly. Had not thought of that. Generally speaking, slowing down the airflow which is what would happen if you restrict the inlet, allows the temperature to rise, depending on the sun ... so may be useful. I had a look at a few. Look pretty substantial, though like you say, may need to build a custom cowling. It's likely I could use a director as well because with the output I've been getting on good days, it would be handy to direct it to other rooms. Changing resistors sounds like a great way of fine tuning the speed. And good idea for finding a cheap fan, I can do that. Sounds like my kinda bargain. I'll see if that works. Haha, solve two for the price of one. Great idea though. Yes, it should make a difference in cold weather. We do get the occasional frost ... but it's been years since I saw anything frozen outside around here, in the way of tapes or pipes or birdbaths. I don't know how they get on with the 'water-in-pipe' evacuated tubes in freezing weather because they wouldn't be practical to drain. Plenty of other hot water systems build in a way of draining them when a freeze is likely. Cheers, Roger |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
That's one reason why you don't see any here in NZ, there used to be a lot around, haven't seen any in years; all replaced by the heat-pipe versions, which don't have any problems with frost. |
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| rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852 |
I'm surprised though ... if they don't work too well in cold temperatures (as in crack when it gets frosty), how did they even get off the ground to start with. I can understand tubes with heat-pipes of course because no water in them to freeze and crack the glass. I did come across an article on a Victorian website about some tests done in the cold and snow at Mt Buller ... see here ... Lots of photos but no explanation. Not sure I understand the point of it ... and perhaps it was just an excuse to make a trip to the snow look like a scientific experiment and therefore tax deductible. The other thing is that the tubes are usually plugged into a tank that holds 100-300 litres of water ... and that would take considerable time to freeze ... though the tubes sticking out by themselves would certainly be the most vulnerable. Looks like though, that if I do end up wanting to use tubes to heat water I would either have to invest in some heat-pipes or make my own ... or try Daves suggestion and use a car heater as a heat exchanger. Cheers, Roger |
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| rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852 |
Todays results were interesting. Because of the forecast being similar to yesterday plus a degree hotter I was expecting big things. In fact we had a high of 46.1 from the outlet and a record instantaneous power output of 1,517.26 watts Because I was hoping for a very smooth output over the day like yesterday ... I decided to remove the reflectors from behind the tubes at midday, when they should be at their most effective ... ... and replace them at 13:00. I was hoping to see a big drop in output temperature ... and we did ... but the outside temp in the sun also dropped at the same time due to cloud cover ... so is hard to say exactly just how beneficial the reflectors are. Maybe someone else can interpret the chart to offer some insight.  Cheers, Roger |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
You probably need to run the test multiple times and when the conditions are more stable to get an averaged meaningful Result. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Agree, its up and down all over the place, difficult to see anything definite. Cheers, Tony. |
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| rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852 |
You're right. Just my lazy self was hoping to get the answer in one easy test. If today looks a bit more even, I'll perhaps try it again. Forecast calls for 'high cloud' and sunny. Just no cloud for an hour over lunch would do it. I've got a few other experiments in mind. I cleaned and wired up a couple of identical fans that I saved from wrecked microwaves years ago. 240V and 35W ... and with a roughly 9.5 meters/sec airflow velocity. I'll work out the cubic meters/min when I do the tests. If nothing else, I've learned there's no point doing one test after the other ... they must be done together otherwise it's too easy to get misled. Conditions change by the minute.  Cheers, Roger |
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