Search found 2855 matches
- 19 Mar 2024, 10:49am
- Forum: Bikes & Bits – Technical section
- Topic: how slow do they get?
- Replies: 5
- Views: 80
Re: how slow do they get?
Given that 'unpunctured' tyres still go flat eventually, it begs a definition of what constitutes a puncture.
- 18 Mar 2024, 6:52pm
- Forum: The Tea Shop
- Topic: Heat in the home
- Replies: 2292
- Views: 133320
Re: Heat in the home
I think I must have been typing that from memory instead of checking.axel_knutt wrote: ↑16 Jan 2024, 5:39pmI measured the cost of running the heating continuously a few years ago, and found that switching off for 7 hours overnight saved about 12% off the gas consumption.
So, the first time I measured this was for 8 weeks in November and December 2017, and alternated the heating between timed and continuous each week to minimise the effect of weather, on the basis that any temperature variations won't be synchronised with the measurement periods. Comparing the ratio of average fuel consumption to average temperature difference for the 4 timed weeks and 4 continuous weeks, I got a 7.9% (not 12%) reduction in consumption with the heating switched off at 00:30, and back on again at 07:30.
This winter I repeated the measurement but by a different method: running the heating continuously for the whole winter and comparing it with the timed running during the three preceding winters. To account for variations in the weather, this time I normalised the fuel use to the heating load in degree days. The result this time was an average consumption of 350W/K on timed, and 383W/K on continuous, giving a similar saving of 8.5%. (Heating load measurement)
I also have a model of the house and heating system on the computer (in the form of a virtual analog computer running on Spice), so I thought it would be interesting to compare what that says against the actual measurements. This time I don't need to worry about the effect of weather variations, because I can repeat the simulation with exactly the same weather temperature for both timed and continuous operation. The result I got this time was a saving of 9.47% using weather data for Oct 2021 and 9.53% for Jan 2022, those months being chosen so as to cover a wide range of heating loads.
In view of the similarity of the results for all three measurement methods, I think it likely that the results are about correct.
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Diffusivity is a property that's normalised to a standard dimension, in the same way that density is weight normalised to a standard volume. What you are arguing is the equivalent of saying that the density of steel has no effect on the weight of a bike because it's just a property of the material, or that the contents of your bath weigh the same as the contents of the local reservoir because the density of the water is the same in both.roubaixtuesday wrote: ↑16 Jan 2024, 6:30pmIf you believe thermal diffusivity is "absolutely fundamental" please answer the point about how it is identical for walls regardless of their thickness.axel_knutt wrote: ↑16 Jan 2024, 5:39pm
Thermal diffusivity is absolutely fundamental: it’s what determines how fast a material can change temperature in response to a change in surrounding temperature, and how fast a change in temperature can propagate from one side of it to the other. What I said above all assumes a static system, but the conditions never are static: as weather and/or heating level change the thermal diffusivity of the walls determines how fast they can follow, and hence the extent to which MRT and level of radiant asymmetry vary, causing transient variations in thermal comfort.roubaixtuesday wrote: ↑15 Jan 2024, 7:54pmThermal diffusivity is a material property. It tells you almost nothing about how a building responds to temperature changes.
A wall constructed of a single sheet of paper and a wall constructed of 10 metre wide coastal redwood trunks would have the same thermal diffusivity.
It is a property of a material, not a property of a building, or even an element of a building.
In the same way that the performance of bicycle frame depends on it's geometry, tube thickness and other design parameters, not just the youngs modulus of the material.
One advantage of computer models is that you can test hypothetical what-ifs that wouldn't otherwise be possible: On the left, the inside and outside temperatures from data logging thermometers are plotted in green and blue respectively, and in red you can see that the modelled inside temperature is a reasonably good fit for the measured data. On the right is the same plot again, but this time it shows the result when I remove all the thermal inertia from the model: unsurprisingly the temperature plummets as soon as the heating switches off, because there's no longer any heat capacity in the building to maintain it.
We can also look at how the thermal properties of the wall affect the heat transfer through the wall: Here, the red trace is internal air temperature, and then from top to bottom are the temperatures of the internal surface of the internal leaf of the cavity wall, the external surface of the internal leaf, the internal surface of the external leaf, the external surface of the external leaf, and finally, the outside air temperature in blue, which I've set to be constant so as not to obscure the effect we're looking at.
What you can see is that the thermal resistance and heat capacity of the brickwork combine to create a low-pass filter, smoothing out the temperature variations between the inside and outside of the wall. Note that not only has almost all of the temperature variation gone, but there's a phase shift: minimum temperature on the inside occurs at 07:30, immediately before the heating comes back on, but the outside surface of the wall doesn't reach minimum temperature until gone midday. All this effect disappears if you don't take proper account of the way that the thermal resistance and heat capacity combine to influence the way that heat diffuses through the wall.
The reason I built the model in the first place is that I was interested in exploring the transient effects as the heating switches on and off.
I don't see what you're getting at, I haven't said that there's no convection or conduction in the room, and I don't see the relevance of conduction through the wall. The point I was talking about is that people are more sensitive to radiant heat, and that asymmetric radiation causes thermal discomfort. Radiant heat's not irrelevant, or even insignificant.Jdsk wrote: ↑17 Jan 2024, 12:13pmI'm all in favour of thought experiments in discussing the physics.axel_knutt wrote: ↑16 Jan 2024, 5:39pm ...
If the walls of a room were at body temperature, then the radiant heat passing from the body to the walls would be exactly balanced by the heat radiated from the walls to the body. Walls colder: there’s a net heat loss, hotter and there’s a net heat gain.
...
But in this one is there any heat flow from the interior surface of the walls to the walls and the outside world? And is there any conduction and convection within the room?
Thanks
Jonathan
See here: An excessively warm ceiling is the most uncomfortable, followed by a cool wall (which is the problem I have in my dining room).
- 17 Mar 2024, 3:06pm
- Forum: Campaigning & Public Policy
- Topic: LTN politics
- Replies: 52
- Views: 3156
Re: LTN politics
The review was published this morning.
https://www.gov.uk/government/publicati ... ood-review
https://www.gov.uk/government/publicati ... ood-review
- 17 Mar 2024, 1:23pm
- Forum: The Tea Shop
- Topic: Just how long will it take to turn this ship around?
- Replies: 39
- Views: 920
- 16 Mar 2024, 7:20pm
- Forum: Health and fitness
- Topic: Max Heart Rate (Setting & Meaning)
- Replies: 42
- Views: 1698
Re: Max Heart Rate (Setting & Meaning)
From above:Psamathe wrote: ↑16 Mar 2024, 5:33pmI have noticed in the past readings I cannot understand - they might be correct as I don't bother taking my pulse manually whilst riding but, high cadence on flat and fairly high HR eg 120. Few mins later slight incline really pushing it, legs burning struggling, breathing hard thinking it's time to back-off a bit and HR down in low 60s!. Thinking about this I concluded I don't have enough knowledge to explain it
The reason the finger clip was showing my heart rate as a third of the correct rate is that my heartbeat had a pattern in which every every third heartbeat was stronger, with weaker ones in between (SwwSwwSww etc), and simple plethysmography catches only the strong ones. My true HR from the ECG was 210, but the rate indicated on the finger clip oximeter was 70.axel_knutt wrote: ↑11 Mar 2024, 6:53pm Plethysmographic monitors are also very good at hiding arrhythmia. My AF doesn't show up at all on them, the paramedic's finger clip was showing 70bpm when my actual HR was 210.
When we got to A&E on that day the triage nurse in the lobby also put an oximeter on my finger just like the paramedic had done before he wired up the ECG, and when she saw 70bpm she decided I was low priority. The paramedics tried to show her the ECGs but she wouldn't listen to them and made me wait until there was a cubicle free in Majors. I could see the paramedics weren't happy about it, but once they'd installed me in a cubicle and got their trolley back there wasn't a lot more they could do, so they left.
It was about another 10-15 minutes before a nurse came along and wired me up to the ECG, at which point she took one look at the 200+ bpm on the screen and just flew out of the door. Within seconds of that I was whisked up the corridor and put in Resus.
Trying to take your pulse manually also causes the same error, as another nurse discovered on a subsequent occasion.
- 16 Mar 2024, 5:46pm
- Forum: The Tea Shop
- Topic: Should the BBC be privatised?
- Replies: 114
- Views: 4350
Re: Should the BBC be privatised?
It can get a lot worse than that, a lot of the programme material contains thinly veiled adverts as well, in addition to the formal breaks. On one Julia Bradbury walking program she was stopping at businesses en route every verse end, and spending several minutes being a platform for the owner to promote his business. IIRC there was about 12 minutes of walking in the 30min slot.
- 15 Mar 2024, 5:36pm
- Forum: On the road
- Topic: Dangerous mudguards
- Replies: 37
- Views: 3311
- 14 Mar 2024, 12:09pm
- Forum: Health and fitness
- Topic: Max Heart Rate (Setting & Meaning)
- Replies: 42
- Views: 1698
- 14 Mar 2024, 12:01pm
- Forum: Health and fitness
- Topic: Max Heart Rate (Setting & Meaning)
- Replies: 42
- Views: 1698
Re: Max Heart Rate (Setting & Meaning)
Before I had my ablation a nurse told me what sort of symptoms I could expect during recovery. As she described them I remember thinking to myself that she was describing the same symptoms I had during cycling that I'd been telling them about for years.Audax67 wrote: ↑13 Mar 2024, 9:56amThree possible sources in my inexpert experience: inflammation due to heart hammering away at inside of rib cage & so forth, strain of rib cage from heaving on bars, and heart problems. Doc told me once that if I could modify the pain by pressing on it or moving it, it wasn't cardiac.Nearholmer wrote: ↑12 Mar 2024, 7:04pmYou get chest pain from a heavy bike ride?hours left me with pretty similar fatigue and chest pain the day after to that which I had from a heavy cycle ride
That wasn’t what I meant by “a bit whacked”, I was talking about not being up for a load of exercise, a bit weary, not actual pain.
- 12 Mar 2024, 6:55pm
- Forum: Health and fitness
- Topic: Max Heart Rate (Setting & Meaning)
- Replies: 42
- Views: 1698
- 12 Mar 2024, 5:27pm
- Forum: Health and fitness
- Topic: Max Heart Rate (Setting & Meaning)
- Replies: 42
- Views: 1698
Re: Max Heart Rate (Setting & Meaning)
I used to see much the same at younger ages.re_cycler wrote: ↑12 Mar 2024, 12:26pmBut what causes a step change at 60Nearholmer wrote: ↑12 Mar 2024, 11:59amDitto, but that was exactly what the cardio chap told me was “not heart healthy once you get past sixty”, so I now only do it accidentally, and it’s noticeable that doing it leaves me a “a bit whacked” for the next 24hrs, whereas if I confine those higher numbers to shorter periods, and they don’t sum to a large amount of time overall, I’m ready to go again the next day.I can maintain 160-165 for periods up to 2 hours, both cycling
It's interesting that after I developed AF I noticed that an episode at 200+bpm for 10-20 hours left me with pretty similar fatigue and chest pain the day after to that which I had from a heavy cycle ride.
After 10 trips to A&E in an ambulance the NHS still didn't accept that it was their advice that there was no need to stop cycling that kept putting me in there.
- 12 Mar 2024, 11:41am
- Forum: Health and fitness
- Topic: Max Heart Rate (Setting & Meaning)
- Replies: 42
- Views: 1698
Re: Max Heart Rate (Setting & Meaning)
That's the point I was getting at above, if you push your exercise anywhere near MHR you know all about it. The professional advice I've read about conducting maximal tests is that they should be used very sparingly, because they take a long while to recover from.
- 12 Mar 2024, 11:00am
- Forum: Does anyone know … ?
- Topic: Cateye cycle speedo/computer adds stealth miles overnight?
- Replies: 15
- Views: 816
Re: Cateye cycle speedo/computer adds stealth miles overnight?
I always use wired ones, but my HRM is interfered with by overhead power lines, passing trains, and a local radio mast.
Something I've recommended numerous times, but once people get in the grip of a marketing fashion the sensible products can quickly become all but unavailable. It's 10-15 years since I last bought a cycle computer, and the choice of wired ones was pretty limited even then.
- 11 Mar 2024, 6:53pm
- Forum: Health and fitness
- Topic: Max Heart Rate (Setting & Meaning)
- Replies: 42
- Views: 1698
Re: Max Heart Rate (Setting & Meaning)
Plethysmographic monitors are also very good at hiding arrhythmia. My AF doesn't show up at all on them, the paramedic's finger clip was showing 70bpm when my actual HR was 210.
- 11 Mar 2024, 5:17pm
- Forum: Bikes & Bits – Technical section
- Topic: "Plus" vs breaker for fairy protection
- Replies: 16
- Views: 878
Re: "Plus" vs breaker for fairy protection
I had a screw that was well on its way to going through the bed of the rim. It was getting hammered by my bodyweight with every rotation of the wheel so I think it would have gone through if it had been a nail instead of a screw.
The most damage I had was from a 4" nail that went in through the tread, out of the sidewall, and then snagged on the brake block with every revolution, so that by the time I'd stopped it had ripped a slot in the side wall of the tyre.