Of socks and men

August 7, 2013, 5:09 am
Filed under: Biofuel, Transport

Just browsed past an article on the difficulties of transforming straw into biofuel for cars. The thought struck that we could step laterally away from the current dominant auto design that uses internal combustion. Wouldn’t it be easier to use straw in an external combustion engine (AKA the steam engine)? Bring it up to date by using something other than water as the heat engine fluid and you should be able to engineer a nice economical vehicle that would be cheap as chips!

Velux Windows – How-Not-To
August 4, 2013, 5:11 pm
Filed under: Building Technology, House Renovation, Structures | Tags: ,


Velux window in main attic room


Velux rooflight installed in landing area

Two rafters have been cut in each case. The landing velux benefits from an two rafters on right hand side but only one on left hand side. The main attic velux only has one rafter on each side. There should be two rafters on both sides of the rooflight.


External view of attic room velux

Looking down the roof: The tile nib sits on the Velux flashing tray and does not engage with the counterbatten.

Looking down the roof: The tile nib sits on the Velux flashing tray and does not engage with the batten.

The tiles are Redland Grovebury concrete tiles. The tiles on the right hand side of the Velux are cut to half width. This leaves a single nib on the back to hook onto a rafter. Unfortunately this sits on the flashing detail so that the tiles are loose and can be pulled downwards. If these tiles slip then they will ultimately fall onto the conservatory roof!

The nibs on the tiles need to be trimmed to allow part of the nib to hook onto the batten.

There is a leak on the lower left hand corner, daylight is visible from inside.

The IKEA like instructions show that an “underfelt collar” should be fitted. This can either be a Velux part BFX or it can be crafted from roofing felt. Fitting instructions for these options are shown in the instructions and Velux have been quoted as saying “On page 11 of the fitting instructions there are two options shown, option A shows the window being dressed using our BFX underfelt collar which can be bought as an optional extra, option B shows the window being dressed with four strips of roofing felt. Either of these can be used to dress the window and they provide a seal between the window and the existing roofing felt. If the window has not been fitted according to the instructions then any problems which occur would not be covered by the warranty and would have to be rectified by the installer.”

Neither option has been installed on our veluxes (this additional layer of felt should be easy to feel beneath the flashing tray on eaither side of the window) so the windows need to be reinstalled with according to the Velux instructions

According to the instructions the head and foot of the reveal should be unobstructed by the noggins. The instructions (Section 4a, page 6) show how to set these noggins back from the revela. This provides better access and avoids the potential hazard of a sharp 90 degree angle at head height.


There is a suggestion that these tiles should all be clipped according to Redlands guildelines The property is above 100m, in Zone 1. Building height between 5-10m. Headlap is 75mm.

Double Glazed Unit Aspect Ratio
May 22, 2013, 7:08 am
Filed under: Building Technology, Structures

I’m looking for guidance on design limits for insulated glazed units for a conservatory roof. Currently we have some units that are 3.5m long by 58cm wide. I have read suggestions that such a 6:1 aspect ratio for a roof unit is excessive. An article by The Window Man suggests that large aspect ratio units are more prone to explosion.  Are there any concrete guidelines?

Cold Winter – Air Locks – Dissolved Air
April 20, 2013, 11:22 am
Filed under: Building Technology, House Renovation | Tags: ,

This winter and spring have been very cold. Not cold enough to freeze the pipes but cold enough to stop our plumbing from filling the toilet cisterns!

The symptoms were that after a cold day or night our toilets would not refill after a flush.

We have our toilets fed from a cold water tank in the loft space. It was an insulated tank so no danger of freezing and this fed through plastic pipework down into the house.

The tank is on the floor above the toilets that had the problems. A toilet on the ground floor never had an issue so it appeared that there was an issue with air locks.

I wondered if a flush in the downstairs toilet caused the water pipes to the upstairs toilets to get drained with air entering via the float valves. With little water pressure in the gravity fed system this seemed like a reasonable possibility.

I tried changing the float valve. This made no difference.

I put check valves in line to physically prevent the lines being drained by other users downstairs. This made no difference.

Serious head scratching ensued. There were no water leaks. Air entry through the float valves had been eliminated. Where was the air coming from?

Then I wondered if the problem could be coming from dissolved air. A small amount of air will always be dissoved in water. When you boil a kettle the water fizzes as the water is warmed as the dissolved air is driven off well before the water itself boils.

The behaviour of water in a kettle actually demonstrates that the solubility of air in water is greater at lower temperatures than at higher temperatures. This might be counter intuitive as we all know that common substances like sugar and salt are more easily dissolved by heating water.

A quck search of google reveals that http://www.engineeringtoolbox.com/air-solubility-water-d_639.html has the data. A few sums later and you can work out that although the mass of air dissolved in water is small this turns into problematic volumes of gas if enough water is warmed to a sufficient amount.

On entering the house the water warms from 0C to nearer 20C. When a toilet is flushed this draws 6 litres of water through the pipes.

The engineering toolbox graph shows that between zero and 20C the solubility of oxygen drops from 14mg/l to 9mg/l so 5mg O2 gets released from each litre of water. This is 0.3mmol so takes up 3.5ml of space at 20C. Nitrogen will also dissolve at a similar rate and all of the other gases in air will too. One flush therefore generates something like 50ml of air and this is enough to introduce an 50cm long airlock of pipework in half inch pipework.

The solution was to take out the tank and connect the plumbing straight to the mains. We had planned to use the loft tank as part of a rainwater harvesting system but realised that it wasn’t really necessary.

Locations in Oracle Warehouse Builder (11gR2)
June 21, 2011, 9:03 am
Filed under: Oracle | Tags:

Connecting a mapping to a physical file so that files can be produced for other processes is a tortuous process. Running with multiple configurations in OWB allows all of the location and configuration detail to be setup in OWB.

OWB allows you to share locations between configurations. Thus a physical directory can be reused in your DEV TEST and PROD environments.

1. Create a location in the Files section of the Locations navigator. Ensure that the OS account that will be used to read the data can actually read the physical directory.

2. Make the location available (in Data Locations) in the relevant Control Center in the Locations Navigator.

3. Make the location in the Data Locations section of the relevant Module in your project in the Project Navigator. i.e. Edit the module and move the location into the Selected Locations list in the Data Locations section.

4. Configure the module to use the relevant location. The location is configured in the Identification section of the configuration.

The results of this process need to be deployed in Control Center.

Open the Control Center where the location should appear under the project container.

5. Register the location. Right click on the location under the project container and select the Register option.

6. Deploy the Connector. The reference to the files location is called a connector in Control Center. Find and select the location within the relevant database and deploy it by selecting the Create option and invoking the Deploy action. This will create a directory object in the Oracle database.

7. Deploy and execute the package that references the location. If this fails then it may be that the service account, that the Oracle database is running under, does not have permissions to the OS share or directory.


Making a change to a location, such as changing a UNC path, requires that it is Unregistered in all of the Control Centers that it has been registered in. If you Edit the Location in the Locations navigator you will see a Registration view that shows all of the control centers where it has been registered.

Comparison Groups
June 8, 2011, 2:00 pm
Filed under: Uncategorized

Comparison groups can either be Family or Friend types. For Families an area may be classified into exclusive groups – Towns may be metropolitan or rural and can’t be both. For friend comparisons such as nearest neighbours an area will have different neighbours its own neighbours. Bristol adjoins BANES but BANES adjoining neighbours aren’t the same as Bristol’s.

Listing partitions in Oracle
June 8, 2011, 11:01 am
Filed under: Oracle, Uncategorized

If you have dba privs then …
SELECT * FROM dba_tab_partitions WHERE table_name = ‘T_Foo;
Else …
SELECT * FROM user_tab_partitions WHERE table_name = ‘T_Foo’;

There are a number of other views that are available in dba and user flavours.