The Science of Length Measurement

Lidl Parkside brand measuring tape. © Eugene Brennan

An excuse to add a post about one of my favourite things, the centre isle of Lidl. I saw these 50 m tapes yesterday. I already have two, bought in other places and had to resist the urge to triple up. These things are useful for measurements in the garden or laying out foundations for sheds. When I was building the garden shed, I used two of these to square the foundations and concrete base (made the timber formwork for the base, placed it into position and then adjusted it by trial and error until the diagonals were the same length). Four pieces of rebar hammered into the ground then marked the four corners and I tied the 4 x 2 formwork to these.

Stud wall frame for my garden shed. © Eugene Brennan

Formwork for the concrete base was adjusted so the diagonals were of equal length. © Eugene Brennan

I built the shed on a concrete slab. © Eugene Brennan

 

Engineering Case Study: Weir Calibration at Mill Stream Bridge

Michael Browne from the EPA makes some calibration adjustments at the hydrometric station on the Mill Stream, Kilcullen, Co. Kildare. © Eugene Brennan

If you’ve ever taken a shortcut through the fields to New Abbey graveyard in Kilcullen, you may have noticed a silver-coloured tube beside the bridge where the mass path crosses over the mill stream. There’s also an adjacent vertical scale with numbers on it, rising up out of the stream. This setup is one of over 800 active hydrometric stations located around the country that collect water flow and level data as part of the EPA's National Hydrometric Programme. The stations monitor both surface water (streams and rivers) and groundwater, providing valuable statistics on water movement. The data is used for managing water resources, infrastructure design and monitoring long-term trends for flooding.
The mill stream station was commissioned in October 1999, but only started recording digitally in 2009. Originally a mechanical chart recorder would have been used to record data with a pen on a roll of paper. The recorder would have been driven by a weight, similar to the one in a grandfather clock. This recorder would have been housed inside the stilling chamber if used, although it's more likely that a digital data logger was used at this station from when recordings began in 2009.

What’s a stilling chamber?

This is the metal tube beside the bridge. It shelters the water’s surface, making it less turbulent, ensuring more accurate stream level measurements.

How do hydrometric weirs work?

These structures are used in hydrometrics to measure flow. They can be rectangular, V-notch, broad-crested etc, typically made from concrete or stainless steel sheets. The type used under the mill stream bridge is a concrete flat-V type. A weir causes water to backup behind it and change in level as flow rate varies. Using an equation specific to the shape of the weir, the flow rate can be determined from the water height. In theory, flow rate can be calculated knowing the cross-sectional area of flowing water and measuring its velocity. However, velocity varies within the flow; faster in the middle of a stream and slower towards the edges and bottom due to drag. Therefore, making a velocity measurement at a single point in the flow would be insufficient. Using a weir simplifies the calculations.
Various types of weirs have been studied and empirical equations developed (i.e. by making measurement from models, not analytical derivations) giving flow rate versus water level. Water level can measured using an ultrasonic transducer that emits a high-frequency sound beam and detects its echo. By knowing the transit time of the beam and the speed of sound, distance can be measured to the water surface (similar to the way an ultrasonic measuring tape works). This measurement is then converted to a flow reading, using an equation specific to the type of weir. Alternatively a depth sensor can be used to measure water height.

Telemetry

Measurements are taken every 15 minutes and telemetry data is transmitted every four hours via a GSM modem back to a central station. Although instrumentation can be powered by a solar panel or a small windmill, this station is powered by a battery. To conserve battery power, a compromise has to be made on measurement intervals, and on-time needs to be minimised: the instrumentation wakes briefly, takes a reading and then returns to sleep.

Stilling chamber containing a depth sensor for measuring water level at the Mill Stream hydrometric station, Kilcullen. Co. Kildare. © Eugene Brennan

Calibration of the Weir at the Mill Stream

I met Michael Browne from the EPA a couple of months ago on a walk through the fields to New Abbey graveyard. I was immediately curious, having worked for a company that had designed, manufactured and installed data logging equipment for local authority sewage and water treatment plants. I got chatting with Michael and discovered he was calibrating the weir. This is done every 6 to 8 weeks or so to make sure readings are correct on an ongoing basis. Vegetation can build up on the weir, causing errors in measurement and so it has to be cleaned. Calibrating the weir involves taking actual flow measurements using a portable flow meter and making corresponding readings for the height of water. Necessary adjustments are then made in the instrumentation firmware (embedded software) to the equation that converts height to flow.
I could have talked shop to Michael all day, but I didn't want to take him away too long from his work. Also the cattle had become curious and had come from both fields to investigate, so the place was getting overly crowded and it was time to move on.…
 

An antenna is mounted on top of the stilling chamber for sending data back to a monitoring station. © Eugene Brennan

Archer 2, handheld computer used for calibrating the system. © Eugene Brennan

Making a Thermocouple: Seebeck Effect Demo

A microammeter. © Eugene Brennan

See previous article about spacecraft power for an explanation of the Seebeck effect. This is a simple thermocouple I made tonight from a piece of scrap copper wire from 1.5 mm² flat T&E lighting cable and heating element wire from an old hairdrier (probably nichrome). The ends of the two pieces of wire are twisted together and the other ends connected to a microammeter. Then the twisted together ends are heated with a blowtorch. It generates 26 mV and a current of about 20 μA (20 millionths of an amp). Lots of these are connected in series or series-parallel (to increase both voltage and current) to make a thermopile for providing power in spacecraft.

Thermocouple made from copper and nichrome wires, joined at the end. © Eugene Brennan

The thermocouple was connected to a microammeter. © Eugene Brennan

....voltage was also measured. © Eugene Brennan

The Mill Stream in 2015, Kinetic Energy and the EPA's National Hydrometric Programme

Flow wasn't quite as strong today, but it could be once all the rain from last night soaks through the ground and makes its way into watercourses. This video was from October 2015 after a long, wet autumn. The water level was up to the 4th rung from the top of the ladder (half a rung spacing lower today in the video at the end of this post). At some stage, I want to do back-of-envelope calculations to see what the max energy output would be if the stream was harnessed. Kinetic energy can be calculated simply by measuring width, depth and speed of the flow to get cubic metres per second and working from there. Surface speed could be checked by timing how long it takes something floating on the surface to pass between two points. Speed wouldn't be constant from surface to bottom of the flow or from centre to the banks because of friction, but the streambed under the bridge is smooth and level, as are the walls, so velocity profile is known for such shapes (This is effectively a rectangular weir, used by the EPA to do hydrometric measurement). 

 

Data for the "Kilcullen Stream" is available here on the EPA's HydroNet website. (Isn't this the "Mill Stream")? 

 

Data available on the EPA's HydoNet website. Image courtesy EPA.

 

What's the tube at the side of the bridge?

 

This is the stilling chamber of what's called a gauging station by the EPA. The stilling chamber is used to make the surface of water less turbulent so that stream level can be measured with an ultrasonic sensor (which needs a level surface to get a reliable echo for measuring distance). I'm not sure whether they actually use a logger here to measure levels continuously. There's no antenna for telemetry, so possibly data is downloaded every so often or more likely transmitted using a GSM modem over the mobile phone network. A Yuasa type battery would power a setup like this because the electronics only has to wake up every 15 minutes or so, take a measurement and then go to sleep again. (Similar to the way a wireless doorbell sounder only wakes up every second or so to detect a button push). There's also a scale at the station for taking manual depth measurements.

More information about the EPA's National Hydrometric Programme is available here.

 

 Mill Stream December 7th, 2024

 

Pocket Weighing Scales From Temu

Pocket scales from Temu © Eugene Brennan

Playing with my latest Temuu gadget. A pocket scales which cost less than €5. Without proper calibration weights, it's hard to tell how accurate it is, but this is what it displays with some weights I bought.
Not sure what I'll use it for 🙄. No, I'm not becoming a drug dealer, but I like measuring instruments.

There used to be a balance in our classroom in the BNS, when it was based in what is now Kilcullen Library, along with glassware and other scientific equipment, hidden away in a cupboard (including a bottle of mercury which is highly toxic). They may have dated from before the war. We never used those items though.

Some information here on the teaching of "Rural Science" and "Nature Studies" in primary schools in the early part of the 20th century. The teaching of these subjects became optional in 1934, more emphasis being placed on using the time for teaching Irish:

ELEMENTARY SCIENCE IN IRISH PRIMARY
SCHOOLS FROM THE LATE 1800s TO THE
PRESENT DAY

Some more photos of what the scales indicated for the various weights I bought.