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

More Bord Na Móna Paraphernalia: Bagger Near Edenderry

A bagger on a bog near Edenderry. © Eugene Brennan

The remains of a bagger on a bog at Mount Lucas near Edenderry, photographed in 2009 (using a dumb phone, so quality not great and I didn't have the digital camera with me). Baggers were like mobile factories, with bucket excavating arms that harvested turf, macerated it and turned it into sausage-shaped sods for burning in power stations. They were powered by a trailing cable connected to local 3.3 kV power lines. This machine was later moved to BNM's Edenderry depot and the transformer removed, I'm guessing because it contained (or they wanted to check whether it contained) polychlorinated biphenyls (PCBs), which are organochlorine compounds, once used for cooling electrical equipment and making carbonless copy paper. They are highly toxic and were subsequently discovered to be carcinogenic.

 

More information on baggers here.

 

There's also a video of a bagger in operation here.

 

© Eugene Brennan

© Eugene Brennan

© Eugene Brennan

© Eugene Brennan

 

The Argument for Data Centres

Data centre. Image produced by Bing Image Creator

Not my argument, this is from Publicpolicy.ie, an "online platform for informing and debating public policy in Ireland", which has recently been incorporated within the UCD Geary Institute for Public Policy.
According to Eirgrid, "By 2031, 28% of all electricity demand is expected to come from data centres and other new large energy customers".

Nuclear Fusion Reactor Up and Running

JT-60SA tokamak nuclear fusion facility outside Tokyo, Japan. JT-60SA.org

The old joke is that fusion is always 50 years away. Unlike fission which breaks up atoms and results in dangerous radioactive waste, fusion pushes atoms and their subatomic particles together, mimicking the process that happens on our nearest star and nuclear reactor, the Sun. This results in conversion of matter to energy and a tremendous release of heat (thermal power stations use that heat to boil water and make steam, which drives turbines to spin the alternators that make electricity). Fusion produces minimal radioactive waste and unlike a fission reaction which can go out of control and result in a explosion as happened in Chernobyl, it's inherently safe because of the conditions required to sustain fusion: high temperature and pressure. If these conditions aren't sustained, the reaction safely fizzles out. While the Sun finds it "easy" to sustain a fusion reaction, due to the immense gravity that squeezes hydrogen atoms together and forms helium as a waste product, creating the conditions for fusion in a reactor on Earth is a huge technical challenge. Up until recently, more energy has been required to create the required high temperatures and pressure than the net energy that could be generated by a fusion reaction. According to Popular Science, in 2023, "the National Ignition Facility (NIF) at Northern California’s Lawrence Livermore National Laboratory achieved a net energy gain for the second time using what’s the inertial confinement fusion method".
The fuels for fusion reactors are tritium or deuterium, both isotopes of hydrogen.
This article from Popular Science, published in Dec 2023, informs us that the world’s largest experimental tokamak nuclear fusion reactor is up and running.

Deciphering NLI Images

Image courtesy The National Library of Ireland

What's written on the samplers (there's probably a correct term for these) from newspapers outside Bardons in this section of a photo from the Lawrence Photograph Collection in the National Library of Ireland? The photo is the one looking up Main Street from the bridge. "Freeman" seems to be legible at the top of one board, so possibly an ad/excerpt from The Freeman's Journal. Maybe AI could decipher the other content? On the other side of the street, M.A. Egan has a display or ad for "Cadbury Chocolate".

I asked Grok, X's AI assistant. It wasn't much help and presumably because of the data it's trained on, it's biased and coming up with words like "New Jersey". It also didn't make the obvious connection with The Freemans's Journal, although I didn't give it any context for the image. So I asked another question, giving it some extra information this time and it made the connection. It hasn't deciphered what looks like "The Land Act..." on the third board. (Officially "The Land Law (Ireland) Acts according to Wikipedia). Next I gave Grok more context, this was its response:

Eugene: Grok can you decipher this image for me? The boards show ads from newspapers, outside a shop in Ireland in the nineteenth century.

Grok: "Thank you for providing additional context! The image shows several boards displaying newspaper advertisements outside a shop in Ireland during the nineteenth century. The photograph is in black and white, which aligns with the photographic technology of that era, and the boards appear to be promoting various publications or notices. Let’s break down the text on the boards as best as I can, given the image quality.

Left Board:

Top Line: "FREEMAN"

This likely refers to The Freeman's Journal, a prominent Irish newspaper founded in 1763. It was one of the leading newspapers in Ireland during the nineteenth century, known for its nationalist leanings, particularly in the later part of the century when it supported Charles Stewart Parnell and the Irish Parliamentary Party.

Second Line: "NEW JERSEY..."

This part is a bit confusing because "New Jersey" typically refers to a U.S. state, which seems out of place for an Irish context. It’s possible this is a misreading due to the image quality, or it could be referring to a specific edition or a story related to New Jersey (e.g., Irish emigration to the U.S.). However, I suspect it might actually be "NEW SERIES" or something similar, which was a common phrase used in newspaper advertisements to indicate a new edition or relaunch.

Smaller Text Below: The text is too blurry to read in detail, but it likely includes publication details such as the price, frequency (e.g., daily or weekly), or a tagline. For The Freeman's Journal, a typical ad might mention its role as a leading voice for Irish interests, its circulation numbers, or its price (e.g., "Price One Penny").

Middle Board:

Top Line: The text is faint, but it appears to start with "THE LAW OF..." or "THE LAND OF..."

Given the context of nineteenth-century Ireland, this could be an advertisement for a publication discussing legal matters, land reform, or political issues. The Land War (1879–1882) was a significant event in Ireland during this period, involving tenant farmers protesting against landlords. A newspaper or pamphlet might advertise itself as covering "The Law of the Land" or "The Land of Ireland" to attract readers interested in these issues.

Rest of the Text: The smaller text is too blurry to decipher, but it might include details about the publication’s content, such as editorials, legal advice, or reports on land disputes.

Right Board:

Top Line: It looks like it might start with "THE" or "THIS," but it’s very hard to read.

This board is the least clear, and the text is too faint to make out with certainty. It could be advertising another newspaper or a specific article or notice.

Contextual Analysis:

Setting: The boards are placed outside a shop in Ireland, which was a common practice in the nineteenth century for newsagents or general stores to display newspapers and attract customers. The Freeman's Journal was widely circulated in Ireland, so its presence here makes sense.

Historical Context: In the nineteenth century, Ireland was under British rule, and newspapers played a crucial role in shaping public opinion. The Freeman's Journal was known for its advocacy of Catholic emancipation early in the century and later for Irish nationalism. Other publications of the time might have focused on local news, legal matters, or land reform, especially during periods of social unrest like the Land War.

Image Quality: The photograph’s grainy quality and the angle of the boards make it challenging to read all the text. Nineteenth-century photographs often have this level of clarity, especially if they were taken with early cameras or if the image has degraded over time.

Interpretation:

The boards are likely advertising newspapers or related publications available for purchase at the shop. The Freeman's Journal is the most identifiable, and it would have been a major draw for customers interested in national news, politics, and Irish identity. The other boards might be promoting local or specialized publications, possibly focusing on legal or land-related issues, which were highly relevant in nineteenth-century Ireland.

If you’d like more specific details about The Freeman's Journal or other Irish newspapers from that era, I can search for additional information. Alternatively, a higher-resolution image might help in deciphering the smaller text! Let me know how you’d like to proceed."

Transformer Cooling Oil and Heathrow Disruption

A typical power transformer with oil cooling tank. Sensenschmied, CC BY-SA 3.0, via Wikimedia Commons.

You're probably aware of the major disruption at Heathrow Airport this morning because of a fire at a nearby electrical substation. According to the BBC, 25,000 litres of cooling oil in a transformer at the substation (20,000 according to The Guardian) had caught fire. London Fire Brigade said this was due to transformer failure igniting the oil. Transformers are ubiquitous components in electrical and electronic appliances and gadgets, and larger types have to be cooled. Just like the way a car engine has coolant flowing through its core to carry away waste heat, transformers have a similar cooling system. Water can't be used, so the cooling medium is usually a specialised high-grade mineral oil.

 

Why do transformers get hot?

 

All electrical conductors have electrical resistance which restricts current flow. You've probably noticed the phenomenon where a power flex in an electrical extension reel gets hot if the reel isn't fully unrolled. Transformers have thousands of turns of wire wound onto laminated iron cores and this wire gets hot (known as copper loss). The iron core of a transformer also gets hot because of magnetic fields inducing current flow in the core (known as eddy currents). The small transformer in a power adaptor can usually air cool, however room-sized transformers in substations because of their size have issues with heat making its way to the surface and dissipating. Without being cooled, heat can build up and melt insulation on the windings, potentially causing the transformer to short out. Large transformers therefore have a cooling system, and oil circulates through or around the device, either by natural thermal convection or by being pumped. This carries heat to the surface of the transformer casing where a heat exchanger (just like the radiator in your car or home) dissipates it to the ambient air. Sometime fans are used to increase airflow through the cooling fins of the heat exchanger. The cooling system is usually sealed, just like in a sealed, pressurised central heating system, so a tank mounted on the transformer is often used to cater for expansion as temperature changes. Sealing is necessary to prevent the oil absorbing water, compromising its electrical insulating properties.

Counter-terror officers are investigating the fire, so it's not known, or at least no information has been released to the public yet about whether the fire was due to a system fault or sabotage by terrorists.

The image above is of a typical transformer with conservator tank mounted on top.

Proposed LNG Power Plant in Kerry

Gas storage tanks at an LNG Terminal. © Copyright Robin Lucas and licensed for reuse under this Creative Commons Licence.

Great news for energy security as reported in this RTÉ News article, but of course some aren't happy because the gas (shipped as liquid methane) that it's hoped will supply it may come from fracking. The plant will also include a 120 MWh Battery Energy Storage System, such facilities being needed nationwide to store energy and release it as needed (although they're controversial as we saw locally). Newer forms of electricity generation such as wind can be unpredictable, so battery storage acts as a buffer to feed the grid when the Sun isn't shining or wind isn't blowing. It's hoped, according to the RTÉ article, An Bord Pleanála won't put a spanner in the works now and appeal the High Court ruling on the LNG terminal. The court overturned An Bord Pleanála's refusal of permission for the facility last September.

There's an error in the RTÉ article. "120-megawatt" should presumably read "120 MWh" or "120 megawatt-hours".

From the Wikipedia Rabbit Hole Day Trip: Bacterial Cellulose

Lightenoughtotravel, Public domain, via Wikimedia Commons

The equivalent of cellulose in other plants (e.g. wood formed from lignin fibres in trees), but made by bacteria. So just like plant cells have a cellulose shell to give them protection and structural integrity, bacteria have something similar. I ended up on this page because I was reading on a homemade tools group about how someone had used the stuff in the same way leather is used, for strapping two parts together. Bacterial cellulose has certain desirable characteristics such as greater tensile strength than plant cellulose and large hydrophilicity (attraction to water) and so may be useful in biomedical applications.

More info in this Wikipedia article.

#MaterialsScience

Arc Flashes

Zaereth, CC BY-SA 4.0, via Wikimedia Commons

Which can cause severe burns. The arc is usually hotter than the surface of the Sun and has the energy of an explosion. A conductive arc can form due to dust, faulty components or shorted terminals in an electrical panel due to a dropped tool. Another cause can be improperly placed test leads which bridge live parts. The huge surge in current through the low resistance arc can lead to catastrophic injuries and damage in the milliseconds before a protective device operates. Personnel working on electrical panels use PPE to protect themselves from the heat and intense UV of an arc flash. See this Fluke guide for more details.

Science With Dr Karl Podcast — Heterochromia, Cold Water Therapy and Time of Death

Dr Karl is on sabbatical, but Bianca Nogrady joins Lucy Smith to answer listeners' questions about having different colour eyes, could we evolve to become fish people and plant cancers. We also learn about a new pump to replace the heart that runs continuously and doesn't beat like a normal heart.You can listen to the latest episode here.

Bicycle Wheel Bearing Maintenance

Bicycle hub with traditional cup and cone bearings © Eugene Brennan.

So the back wheel on my bike was making grinding noises on the way back from my cycle along the canal to Sallins last Sunday. These rims are 1997 vintage and older wheels have cup-and-cone bearings. The advantage of these is that the parts can be easily replaced when they wear: the axle, the cones and the steels balls the cones turn against. Unfortunately they're also prone to grit and grime getting into the bearing, especially when a hose or power washer is used to wash muck off a bike, as this can drive the stuff inside. Newer hub bearings are sealed and probably use roller or ball bearings. However when the bearing wears, the whole wheel has to be replaced. The hub and axle alone can be replaced, but it means undoing all the spokes, then rebuilding the wheel with the new hub and replacing the spokes, which is not a job for amateurs. That's because all the spokes have to be properly tensioned to ensure trueness of the wheel.

I cleaned all the balls from the bearing (a magnetic pick-up tool is the best way of getting them out of the grease), the cones and the cups which are bowl-shaped steel inserts in the aluminium hub. Neither were excessively pitted nor worn. The axle was a bit bent, but that often happens due to impacts with a bad road surface and applying too much force to pedals when climbing steep hills. Then I re-greased everything and put the bearing back together. It's still a bit rough, but should last for another while. I have a spare pair of new sealed bearing wheels to act as replacements.

Spacer to accommodate freewheel cogs. © Eugene Brennan

Components from bearing. © Eugene Brennan

Trainee bicycle mechanic. © Eugene Brennan

Cup part of cup and cone bearing. © Eugene Brennan

 

Septic Tank Inspections

Schematic of a septic tank. Image courtesy Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph., Zurbrügg, C., CC BY-SA 4.0 , via Wikimedia Commons

The smell of overflowing septic tanks is something I encounter regularly, cycling the highways and byways of Kildare. Two incidences yesterday in the Suncroft area and one close to Narraghmore Game Reserve. Maybe there are other issues that cause smell though like bad drainage or older systems that don't agitate the contents of the tank? Agitation increases the oxygen content of liquid in a septic tank, improving the rate of aerobic bacterial growth and speeding up the digestion of sewage. This recent article from the Kildare Nationalist by Finian Coghlan fills us in with the details of the inspections.

Plate Tectonics and Finger Nails

Image created using by Bing Image Creator.

Plate tectonics is a theory, superseding and encompassing the earlier continental drift theory, that the Earth's continents make large scale movements over geological time. The process is really slow, and occurs at the same rate finger nails grow: about 0.1 mm a day. That's 36.5 mm a year, 36.5 m per millennium and 36.5 km per million years or 3650 km in 100 million years. Ireland, or more correctly the parts that make up Ireland, were once located south of the equator in tropical seas. (I found a rock once in the midlands, when helping to dig a deep hole. The rock was embedded with large fossilised cockles). Over millions of years the parts drifted northward and merged, to make the island. The dividing line between the northern and southern parts of the island of Ireland runs roughly down a line between Louth and Kerry.

Force Between Magnets Over a Distance

Public domain image via Pixabay

As you probably know, like poles of magnets repel and unlike poles attract. The force between two magnets increases as they're brought closer together, but how does it vary? Many things in nature follow an inverse square law. So the luminous intensity of an omnidirectional (the same in all directions) light source or loudness of an omnidirectional sound source follows an inverse square law. If you move 3 times further away from a sound than your original position, the loudness reduces by a factor of 9. At 100 times the original distance, the loudness reduces by 100 x 100 =10,000 times. Similarly for the brightness of a light. That's because the sound or light flux is passing through an imaginary sphere and the surface area of a sphere depends on the square of its radius (4πr²). In this Quora post, we discover that for a magnet, the scenario is a lot more complicated because fields aren't uniformly shaped.

Tree Manslaughter

Dead apricot tree. © EugeneBrennan

So I tied a rope around a limb of this apricot tree that I grew from a stone around 2003, because it was growing across the driveway and the rope was to pull it inwards. Nothing wrong with that and usually you thread the rope through something soft like a scrap of garden hose so it doesn't damage the bark. Then over time, the tension normally goes out of the limb and it stays in place and the rope can be removed. However I left the rope in place, thinking it would be swallowed by the bark, just like what sometimes happens to park benches and the vascular channels in the bark would merge and create a bypass for sap. But apparently not. What happens is that a tree actually gets strangled as the trunk expands and flow of sap and nutrients can't get past the bottleneck. So I queried it on a Facebook gardening group and was accused of murder and was lucky to escape without being virtually lynched.

I think the tree looks nice though even though it's dead.

© Eugene Brennan

Ardnacrusha to Dublin Transmission Line and Mystery Engine House

Image courtesy Military Archives.

Just noticed the base of one of the pylons of the 110 kV line from Ardnacrusha to Dublin in the top left of this photo of Kilcullen Bridge from the military archives (Click on the link in the image caption to open the image viewer which has a zoom facility). Ardnacrusha hydro-electric power station was commissioned in 1929, and presumably the transmission line dated from then. The line was dismantled in the 60s or maybe early 70s? When the photo was taken, power was delivered directly from Newbridge/Kildare to Kilcullen at 10 kV and there's a transformer in Nugent's field for dropping voltage from that 10 kV to around 230 V. There was no substation in the town at the time, or at least when the last edition 6" OSI map shown was drafted. The map is post-1939 as the Nicholastown housing estate has been built. The 38 kV station at Sunnyhill we have now is fed from the 110 kV substation on the N81 near Stratford-on-Slaney.

The record in the archives at the link below also has an aerial view of the bridge looking south. On desktop/laptop, right click and open the image in a new tab to view at full size. There isn't a zoom facility in the viewer.
I still haven't found out what was housed in the "Engine Ho.", located approximately where Scoil Bhríde is now. A stationary engine for pumping water from a well for the convent? Or maybe a machine for making meal for feeding animals on the convent "farm"? Or did the convent have their own electricity and this was where the engine and generator was located? Maybe someone knows.

Image courtesy Tailte Éireann (OSI)

 

Well Fancy That!

St Brigid's Well, Kilcullen. © Eugene Brennan

KCA have added a nice new gravel ramp leading up to the bottom step at St. Brigid's Well which will help us older or short-legged folk negotiate the incline. The riser of the step was always quite tall. The others are a bit tall too. The gravel ramp should be safer than the existing stone one which is probably risky in the winter because of ice and algae. The steps are somewhat slippy, possibly because the hard rock has become worn and polished from traffic. In winter the situation is worse because of algae, but KCA (I presume) have regularly sanded the steps which helped and made them safe

St Brigid's Well, Kilcullen. Ramp added to reduce step height. © Eugene Brennan

On This International Women's Day, Remembering Irish Pioneers of Science and Technology

Jocelyn Bell Burnell, Kathleen Lonsdale, Mary Parsons, Kay McNulty and Mary Ward. See below for Image attributions.

 

Jocelyn Bell Burnell (1943 - ) who as a graduate student, discovered astronomical objects called pulsars in 1967.

 

Kathleen Lonsdale (1903 – 1971), a crystallographer from Newbridge, who used new techniques such as X-ray diffraction and Fourier spectral methods to investigate hydrocarbons. She was also the first woman to become a member and president of several male-dominated scientific institutions.

 

Mary Parsons, Countess of Rosse (1813 – 1885). An amateur astronomer and blacksmith and a pioneer in making photos from wax paper negatives.

 

Kay McNulty (1921 – 2006) Software engineer and one of the six original programmers of ENIAC, a room-sized early computer, used by the US military to calculate artillery firing tables.

 

Mary Ward (1827 –1869) Naturalist, amateur astronomer, microscopist, author, and artist who wrote several books about her discoveries in microscopy including A World of Wonders Revealed by the Microscope.

 

Image attributions:

 

Photo of Jocelyn Bell Burnell at Launch_of_IYA_2009,_Paris_-_Grygar,_Bell_Burnell.jpg: Astronomical Institute, Academy of Sciences of the Czech Republic derivative work: Anrie, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons.

 

Photo of Kay McNulty by Zebbie, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

 

All other images, public domain.

How Much Does the Length of a Day Vary By?

Created with Bing Image Creator.

Every four years we have a leap year and an extra day is added to the calendar in February. That's because the time it takes for the Earth to orbit the Sun is approximately 365.25 days. Considering the Earth's orbit as approximately a circular path around the Sun, If we didn't compensate for the extra quarter day, the Earth would end up being a little short of a full 360 degrees of its way around the Sun on the 31st December (a quarter day short). Eventually 31st of December would fall earlier and earlier in the seasons until it ended up in the summer. By adding an extra day every four years, the Earth is allowed to "catch up" in its orbit. Similarly, a day, or the time it takes the Earth to revolve on its axis isn't exactly 24 hours long and is influenced by environmental and geological events. In fact in 2025, the length of time between the longest and shortest day was approximately 1.6 mS. It mightn't seem like a lot, but to keep everything in sync, leap seconds have to be added onto Coordinated Universal Time (UTC) every so often. The leap second was introduced in 1972. The most recent leap second adjustment was in December 31, 2016. However leap seconds are controversial and cause issues with critical systems that rely on accurate timekeeping. 

More info here.