The Humble Centimetre

Discussion in 'General Discussion' started by Ashley Hill, 9 Feb 2020.

  1. Ashley Hill

    Ashley Hill Member

    Messages:
    252
    Joined:
    8 Dec 2019
    Location:
    Ashley Hill
    Listening to today's weather forecast the presenter reported that some areas had received over 100mm of rain. Why not say 10cm? Industry is no better,often quoting sizes in 1000s of mm rather than so many metres and centimetres. Is this because 100mm might sound more impressive than just 10cm,or the current media trend to "big things up". Is it perhaps a desire to be as accurate as possible in reporting? What do members think and are there any similar examples?
     
  2. Registered users do not see these banners - join or log in today!

    Rail Forums

     
  3. DaleCooper

    DaleCooper Established Member

    Messages:
    3,403
    Joined:
    2 Mar 2015
    Location:
    No.664 - Next door to the Beast
    I prefer mm to cm, perhaps that's from working in engineering where all drawings were in mm. Note that we tend not to use centi, deci, deka or hecto very much, it is more usual to use multiples and submultiples of a thousand - micro, milli, kilo, mega etc. I don't think you will ever hear of centiamps or hectovolts.
     
  4. Ashley Hill

    Ashley Hill Member

    Messages:
    252
    Joined:
    8 Dec 2019
    Location:
    Ashley Hill
    I agree,with engineering and similar, working with exact tolerances needs extreme accuracy. Daytime telly etc doesn't. I would be quite happy being told 10cm rather than 100mm.
     
  5. RichT54

    RichT54 Member

    Messages:
    209
    Joined:
    6 Jun 2018
    Location:
    UK
    It's because millimetres (or tenths of a millimetre if less than 1 millimetre) are the units that rainfall amounts are actually measured in.
     
  6. DerekC

    DerekC Member

    Messages:
    1,101
    Joined:
    26 Oct 2015
    Location:
    Hampshire
    I agree with @DaleCooper - in fact I can recall as an engineering student being told that distances are measured in millimetres, metres and kilometres.
     
  7. AM9

    AM9 Established Member

    Messages:
    7,841
    Joined:
    13 May 2014
    Location:
    St Albans
    Because weather presenters are scientifically trained (at least most of them are qualified to inform us on what is a very scientific subject) so they report things in the legally corerect units, i.e. SI units.
    No, The UK legally uses the MKS (metres, kilogrammes, seconds) standard, adopted in 1889, as the base units. Every other dimension in length and mass can be expressed in units 1000 times or 1/1000 times as convenient, therefore apart from metres and kilogrammes, kilometres (1000m) and millimetres (1m/1000) and we use tonnes (1000kg) grams (1kg/1000), milligrams (1g/1000). Like weight volume It's very easy for anybody who can count in decimal to understand. Only the old* have any excuse to not have a mental grasp on the size of things expressed in metric units, and even then, it is ridiculous to protest that having at least a basic understanding is 'difficult'.
    * All schools were legally required to teach metric units as the primary system of measurement since 1974, so as the leaving age was raised to 16 in 1972, those who weren't given the mandatory metric education would be at least 63 years old.
     
  8. DaleCooper

    DaleCooper Established Member

    Messages:
    3,403
    Joined:
    2 Mar 2015
    Location:
    No.664 - Next door to the Beast
    Actually I'm quite old (66) and in a way I have the advantage of being bilingual, ambidextrous or. whatever the word is for being comfortable with either metric (SI) or imperial.
     
  9. Ashley Hill

    Ashley Hill Member

    Messages:
    252
    Joined:
    8 Dec 2019
    Location:
    Ashley Hill
    At school I was taught in metric. We measured in mm,cm and metres. But if something was 100mm we said 10cm. If it was 105mm we referred to is a 10 and a half cm.
    So why has the cm fallen out of use?
     
  10. DaleCooper

    DaleCooper Established Member

    Messages:
    3,403
    Joined:
    2 Mar 2015
    Location:
    No.664 - Next door to the Beast
    Because of abominations like "10 and a half cm" would be one good reason.
     
  11. AM9

    AM9 Established Member

    Messages:
    7,841
    Joined:
    13 May 2014
    Location:
    St Albans
    Me too, I can generally flip from metric to imperial and back (using simple approximations e.g. 100mm = 4 inches or 1lb = about 45% of a Kg. When I was at primary school, our exercise books had various equivalents table on the rear cover. The key ones that I remember were 1oz* = 28g (even though I didn't know what a gram was), 1 metre = 39.4 inches. Then there were the quirky ones tables, rod, poles, perches, chains, cubits.
    * avouirdupois of course :)
     
    Last edited: 9 Feb 2020
  12. AM9

    AM9 Established Member

    Messages:
    7,841
    Joined:
    13 May 2014
    Location:
    St Albans
    I remember in industry when manufacturing drawings were drawn on pre-printed linen and melinex masters. These would have standard dimensioning tolerances marked, e.g. inches with one decimal point (dp) were +/- 0.020 in., two dp +/- 0.005 in. and 3 dp +/- 0.001. The transition from Inperial too Metric was usually organised on a project basis when new metric masters would be used. There was at least one instance where a whole set of machining drawings were sent out effectively asking for something like 65mm +/- .005 inch!
     
  13. DerekC

    DerekC Member

    Messages:
    1,101
    Joined:
    26 Oct 2015
    Location:
    Hampshire
    Centimetres do have the advantage for young children of being relatable to the size of things they encounter, like fingers, books, balls etc, without having to worry about hundreds and thousands. I learned both - imperial units at primary school and SI units at secondary and in college. My children went to school in the 1980s/90s and learned only metric units, only to find that the world outside still loves its feet and inches, pounds and stones, pints and gallons … and of course miles! I think the unending use of two parallel sets of units is the source of much more confusion than centimetres and millimetres ever create. And of course with increasing Americanisation post-Brexit, the problem is likely to get worse.
     
  14. hooverboy

    hooverboy Established Member

    Messages:
    1,164
    Joined:
    12 Oct 2017
    there is definitiely a media trend to big things up.
    lashed by 80mph gusts is a little bit of structural damage, it's not particularly life threatening.

    as we're talking about weather at the moment, I don't remember too many named storms growing up when I was a kid.
    I do remember the tail end of hurricane charlie in 1987 quite vividly(100mph)!..I was camping in the lake district at the time.
    I had to evacuate at 4am because the nearby river was about to burst its banks and flood us all out.
    The bottom of my sleeping bag was floating on about 3 inches of water.
     
  15. TrafficEng

    TrafficEng Member

    Messages:
    417
    Joined:
    13 Nov 2019
    Location:
    North of London
    Schools start by teaching using cm because at that age children are also learning the basics of counting and measuring 100mm is quite difficult if you are only confident using numbers up to (say) 20.

    If you are measuring something arbitrary for learning purposes - like the span of your hand - then using whole numbers of cm is accurate enough. It then provides a route to teaching the concepts of fractions and estimating, so your measurement becomes (say) six and a half centimetres.

    When you get more confident handling larger numbers, and the need for accuracy grows, it makes sense to switch to using mm as these provide for a good degree of accuracy for most purposes whilst not being difficult to use for larger measurements.

    A good reason for not using cm is the potential for confusion. For example, if weather presenters said 10cm of rain had fallen it would be quite possible and reasonable to mishear that as 10mm, the error isn't immediately obvious because a factor of 10 difference is within expectations. If everyone sticks to mm or metres then the difference factor is 1000 and an error should be obvious just from the numbers. If we get 10 metres of rain we ought to be very worried.

    Likewise, making it easy to spot (and avoid) errors is one of the reasons why cm are frowned on in engineering and construction. This is even more important where imperial measurements are still used or familiar. Going to a timber merchant and asking for 2m of 4 by 2 could lead to disappointment if what you really wanted was 40x20mm. :o
     
  16. TrafficEng

    TrafficEng Member

    Messages:
    417
    Joined:
    13 Nov 2019
    Location:
    North of London
    I'm not sure the former is such an issue. Most children come out of school quite confident in measuring temperatures in either celsius or kelvin and to know which to use when, as well as converting between them. They should also be ok with measuring angles in radians instead of degrees, even if they never use a radian again in their life.

    The ability to measure in two different systems and convert between them is a skill similar to learning two different languages. The important thing is to make sure the learning of the second system/language isn't holding back the learning of the first one. And if learning the second system/language assists in understanding the first then so much the better.

    As per my previous post, the confusion between centimetres and millimetres has the capacity for mistakes to be made, potentially with serious consequences. Whereas mixing millimetres and inches is less of a problem unless the people involved don't have a good feel for what is being measured and what a 'sensible' measurement for that purpose looks like.
     
  17. DaleCooper

    DaleCooper Established Member

    Messages:
    3,403
    Joined:
    2 Mar 2015
    Location:
    No.664 - Next door to the Beast
    Metric and Imperial are child's play but I'm crap at languages.
     
  18. krus_aragon

    krus_aragon Established Member

    Messages:
    5,558
    Joined:
    10 Jun 2009
    Location:
    North Wales
    That's cause they only started naming them a year or two ago.
     
  19. DaleCooper

    DaleCooper Established Member

    Messages:
    3,403
    Joined:
    2 Mar 2015
    Location:
    No.664 - Next door to the Beast
    Before that they only had their "ineffable effable effanineffable deep and inscrutable singular name" and nobody knew what it was.
     
  20. John Webb

    John Webb Established Member

    Messages:
    1,675
    Joined:
    5 Jun 2010
    Location:
    St Albans
    I learnt imperial measurements at primary school (1951-57) cgs (centimetre, gram, second) 1957-61 and mks (metre, kilogram, second) 1961 onwards in the leadup to O-levels. We'd settled firmly on mks by the start of A-levels, thank goodness!
     
  21. najaB

    najaB Veteran Member

    Messages:
    21,158
    Joined:
    28 Aug 2011
    Location:
    Scotland
    Because there will be some days where the total might only be 10mm or less. So it's about being consistent. While it would be perfectly accurate, I don't think many people would understand if they said that the rainfall total was over 350 light pico-seconds.
     
  22. Ashley Hill

    Ashley Hill Member

    Messages:
    252
    Joined:
    8 Dec 2019
    Location:
    Ashley Hill
    If I'm making something I never measure in mm. It's always 1m 34 and a half cm,I only add the mm to be accurate,134cm and 6mm. It may be daft but it works for me. I can do it in imperial too. Like DerekC mentioned,I went to school in the 70s/80s learning metric whilst all around me used Imperial.
     
  23. najaB

    najaB Veteran Member

    Messages:
    21,158
    Joined:
    28 Aug 2011
    Location:
    Scotland
    Well 134.6cm is the same as 1346mm or 1.346m so it really doesn't make much difference how you express it. And that's the beauty of metric - switch between units by just moving the decimal point. :)
     
  24. GRALISTAIR

    GRALISTAIR Established Member

    Messages:
    3,827
    Joined:
    11 Apr 2012
    Location:
    Dalton Georgia USA
    Same. 63 in April. Being in the USA I have had no choice but to be bilingual and can think in US, Imperial or metric. Still prefer Joules to calories though!

    millimetres is eminently sensible for weather imho
     
  25. DaleCooper

    DaleCooper Established Member

    Messages:
    3,403
    Joined:
    2 Mar 2015
    Location:
    No.664 - Next door to the Beast
    It's on days like that you really need your quantum umbrella and Schrödinger wellies.
     
  26. GB

    GB Established Member

    Messages:
    5,563
    Joined:
    16 Nov 2008
    Location:
    Somewhere
    CM is still used when referring to snow fall.
     
  27. najaB

    najaB Veteran Member

    Messages:
    21,158
    Joined:
    28 Aug 2011
    Location:
    Scotland
    1cm of unpacked snow is equivalent to 1mm of rain so it kinda makes sense.
     
  28. furnessvale

    furnessvale Established Member

    Messages:
    3,637
    Joined:
    14 Jul 2015
    Surely by 1987 your sleeping bag would have been floating on about 76mm of water, not 3 inches! :D
     
  29. PeterC

    PeterC Established Member

    Messages:
    2,415
    Joined:
    29 Sep 2014
    I switched from imperial to SI when moving from primary to secondary in 62.

    Kids today, getting all their arithmetic in base10 don't know they are born. In my day you had to cope with numbers in base 12, base 16 and base 20 in a single piece of mental arithmetic just to buy some fruit and veg.
     
  30. Lucan

    Lucan Member

    Messages:
    617
    Joined:
    21 Feb 2018
    No it is because they are using the mks system of units, as we all should. In this, multiples or divisions of 1000 from the basic units are preferred. The basic unit of length being the metre, preferred subdivisions are the millimetre (1/1000 of a metre) and the micron (or micrometre, 1/1000 of a mm). The preferred multiple is the kilometre (1000m). Similarly with weight : the kilogram being the base unit of mass, it divides into the gram (1/1000 of a kg) and the milligram(1/1000 of a gm), and the preferred multiple is the tonne (1000kg).

    The cm is part of the old cgs system (yes, there is more than one metric system). I am an engineer, and I think of the cm as a dressmaker's unit :rolleyes:
     
  31. DarloRich

    DarloRich Veteran Member

    Messages:
    23,778
    Joined:
    12 Oct 2010
    Location:
    Work - Fenny Stratford(MK) Home - Darlington
    How many inches of rain are we talking about?
     

Share This Page