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Pumping - Gauges

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Hi all, I'm currently in development and have a question about Pump gauges. I know that we have two types of gauges: the Bourdon and the Diaphragm. During a pumping drill the other day, my gaffer asked how these work mechanically, and it threw me. I know how to read them (in regards to pump operating), but actually understanding the mechanics behind them, I had no clue. I've been trying to find out the answer using Google, YouTube, etc., but all of the explanations go over my head, probably due to the fact that they're discussing engineering rather than a fire pump, so there are more things involved. I've also looked in the Hydraulics, Pumps and Pump Operation (Book 7) of the Manual of Firemanship; however, this was published in 1986; therefore, I only want to take it with a pinch of salt as I don't know if it's a little outdated now or if the gauges are slightly different mechanically from back then. 

Is anyone able to discuss the mechanics behind each one for a fire pump and how that's used in correlation to pump operation and revs, etc?

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Bourdon type: imagine you have a length of hose lying on the ground in a spiral, branch closed.

You start pumping water into the length.  As the pressure increases, what does the hose do?  It tries to straighten out.

This is how a bourdon gauge works:  there is a spiral tube inside the gauge with a needle attached, as pressure increases the tube tries to straighten & the needle moves.  The tube is attached to a spring, so when pressure decreases it coils up again.

Diaphragm type: imagine two balloons, pressed side by side inside a glass fishbowl, when you look at them they look a little like: ( | )

As pressure increases in one balloon it gets bigger and compresses the other balloon: (|  )

As pressure decreases in one balloon it gets smaller and the other expands: (  |)

This is how a diaphragm gauge works, needle is attached to the diaphragm (the line where the balloons touch).

This is why Bourdon tube gauges have unequal sides (ie. large positive reading area, small negative), where as Diaphragm ones are equal both sides of zero.

Hope that makes sense?


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Perfect, that's exactly what I'm looking for. Thanks for the response. I spent all that time searching, and all I had to do was a message on here, haha. Thanks

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Obviously the needles are actually attached by little cogs & gears and whatever & it's not actually balloons, but that's the basic principle!

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26 minutes ago, Jack.B said:

Perfect, that's exactly what I'm looking for. Thanks for the response. I spent all that time searching, and all I had to do was a message on here, haha. Thanks

Could you have asked the question then and there? You aren't expected to know everything as a trainee and this feels like pointless information to fill your head with.

Of course, please take all the information in as you wish, but I am not sure learning the inner mechanics of a pump gauge will make you a better firefighter. Fire services seem to get fixated on these strange little details, and we have had trainees come on to the station who could tell us everything you need to know about a piece of equipment, but are not able to use it. 

Just my thoughts.

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Posted (edited)

He did explain there and then, but I must admit, reflecting on it, I mixed it with other information and confused myself, so I wanted that clarification. A few of the guys on my Watch have said that, too, that it's not really something I need to know, but personally, that makes me feel more confident knowing the ins and outs about the most important things but making sure I know the basics inside out. If you know the basics, why not further develop your knowledge? I also feel that by understanding how something works, it's much easier to fault find and read if there's a problem, especially under pressure. There's always the interest in 'why' something happens, 'why' it does what it does, etc. For me, anyway.  

It works both ways, I think. You need to know the theory but be excellent at practically performing it, too. Practice both, and you're more likely to perform well and be reliable. A lot of trainees (myself being one now) leave training school with much less information than the older guys and girls did, and it's very noticeable when discussing things and seeing it in the yard or fireground. Actually understanding concepts and being practical is probably what we need, rather than the current culture of 'this is here, but you will never use that, so don't worry about it'. Yet that time comes when you do use it or asked about it, and you stand there and go, 'I don't know what I'm doing'. Now I look like an idiot with a terrible excuse of "well I've been told many times not to worry about that". 

Edited by Carl
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I agree with you on the  "this is here, but you will never use that, so don't worry about it" statement. There seems to be a lot of that in training school, and I remember being told in training that I probably won't use the 135 in anger, but I have dozens of times.

You say about fault finding with equipment, and that's fine, but being on a job where a gauge needs fixing, and you have the time, skill, or tools to fix it is slim to none, I understand your point though.

It's good to have a further underpinning knowledge of equipment, but you already know the basics (how to read the gauge) understanding the inner workings is something else entirely. Where does it end? understanding the inner wiring of a thermal image camera? 

Please do not take this the wrong way, I see this thing all the time in real life, where recruits get pinned down by trying to learn a bit of equipment and memorise all manner of key points like they are reading directly from the technical note. It's a fault in training school I think, where you are tested on knowing the weight, dimensions etc of equipment, more than you are tested on actually using it.

I don't really disagree with anything you have said, but I have just seen the other side, and as I said before, when people can tell you absolutely every technical detail on a piece of equipment without actually being able to use it.

The point I am (badly) trying to make, is to bogged down with this kind of thing at the expense of being practically good at it.

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