AWESOME @Grumpy_Old_Man STICK IT to them they’ve earned it!
Sasquatch
Not really. My tubes cost ~$1000 to replace. This tool will last decades.
The alternative is to hire someone like me to come and fettle your laser, and I charge $120 an hour.
Without a meter you are only guessing.
There’s so much more to it than just reading the power output. For example:
Use it between the tube and first mirror to give you initial tube reading, along with an RMS multimeter that measures the precise output at a specific, precise mA.
Move to the next position after the first mirror, and you can read your mirror losses. Same for next position, then remove your lens and read at the final position to give you a reading of your tube and mirrors.
Take readings all over the extents of the bed and you can get a better understanding of your mirror and chassis alignment.
And, of course, it allows you to keep track of your tube power over time.
If, for example, your machine suddenly loses power, it can take just a couple of minutes to see where that loss is occurring without having to guess, or fiddle and potentially cause some other issue.
If course if you have a $300 K40 or 5W diode, it is a significant investment.
As far as tools go, I have a hammer that cost nearly as much and a spanner that cost twice that. Bell-Laser are not a price-gouging site and they hand-make all their meters in the US. The process of calibration is not insignificant.
Go look at some hand-made tools from the US and then compare the price.
There are other, cheaper tools - plastic, digital displays, etc. but they don’t have the reliability or calibration simplicity of the Mahoney. It’s so basic, it will work optimally, forever.
The Model T of the laser meter world. 
An ammeter is half your diagnostic.
It only shows what power is coming from the PSU.
If you are, for example, showing 25mA at your ammeter, but you can’t cut paper, you are no wiser as to the reason.
It could be a faulty tube, dirty tube, faulty or dirty mirrors or lens, alignment, mechanical, whatever - all you know is the power output of your PSU.
People put so much faith in an uncalibrated Chinese ammeter - I’ve never worked out why. I can’t say I have ever used the one installed on all of my machines.
A/ They’re unreliable - I have read differences of nearly 5mA from real, calibrated tool readings. That’s enough to destroy a tube when you think you’re putting out 18mA and you’re really putting out 23.
2/ Why? What do you think you are you reading and what do you think it is telling you?
iii/ So you can see it is putting out ~20mA - so what? How does that change what you are doing?
I have a dial just below mine on my latest machine. I use that somewhat regularly to run a second pass, or to fix a problem - I can turn power off entirely until I reach the point I want to resume cutting, or I can run a light second pass to compensate for environmental conditions, etc. - but as to the meter itself, I guess it shows I’m putting out something.
I prefer to calibrate the machine from new, then I know what the upper limit is and can happily tell the software to run flat out at 100% and be sure it’s not going to be damaging the tube.
I use a screwdriver and a multimeter and the Mahoney power probe.
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I wasn’t serious. Just having fun. It really is not expensive.
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Thanks for the replies guys! That will prob be my next buy
Still learning 
Calibrating an ammeter is no big challenge
But using a co2 laser machine (K40 / 50 …) without ammeter is a challenge that ends up with a much shorter life of one’s tube.
And even with a 10% deviation from a Chinese ammeter, it will show “good enough” for our purposes.
Skål
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Seems like a dial is also a good idea… to quickly do a second pass with lower settings… on the other hand… that isn’t a biggie
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I have one of these. Don’t know if they’re any better or worse than the others. Made by Macken.
https://www.kenteklaserstore.com/digital-laser-power-probe-kit.aspx
Same website, anyone used this:
https://www.bell-laser.com/chinese-laser-focus-lens-znse
Looks quite impressive on paper…
Well, I picked this up:
Ordered the 0-100w version as I don’t think I’’ be buying a +100w laser anytime soon!
Any other recommendations?
Nice post!
Thank you.
The 200W one would have been more useful(you WILL get a bigger machine :), but not to worry, it’ll be fine. make yourself a little stand. I’ve got an overly-complex design, if you want it, but it’s a good exercise in design to make your own.
If you haven’t got a decent RMS multimeter and you have a bit of spare cash, one of those is nice.
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