Mahoney CO2 laser power meter

Mahoney CO2 laser power meter

posted in: Laser cutting | 2

I recently bought a Mahoney laser power meter from “Bell Laser” (www.bell-laser.com , also known as www.2laser.com), to establish whether my “50W” Chinese laser cutter was actually producing anywhere near 50W.

mahoney_100w_meter_front

 

The meter was on a special offer price of $95, reduced from $179 (USD), however its unclear whether this price is really always $99 and the $179 price is just to make you think you have getting a good deal.

Bell Laser have a video showing the meter in use here https://www.youtube.com/watch?v=Qhq122YiOwQ

 

Bell Laser list their address as Seattle, WA 98104 (USA), however this appears to be somewhat deceptive, as the online payment when buying the meter appears to be to a Chinese company (Na Tang, also know as Bing.tang), and the meter its self was shipped from directly from Hong Kong.

The time between buying the meter on the 2laser website and shipping was 8 days. It actually only took 3 days in transit as it was sent it by DHL, but I’m not entirely sure why it took 8 days just to send it out.

Packaging of the meter is strange. Its put inside a 4 inch diameter plastic drain pipe, which has ink jet printed label stuck completely around it.

mahoney_meter_packaging
The meter was wrapped in very dusty old bubble-wrap, but there was wasn’t enough bubble-wrap to hold it snugly inside the tube.
The ends of the pipe were capped with 2mm plywood disks which look like they were laser cut, and flimsy polystyrene sheet was stuck to the inside
The end caps are held loosely with tape, but both of mine had fallen off in transit and the meter was rattling around inside the tube.
The whole lot was wrapped in piece of corrugated cardboard which had loads of clear tape wrapped around it.

Overall, the packaging was disappointing, and although the plastic drain pipe stopped the meter being crushed, there wasn’t much to stop the meter rattling around inside the drain pipe, so its lucky it was not damaged in transit.

In terms of its operation its exactly the same as the Macken meter, as shown in this video https://youtu.be/DrbQdIAQl4s?t=70

The meter initially needs to have its reading zero’ed. This is done using a small spanner, (which is supplied with the meter), to turn a nut / screw on the back of the meter, which rotates the dial inside the meter.

mahoney_100w_meter_back_spanner mahoney_100w_meter_front_spanner

You can only zero the meter, after the reading has settled as the meter temperature matches the current air temperature.

Once the meter has settled and has been zero’ed, you shine the unfocused laser on it for 37.7 seconds (this the time for my meter), and then wait for the reading to settle (for up to another 25 secs), before reading it.
Like all meters of this type, it is a “Calorimeter”, (https://en.wikipedia.org/wiki/Calorimeter)  made of a block of metal (in this case Aluminium) which has a known mass and known absorption of light at the wavelength generated by the CO2 or YAG laser.

When the laser light hits the calorimeter for a specific amount of time, it will heat proportional to the amount of power / energy being absorbed into the calorimeter.

Each meter is supposed to be calibrated in the factory, against a known standard, and the time the calorimeter needs to be heated in order for the gauge to read the correct value is engraved on the back of the meter.

In my case this time is 37.7 seconds. However I don’t actually have a way to know whether the meter has been accurately timed by Bell Laser,
But the whole point of paying $99 (Special offer price) for what is effectively a meat thermometer attached to a block of anodized aluminium, is that it is calibrated, as the cost of the components must cost less than $10.

 

The calorimeter block on my meter is 30.2mm x 10.2mm x 44.8mm. This size appears to be identical to the 200W meter sold by Bell Laser. But the 200W meter has a different scale plate fitted and has a much sorter exposure time (approx half of the time for the 100W meter). The 200W meter also costs $99 on special offer of $199 at full price, but this appears to be somewhat of a rip off, as its identical to the 100W meter apart from having a different scale dial on the meter.

 

As a matter of interests, I did a temperature / scale test, and the scale of my 100W meter seems to be 0.52 deg C per Watt.  I calculated this number by zero’ing the meter in cold water, and measured the temperature on a digital thermocouple thermometer (which read 9 deg C), I then poured in boiling water little by little until the meter read 100 W, and the temperature of the water was 61 deg C.

Hence a temperature rise of 52 deg C reads as 100W.
At the moment I only have the Mahoney meter so can’t double check these results, but my “50W” laser appears to output a maximum of 37W at the tube, and I get 32W after all 3 mirrors.
This is consistent with what other owners of the same laser cutter have reported and is consistent with the machine having a 40W tube rather than the advertised “50W” tube.

I will do a follow up post to this in the next couple of weeks, as I have bought a “DoHICKY” calorimeter from Russ of Sarbarmedia https://www.youtube.com/channel/UCqCyShJXqnElPTUnxX0mD5A

As the DoHICKY is calibrated against Russ’s “Macken” brand meter, so I will have something to compare my results to.

2 Responses

  1. G. Michael Bridge
    |

    Did you ever run the comparison against Russ’s DoHICKY ?

  2. Roger Clark
    |

    Yes.

    Both the Mayoney and Russ’s DoHICKY both gave the same values. Which gave me faith in both of them, especially considering that the DoHICKY is calibrated against another commercial calorimeter meter (I can’t recall the brand, but I know it was more expensive than the Mahoney and also supposed to be better or more accurate)

    I also did some experiments making my own DoHICKY, using high temperature resistant black paint.

    This worked reasonably well, but the black paint I had (its for car engines), seemed to have a absorbution of 80%, instead of the near 100% for anodised aluminium.
    So ultimately if anyone wants to make their own DoHICKY, you would need to get the aluminium anodised.

    I also did some other experiments with computer heat sinks as the calorimeter, as you can get black anodised heat sinks quite cheaply, and just baffle the fins to prevent heat loss.
    This works OK, but finding an ideal shape heat sink is a problem, as the optimum shape is circular cross section like the DoHICKY.

    So currently I’ve parked the idea of making a high performance calorimeter

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