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Thread: Would a kenometer be a good choice for a new hobby LPM

  1. #1
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    Default Would a kenometer be a good choice for a new hobby LPM

    YES - ive seen the drama topics on both LPF and PL about the kenometer.
    But i really cant care much about some dust under a screen, or glue inside, as long as it works as intended and doesnt fall apart on the real things(instead of prototype)
    I just wonder if it is a good unit for a hobbyist. most important is accuracy and ability to make graphs on pc. i also like how the kenometer has a very quick response time.
    I am comparing it to a laserbee 2. got a offer for both of them.
    Althought my old laserbee usb died...poor quality product for sure as well. loose cables, the sensor or whatever it is fell off the heatsinks during measuring(below 2.5w)
    So in my book and head, both products are kinda on the bad side here, but i have to chose between 2 evils because those offers i had are from used products. i really do not wanna spend 400usd or more for a LPM shipped.

    What are your guys opinions?
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    Quote Originally Posted by borgqueenx View Post
    ...the sensor or whatever it is fell off the heatsinks during measuring(below 2.5w)
    Off, or apart? Did you see a flat sheet area or a lot of little blocks or pads?

    2.5W isn't much power, as a heat source goes, but if the sensor is small enough you can still raise it above 136°C if there is a laser shining on it long term, and if there isn't enough conduction or convecton to carry heat away, because even a black casing will not radiate very strongly if the difference is only a bit more than 100°C or so, so in a room that might be 25°C with local air temperature around the sensor at least 20°C hotter, you might find the sensor itself going over 136°C at which point a TEC based thermopile will fall apart if it wasn't built specifically for higher temperatures. Most sensor bodies are designed to make a thermal mass to stabilise readings, NOT to get rid of heat fast (because that would destabilise them). So all sensors are vulnerable. Some epoxies have even lower temperature failure, or will at least go soft enough to cause a fairly dense TEC to fall off under its own weight.

    If in doubt, go for fast response so you don't have to keep high power running into a sensor for long. Failing that, go for a large barreled sensor. Compared to the Ophir heads, let alone Laserbee heads, the Scientech 360001 is a beast of a barrel, and even that is maybe rated only for 20W long term. And it STILL is smaller than the average coffee mug, and 20W is enough to heat coffee in that mug till it's hot, if you wait long enough.. So a tenth of the power can surely heat beyond water's boiling point in any sensor with less than a tenth of the surface area of that coffee mug if there is warm still air around it.

    It's odd that there is an idea it gets harder to measure laser power over several watts, but it actually gets easier to think about, not harder. Heat is heat.
    Last edited by The_Doctor; 10-19-2013 at 01:21.

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    well i like the fast response time, but also like to see stability of the laser units themselfs. so i would like to keep them running for half an hour to see what they do.
    i wont do that every time, but i am curious if my projector is stable.
    Can an ophir have that long time without problems?

    And it fell apart, not off...its still wired...but not glued or taped or stuck to the heatsink anymore.
    i see white stuff with codes on it.
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    By 'apart', I mean, do you see two rectangular thin white slabs of ceramic, separated now, revealing lots of little thermopiles? If so, heat is definitely the problem.

    I'm not sure about Ophir heads, it's too long since I looked for info, but I think some of them use a radial array of thermocouples and not a TEC, and may well be rated for more heat.

    Either way, the trouble with power ratings is that context is everything. If for any reason the heat can build up in ways the maker didn't intend or expect, then all bets are off.

    White stuff with codes suggests you may be seeing the outside surface of an intact TEC. In this case heat may still be the problem. Not that there is too much, exactly, but that thermal contraction and expansion of repeat use might have weakened an epoxy bond. Ceramic expands so little that almost anything else will, in comparison. In this case you might get by sticking it back with a smear of arctic silver epoxy (good for more heat than a TEC will usually take). But if a TEC comes apart then only a new one will do.

    For long term monitoring, you could point a small DC fan at the back of the sensor head. So long as the thermal equilibrium settles (keep doors closed, other draughts to a minimum), you might get away with a stronger input power for longer. But it really pays to watch the temperature of the sensor... A tiny PT100 platinum thermistor might be worth adding, as the resistance has a well-defined change you can read directly on a good millivoltmeter (with a small constant current through the PT100 element) to get temperature without disturbing the power reading from the sensor.

    Edit: a carbon-black aluminium block with a PT100 thermistor on it might be a basis for a power meter in itself, even without the thermopile. No good for a few milliwatts, but for a few watts it might be excellent, and cheap to make.
    Last edited by The_Doctor; 10-19-2013 at 02:14.

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    so how hot is normal? i have a laser thermometer that works wirelessy, and a wired thermometer, both to measure temperature on a object.
    im not looking to fix my laserbee since also some wires are loose etc, i just dont trust it. maybe il sell it after i got a other LPM.

    just looking into if its a right choice of buying a other laserbee LPM(the laserbee 2 of 3,2W) or the kenometer pro.

    but thanks for the advice so far, good to learn about heat and how that stuff works.
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    Well, I did tell you that a TEC-type thermopile is usually good for no more than 136°C. I said more, but I had no choice. Whole books (thin ones, but plenty info in them) have been written by Marlow and Melcor about heat management and mounting of TEC's.

    The simplest answer is that if the sensor body ever feels as hot as a fresh mug of coffee, take care. Because the sensor plate WILL be a lot hotter.

    There are people here who can give better advice than me on specifics, I'm sure of it.

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    One work around would be to take a small percentage reflection from the main beam via beamsplitter or even a simple piece of non ar coated glass. You can calculate your percentage of power reflected by taking a quick measurement with and without and then factor that into your longer term stability measurements.

    The ophir sensors usually suggest heatsinking when measuring over a certain amount. Each sensor line is different.

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    If you want to measure stability, then TEC based LPM's are out the window. You're only left with an Ophir based LPM for that sort of power.

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    Quote Originally Posted by Things View Post
    If you want to measure stability, then TEC based LPM's are out the window. You're only left with an Ophir based LPM for that sort of power.
    You can do it. Add an amplified differentiator, mix its output with a copy of original input, balance them for early steady reading. I've done it. You won't measure 'jellybeaning', the fast flickering DPSS crystal based instabiilty that makes tiny beads (jellybeans) in rapidly drawn lines and fingers in scanned beams, but if you open your own fingers and pass them across the beam fast enough that all four pass through within a half second, you'll see the TEC based meter register strong pulses. That's definitely fast enough for many good stability measures. To do significantly better, you'd need a photodiode. (And a neutral density filter, no doubt, for the power.. Or do what m0f said, that being cheaper.)

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    i just saw a post on LPF from someone selling a ophir based LPM. you'd be way better off with that than a kenometer.

    Quote Originally Posted by borgqueenx View Post
    YES - ive seen the drama topics on both LPF and PL about the kenometer.
    But i really cant care much about some dust under a screen, or glue inside, as long as it works as intended and doesnt fall apart on the real things(instead of prototype)
    I just wonder if it is a good unit for a hobbyist. most important is accuracy and ability to make graphs on pc. i also like how the kenometer has a very quick response time.
    I am comparing it to a laserbee 2. got a offer for both of them.
    Althought my old laserbee usb died...poor quality product for sure as well. loose cables, the sensor or whatever it is fell off the heatsinks during measuring(below 2.5w)
    So in my book and head, both products are kinda on the bad side here, but i have to chose between 2 evils because those offers i had are from used products. i really do not wanna spend 400usd or more for a LPM shipped.

    What are your guys opinions?
    suppose you're thinkin' about a plate o' shrimp. Suddenly someone'll say, like, plate, or shrimp, or plate o' shrimp out of the blue, no explanation. No point in lookin' for one, either. It's all part of a cosmic unconciousness.

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