25 – Measuring Laser Tube Power (34:09)

The Lightblade Learning Lab with Russ Sadler

The Lightblade Learning Lab is a series of videos that Russ did for Thinklaser Limited based on using the Lightblade 4060 Laser Cutting and Engraving Machine. Thinklasers Lightblade 4060 has a 400 x 600mm bed size and was supplied with a 60W EFR laser tube. In this session, Russ devises a DIY solution for measuring laser tube power.

Measuring laser tube power
Measuring Laser Tube Power

Contents

  • (This video uses a test program called doHICky-timer.rld and the RLD file is available here http://bit.ly/2ywX8jU)
  • Laser tubes will gradually lose power
  • Turning the speed down to compensate or finding out where the power loss is
  • Characterising the tube: comparing the power demanded on the user interface, the current shown on the ammeter and the actual power coming out of the tube.
  • The pre-ionization zone
  • Ways to measure power – power meters
  • Macken Instruments high-end products
  • Or for a home-made version see RDWorks Learning Lab 53
  • Loading the test program onto the machine
  • Looking at what the program does
  • Use of a tray of water as an energy absorber
  • Changing the parameters of the program for speed and power
  • Care when testing the power
  • Bucket of water to get the test probe to ambient temperature
  • Looking at the controls of the temperature meter
  • How to hold the probe during the test
  • Performing the test
  • Recording the results including:
    • Laser power (%) as set on the machine’s controls
    • Start temperature (degrees C)
    • End temperature (degrees C)
    • The current while the test is running (mA)
  • Repeating the test with different power settings
  • Charting the results using a spreadsheet
  • Analysing the relationship between actual power delivered and requested power (as defined by the current)
  • Determining the real maximum current of the tube
  • Looking at the pre-ionization zone between around 9-20% power and looking at the actual beam in the tube
  • Review of results
  • Looking forward to the next project – a keyboard protector

My thanks go out to Tom at Thinklaser for giving permission to embed these videos on this site. If you are looking for a new laser machine from a quality supplier, then I would suggest you check out their website: www.thinklaser.com.

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Video Resource Files

doHicky-timer.dxf

doHicky-timer.rld

doHicky-timer.rd

DoHichy Calibration Data table

Dohicky Calibration Chart

Tube Characterisation Data

There are no resource files associated with this video.

External Resource Links

www.thinklaser.com

www.rdworkslab.com

What can a laser cutter cut?

There are no more external resource links associated with this video.

Transcript for Measuring Laser Tube Power

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00:15

well welcome to another light blade

00:18

Learning Lab

00:20

in the last session we changed the tube

00:23

in this machine

00:26

and today we’re going to look at ways of

00:30

characterizing that – over time the tube

00:34

is going to gradually decrease in power

00:36

no it was just drop off the edge of a

00:39

clip unless something seriously

00:41

mechanical goes wrong with the tube and

00:43

that’s very very great most of the time

00:46

what will happen is this tube will run

00:47

perfectly okay for months and months and

00:50

months you may determine that you’ve got

00:52

a little bit less power losing power can

00:56

happen at several places throughout this

00:58

system as we’ve talked about before in

01:00

earlier sessions you have three mirrors

01:02

you have a lens and each one of those

01:06

stages typically you would expect to

01:09

lose maybe three percent power now I’m

01:12

not going to get too anal about that

01:13

number because you know three percent is

01:15

just an average if it gets to five

01:17

percent loss ie you’ve got one two three

01:20

four features four five is twenty

01:24

percent that’s one-fifth of your power

01:26

that you’re losing before you get down

01:28

to your work surface and that would be

01:31

very noticeable if you’re used to

01:33

cutting for instance something at ten

01:35

millimeters a second and then all of a

01:38

sudden you find that if they’ll you cut

01:39

through what do you do well your first

01:43

reaction might be well I’ll put more

01:45

power into the system well if you’re

01:48

already burning it be a maximum

01:49

allowable current you can’t do that

01:52

because you saw short in your tube life

01:54

the only solution is to do one of two

01:57

things

01:57

short-term if you’re in a hurry you

Transcript for Measuring Laser Tube Power (Cont…)

02:00

might decide to turn the speeds down so

02:03

that you get good cutting it

02:05

because that’s a temporary fix what you

02:07

really got to do is to go into your

02:09

machine and find out whether you’ve got

02:11

lots of power on your tube or whether

02:14

you’ve got lots of power because of one

02:16

two or three mirrors being dirty or

02:19

maybe you need to clean or you might

02:22

need a new things you’ve got to find out

02:26

where the problem is and that’s

02:29

partially what we’re going to be talking

02:31

about today

02:33

now characterizing the tube basically

02:36

means what we’re going to do is we’re

02:38

going to try and establish the

02:40

relationship between the tower that

02:43

we’re putting on the keyboard the

02:45

milliamps that we can see on the a meet

02:48

up that’s on your machine and the

02:50

physical lots of power that are coming

02:53

out of the tube so we’ve got for a

02:56

draught that relates all those three

02:58

things together and in addition to that

03:00

there’s a very interesting section of

03:03

the tube performance called a pre

03:07

ionization zone which is where the tube

03:10

is just starting to fire and it can’t

03:14

make up its mind whether to run on a

03:16

full beam or a partially there the

03:19

strange startup characteristic that

03:22

varies between tubes and it varies

03:25

between power supplies and machines but

03:27

it is important that you understand

03:29

where that zone is on your machine

03:31

because it can be very useful and it can

03:34

be very annoying at times when I first

03:37

got my original China blue laser machine

03:41

it had a very very poor quality tube in

03:44

it and I struggled for a long time to

03:47

prove that the power in that tube was

03:51

not as claimed I spent a long time

03:53

trying to devise ways of measuring power

03:56

through calorimetry techniques and in

03:59

the end I had to give up and I searched

Transcript for Measuring Laser Tube Power (Cont…)

04:02

the internet for a reasonably priced

04:05

power meter that enabled me to measure

04:08

the power coming out with my – on the

04:10

surface of it it looks like a meat

04:12

thermometer with a blob of aluminium

04:15

Harold itíd to the stem and in essence

04:18

that’s what it is but it’s a very

04:21

expensive piece of equipment relatively

04:24

speaking for a hobby use this cost me

04:27

around about three hundred and fifty

04:30

dollars because I couldn’t buy anything

04:32

like this in the UK I had to get it

04:34

specially imported from the states this

04:36

is a company called Mackin Instruments

04:38

and this is the cheap end of their

04:41

product range think Laser don’t use

04:44

anything like this because they’ve got

04:45

very sophisticated equipment that’s

04:47

capable of operating at different

04:49

frequencies different wavelengths of

04:51

light and their kit is in the region of

04:54

about maybe two and a half to three

04:56

thousand pounds that’s not the sort of

04:58

price that you can afford to spend to

05:00

check the performance of your machine

05:01

here you don’t need a piece of

05:03

sophisticated digital measuring

05:05

equipment to measure the power on your

05:07

machine what you really want to

05:08

something which is maybe not quite as

05:10

accurate something maybe something like

05:13

this that will do the job but you know

05:17

it doesn’t cost a fortune oh I invested

05:19

in this and it did a superb job of

05:22

telling me everything I wanted to know

05:23

about the machine it’s still out of the

05:26

price range of many people and so

05:28

consequently on this site here Rd Works

05:32

Learning Lab 53 I’ve done an analysis of

05:36

how this works and I’ve designed my own

05:39

simple version cheap version that will

05:42

do the same task as this this reads out

05:46

what’s directly this does not read out

05:50

what’s directly it’s it’s a compromise

05:52

we get the same answer but we have to

05:55

put a little bit more effort in what

05:57

we’re going to be doing today is to use

05:58

this piece of kit and I’ll show you that

Transcript for Measuring Laser Tube Power (Cont…)

06:00

relatively speaking it is still very

06:02

simple to use

06:03

now whether we use this piece of

06:05

equipment or this piece of equipment

06:08

they’re both based on in essence heating

06:13

up this block of anodized aluminium

06:17

and to do that you have to put the power

06:20

in for a fixed period of time the fixed

06:24

period of time is something that we’ve

06:27

generated a program to do and all the

06:29

information about that program is

06:31

available on the 5th place website think

06:36

Lisa can supply this piece of equipment

06:38

so before we to start any testing we

06:42

need to put a test program into the

06:45

machine and that test program let’s find

06:47

it there we go it’s actually called

06:51

doohickey and here’s what it looks like

06:54

now we’ve got a starting point here

06:58

which is the blue and the first thing

07:02

that happens is at a speed of one

07:05

millimeter a second we Travis outwards

07:09

by about 10 millimeters so that means

07:13

it’s going to take 10 seconds to run up

07:16

the blue line and at the end of that

07:19

period 10 seconds we start running

07:22

around this red spiral getting closer

07:25

and closer towards the center now that

07:28

red spiral has been very carefully

07:31

designed its length and its speed has

07:34

been set to 14 millimeters a second

07:37

because that will give us the correct

07:40

amount of exposure time for the

07:43

calibration to work

07:47

so we don’t have to panic when we press

07:50

the start button because we get nothing

07:53

happening over the blue line for the

07:55

first 10 seconds we’ve got the power set

07:57

to 1 percent and 1 percent will not work

Transcript for Measuring Laser Tube Power (Cont…)

08:01

the laser so we have a little bit of

08:04

relaxation time from the time we press

08:07

the button to the time we go around to

08:08

the back of the Machine and get ready

08:11

for the test to start now it really

08:16

doesn’t matter where you place the laser

08:17

head but somewhere into the middle of

08:19

the Machine is a good idea

08:21

and then press your origin button

08:23

whenever I’m firing the laser down into

08:26

the Machine I always like to put a

08:29

little tub of water in the bottom of the

08:32

machine because let me just show you so

08:37

after 10 seconds the power switch is on

08:39

and we’ve got it set to maximum power at

08:42

the moment which is 67%

08:46

and there we go you can see the effect

08:48

of the energy going into the surface

08:51

it’s actually causing the water to boil

08:55

and produce steam

08:57

[Music]

09:07

so that energy would normally be

09:09

partially reflected off this surface and

09:11

that’s a risk I don’t want to take so I

09:14

always fire my beam into a little puddle

09:17

of water heating affecting the water is

09:19

virtually nil because there’s relatively

09:21

speaking such a small amount of power

09:23

there but it is just causing the surface

09:27

of the water to heat up so much that

09:29

evaporates it only got to go maybe half

09:33

a millimeter into the surface of the

09:34

water and it has no effect at all okay

09:37

so that’s the program set up and all the

09:39

safety precautions so we need to close

09:41

the lid now what we have got to do now

09:44

is to keep changing the parameters in

09:48

this program and to do that we shall

09:51

press enter enter and what we’re trying

09:55

to do is we’re going to change the

09:56

parameter that the power parameter on

09:59

the red layer so we need to use the Z

Transcript for Measuring Laser Tube Power (Cont…)

10:03

Joo button where we use the arrow button

10:05

initially because the blue there tells

10:07

us that we’ve got this selected so we

10:10

can select the red layer so we come down

10:12

to power by using the Z due button now

10:15

the first thing that I’m going to do is

10:16

to wind the power on this machine and

10:19

this is minimum power I’m going to put

10:21

it to 90% and then we’re going to step

10:24

down to maximum power and we’re going to

10:27

set that to 90% as well the speed set at

10:31

40 millimeters a second and the power

10:34

set to 90% so that’s our first test that

10:38

we’re going to run now the fact that I’m

10:40

running this at 90 percent means that I

10:42

shall probably be driving this milliamp

10:44

meter more than the recommended maximum

10:48

for the tube I should be over driving

10:50

the tube but don’t let it worry you for

10:53

such a short period of time this is not

10:56

going to damage your tube there’s

10:58

warnings are all sorts of other covers

11:00

but there isn’t a particular warning on

11:02

here

11:03

but believe me it’s just as dangerous to

11:06

take this cover off and use the machine

11:08

as it is to undo any of the other panels

11:11

so we’re basically going to beat the

11:15

safety system here so I need to

11:17

reinforce that you must at all stages be

11:21

very very careful and I will try and

11:23

explain to you how you can do this

11:25

safely okay now I’ve got to open the lid

11:29

to show you what’s going on but you will

11:34

have to operate this with the lid closed

11:36

because of the safety systems preventing

11:38

the tube from firing when we open the

11:40

lid now one of the most important things

11:43

that we’re going to need is a bucket of

11:46

water now this water here is has been

11:50

sitting here for seven days and it has

11:53

assumed an ambient room temperature now

11:57

that’s most important we need a

11:59

consistent starting point when we do

Transcript for Measuring Laser Tube Power (Cont…)

12:02

every one of these tests using this

12:04

equipment or even using this equipment

12:08

because they’re based on a temperature

12:12

change away from room temperature okay

12:16

before we start let’s take a quick

12:18

close-up look at this little instrument

12:20

now the top button here is an on/off

12:24

button and that’s no registering the

12:27

temperature that it’s seeing in the

12:30

black doohickey probe beside my hand

12:33

there

12:35

now the button at the top right hand

12:37

side we can choose degrees C degrees F

12:42

or degrees absolutely we’re going to use

12:45

the degrees C scale which is the default

12:47

scale and we can see that just up at

12:49

this top right hand corner here now

12:51

underneath that we’ve got a whole bunch

12:54

which we don’t need to use but the one

12:57

button that we do need to use is this

12:58

button that I’ve marked black and I’ve

13:01

marked it black because it’s very

13:03

confusing and sometimes you might press

13:06

the wrong button

13:07

because you read that the number is

13:10

underneath the button and so you think

13:12

we’ll max mean average let’s press this

13:14

button because that’s what you can seed

13:15

and effect that’s wrong so if I’ve

13:18

marked that button so we never make a

13:20

mistake the reason why I’ve marked that

13:22

button is because if we press it once we

13:25

get max if we press it a second time we

13:29

get min and if we press it a third time

13:31

we get average when we turn the power

13:34

off it resets everything and we turn the

13:39

power on and we get nothing so the first

13:41

thing that we must do is press the black

13:43

button which says max and what that will

13:47

now do is that will register the maximum

13:49

temperature that ever the seen by the

13:52

doohickey probe and that’s an important

13:55

number that we need to capture now when

13:59

it comes to doing the test itself what

Transcript for Measuring Laser Tube Power (Cont…)

14:02

we’re going to do is we’re going to put

14:03

this little lollipop in front of the

14:06

laser beam coming out of the tube just

14:09

here

14:10

but I’m going to move it around just so

14:13

we don’t get any intent peeking on one

14:15

particular spot so under all

14:17

circumstances make sure that you keep

14:20

your hand resting on this bottom corner

14:21

of the enclosure because that will

14:25

ensure that your hand is never in the

14:27

later theme park itself even though

14:30

you’ve got the heart covered over by

14:32

this if for any reason you jump somebody

14:35

sneezes somebody finds you you don’t

14:38

want to do this and finish with the beam

14:41

on your fingers so safety safety safety

14:44

please look and think what’s going to

14:48

happen okay

14:50

safety let you out of the way let’s get

14:52

on to do the job okay so the first thing

14:56

that we’re going to do is we’re going to

14:58

put the probe into the water bar and

15:01

we’re going to swirl it around turn the

15:05

meter off and back on and that will be a

15:09

live temperature the doohickey is misery

15:15

and when it’s stable and at the moment

15:20

is 23 22.9 get up take point two point

15:27

three in the fury see it’s stable and

15:30

it’s at around about twenty two point

15:32

eight I’m going to turn that meter off

15:37

now and I’m going to walk down to the

15:40

front of the machine and I’m going to

15:42

press the start of the program back okay

15:46

now we’re in our immediate panic we can

15:50

turn the meter on and we press the max

15:52

power 23.1 is the number that I want to

15:57

sit here we go we can see the belt movie

Transcript for Measuring Laser Tube Power (Cont…)

16:03

and that tells us that we are got

16:07

something going on

16:12

23.1 was our starting temperature

16:14

remember

16:20

there we go it’s finished so we need a

16:24

little piece of paper to write this

16:26

information down 90% power 23.1 was the

16:34

starting temperature now we can take a

16:36

look on the meter and we can see that it

16:38

says fifty five point four okay that’s

16:42

two pieces of vital information that we

16:44

need the starting temperature rule and

16:46

the finishing temperature now there’s

16:48

one other thing that we need to go and

16:50

look at now because we don’t have six

16:52

pairs of hands or five pairs of eyes if

16:56

you’ve got some help then maybe somebody

16:58

else can read this for you but if you

16:59

haven’t got help then you’ll need to run

17:02

the program again as I will do here and

17:05

of course it’s safe to run it because

17:07

we’re running into a bucket of water and

17:10

what we shall need to do now is to make

17:12

a note of the current once the program

17:17

is running 24.5 probably okay so that’s

17:21

basically what I’m recording power 90%

17:24

start temperature twenty three point one

17:27

maximum temperature that we established

17:29

fifty five point four and our milliamps

17:32

tricky four point five those are the

17:34

four numbers that we need to record so

17:37

we no need to come back to here and we

17:40

need to press the enter button and enter

17:43

again press the arrow key to get across

17:46

to the red and then the zi jue to get

17:48

down to the minimum power button and at

17:51

that point we’re going to change the

17:52

minimum power to eighty ten percent

17:56

steps is good enough to get a reasonably

17:59

good calibration graph you could run it

Transcript for Measuring Laser Tube Power (Cont…)

18:01

at five percent two percent it really

18:04

depends on how fussy you are so Max and

18:08

min are both set to 80 percent ater

18:10

enter and now we go through the same

18:14

procedure again and we do that for 70 60

18:17

50 40 30 all the way down to 10%

18:22

now at 10% things start to change just a

18:26

little bit I’ll carry on and do all the

18:28

test results down to 10% and then we’ll

18:31

come back and have a look a bit closer

18:33

at what’s going on at 10% the last set

18:35

of data for 90% it’s slightly different

18:38

to the one that you’ve seen on the film

18:40

because I went away for a meal and when

18:42

I came back I decided that I’d start the

18:44

whole thing again because I do like to

18:46

make sure that I get consistent set of

18:48

results with no time lag in between so

18:52

this set of data is a consistent set of

18:54

data so there was a temperature rise

18:56

injected into that little doohickey by

18:59

virtue of the amount of power that was

19:01

being fired at it he made a thief that

19:02

was being fired at it because what we’re

19:05

going to do we’re going to put the final

19:06

temperature into a calculation 57.5 and

19:13

then we’re going to subtract the start

19:17

temperature from it which was twenty

19:19

four point four

19:24

and that gives us an answer of thirty

19:27

three point one now that’s thirty three

19:30

point one degrees C now for the speed

19:34

that I’m running this at there is a

19:37

multiplication factor of two so if I put

19:41

times two into there the answer that

19:44

comes out at sixty six point two now

19:47

that’s sixty six point two Watts there

19:51

is a calibration chart that you can work

19:54

to but to be honest if you’re running at

19:57

fourteen millimetres a second then just

Transcript for Measuring Laser Tube Power (Cont…)

20:00

taking the difference and multiplying it

20:02

by two gives you the Watts it’s a nice

20:05

simple for most mental calculation this

20:09

doohickey works over three different

20:11

ranges it works up to forty forty Watts

20:14

up to 80 watts and up to up to one

20:17

hundred and sixty watts none of this

20:20

stuff is very complex maths if you like

20:22

you have to be a little bit

20:23

inconvenienced but then the doohickey is

20:25

a very cheap way of measuring the power

20:28

and to be honest you don’t have to worry

20:30

about these calculations because I’ve

20:32

done a spreadsheet here which subtracts

20:35

these two numbers from each other and

20:37

produces a temperature increase and that

20:39

temperature increase is multiplied by a

20:41

factor of two here to give you the power

20:44

output

20:46

and then we also at the same time

20:48

checked that percentage power and the

20:52

watts against the current that was

20:55

showing on the current meter and so

20:57

there we are we’ve drawn this

20:59

characterization for the tube now this

21:03

blue line here is power watts and this

21:08

red line here is current milliamps now

21:12

technically this machine does not

21:16

deliver power when you put percent power

21:20

into your program what you’re really

21:23

doing is putting in a percentage of five

21:27

volts DC and that five volts DC is what

21:32

is used a percentage of that five volt

21:35

DC is what is used to control the

21:37

current that is allowed to flow through

21:40

the tube that current should therefore

21:43

be linear and give or take a little bit

21:46

of my estimation and the accuracy of the

21:50

milliamp meter as you can see it is it

21:52

is pretty well approximately linear so

21:54

the power that comes out of this tube is

21:56

not directly related to the current that

22:00

you’re using to drive the tube with this

Transcript for Measuring Laser Tube Power (Cont…)

22:02

is not a linear relationship people get

22:05

rather confused and they think that I’ve

22:06

got the 60 watt – therefore if I put in

22:10

50% power I’m going to get 30 watts out

22:13

well look 50 percent power is in fact

22:16

going to generate something like about

22:18

55 watts

22:19

that’s not halfway up the scale and you

22:22

can see why it’s not halfway up the

22:24

scale because this is a nonlinear

22:26

characteristic now it’s a very strange

22:28

characteristic because as I said we work

22:31

down to 10% power and I hoped that I

22:34

would get down to 10% power and you can

22:36

see that I’ve just about got there

22:38

because probably if I drive this down

22:41

much lower I might be able to get down

22:44

to possibly nine and a half percent and

22:48

I should get no power out of this device

22:50

at all so as you can see it is quite a

22:53

rapid increase to start with and then it

22:55

drops off into a much shallower curve

22:57

the other vital piece of information

22:59

that we must get from this graph now is

23:02

what is the maximum current that we can

23:06

allow to drive this machine at you would

23:08

normally expect to be able to drive a 60

23:10

watt tube to a maximum drive current of

23:13

22 milliamps so let’s just find where 22

23:18

milliamps is is here okay now 22

23:23

milliamps

23:29

if we draw them up there like that

23:33

we get two bits of information one is

23:38

75 76 77 maybe 78 percent

23:52

and the other piece of information is

23:54

that at 78% we’re getting

Transcript for Measuring Laser Tube Power (Cont…)

24:02

sixty-two for

24:10

so this to a 60-watt tube is delivering

24:13

64 watts at 78% power

24:19

and provided we don’t program seven more

24:21

than 78% into this machine then we shall

24:25

not be exceeding the 22 milliamp limit

24:28

for a 60-watt – now those are crucial

24:32

pieces of information that you need to

24:35

know and you would not be able to get

24:36

this information without the use of a

24:39

power meter so now we’re very

24:41

confidently characterized this machine

24:44

now there’s one other piece of

24:46

information that we might not get on

24:49

this graph here and we may draw a

24:51

completely separate graph to look at

24:53

what’s going on in this little area down

24:56

here and this is something called the

24:58

pre ionization zone now that normally

25:02

happens somewhere in the region of maybe

25:04

four to six milliamps we’re going to run

25:07

and we’re going to check what’s going on

25:09

on this machine between nine and

25:11

thirteen and fourteen percent now we’re

25:14

going to run the same test again but

25:17

what we’re going to do this time is to

25:19

take a look at the tube itself we’re not

25:22

going to worry about the power because

25:23

we can see what the power is doing now

25:25

to do this I’m going to turn the lights

25:27

out so that you can see what’s happening

25:29

in the tube and here we go oh we’ve got

25:31

something happening there at 9% power

25:33

you can just about see right at this end

25:36

here a very faint glow so we have got

25:40

some power there at 9%

25:43

and yes we’ve just got a small amount of

25:46

color at this end of the tube here now

25:49

if we start looking along the tube we

25:52

should find that we have got no no real

25:56

discernible beam along the middle of the

25:58

tube just at the end here

Transcript for Measuring Laser Tube Power (Cont…)

26:02

so here’s what 11% looks like and now

26:05

we’re starting to get more along the

26:09

tube but it still isn’t all the way

26:11

along the tube it’s only part way along

26:13

the tube up to this label here look

26:19

and you can see how the beam is jumping

26:21

around at the end there okay now this is

26:24

12% you can see the beam is getting

26:28

stronger and it’s also getting longer

26:30

look it’s gone beyond that label now

26:32

it’s nearly making it all the way along

26:36

the tube

26:41

and again look how jumpy the beam is at

26:43

the front here we know going up to 13%

26:46

which is where we think the beam is

26:49

nearly going to make it along the tube

26:54

he’s still jumping around a lot at this

26:56

end as you can see and there is just the

27:01

faintest glow that goes more or less all

27:04

the way along the tube look

27:07

but it’s still not what I would call a

27:10

continuous beam because look we’ve still

27:12

got it jumping around at the end there

27:14

and that jumping around is a

27:16

characteristic of this pre ionization

27:19

now this is 14% yes you can see we’ve

27:22

still got a jumpy beam there and we’re

27:26

getting a much stronger beam and it’s

27:28

more or less well it is all the way

27:31

along the tube now but

27:35

we still got this jumping characteristic

27:37

there on that cathode well we’re now at

27:41

15% it’s still a bit jumpy there and

27:47

we’re getting quite a good solid beam

27:48

all the way along but you can see how

27:52

it’s the whole thing is a little bit

27:53

jittery

27:56

okay now this is 16% and look we’re

Transcript for Measuring Laser Tube Power (Cont…)

28:00

switching pretty good now we’ve got a

28:05

steady beam can you see that it’s

28:07

flipping but it’s basically a nice

28:10

steady beam it’s no longer jittery and

28:17

just to verify that here we are driving

28:19

it twenty percent and as you can see

28:22

we’ve got a nice steady pink beam all

28:25

the way along here and a little bit of

28:28

movement on the cathode but it’s not

28:30

jittery movement it is just like a

28:33

little teeny-weeny pulsing movement now

28:36

we should be able to go backwards and we

28:38

should be able to find that at 15% that

28:41

different characteristic fifteen percent

28:44

was the upper threshold of this pre

28:47

ionization zone

28:50

now you can see that jittery behavior

28:53

[Music]

28:55

on the cathode and if you listen

28:57

carefully

29:00

you may be able to hear it hissing now

29:02

look it’s gone to steady mode now so

29:05

it’s on a on a cusp of being stable and

29:09

unstable at 15% so therefore I think the

29:13

limit probably is 14% so let’s just drop

29:16

back to 14% nothing here we are at 14%

29:21

as you can see it’s it is a week looking

29:23

theme it isn’t a lovely solid pink beam

29:26

it’s a bit with speech

29:30

but it’s just jumping around

29:31

characteristic that is the thing that

29:34

gives away this pre ionization okay so

29:40

there we are anywhere between nine and

29:42

fourteen percent for this particular

29:45

tube is the pre ionization zone

29:48

okay so I’ve marked the pre ionization

29:50

zone on this graph now as you can see is

29:53

between nine and fourteen percent and at

29:55

fourteen percent it means that we’re

29:57

actually somewhere in the region of

29:59

around about 12 watts so we don’t get

Transcript for Measuring Laser Tube Power (Cont…)

30:03

steady power out of this tube until

30:05

twelve watts now if you really want to

30:09

be exploratory and fussy you can check

30:12

one percent points through this range

30:16

and out the other side but you’ll find

30:19

that it really doesn’t do anything

30:21

strange it tends to just follow this

30:24

curve so there’s nothing strange about

30:26

the power output curve in relation to

30:29

the percentage power or basically in

30:33

relation to this steady rise in drive

30:35

current but this characteristic here has

30:40

got some interesting and strange

30:41

properties and that’s something which

30:43

should discuss in a future session ok

30:46

well armed with this piece of paper now

30:49

we’ve got a great deal of confidence

30:51

about how we’re going to be able to use

30:53

this machine now remember back to when

30:56

we were doing parameters and I said the

30:58

real thing that you need to do is to

31:00

measure power and instead of specifying

31:03

percentages in your descriptions of

31:07

parameters you need to put in real power

31:11

but this is how you’ll be able to do it

31:13

you’ll be able to refer to this brass

31:15

nickel

31:16

and if you have parameters in there for

31:19

a previous to you and my previous to you

31:21

ran at 67% this one runs at 78% so it’s

31:26

a completely different graph but it’s

31:28

not going to be a difficult job for me

31:30

to run through my parameters and correct

31:32

those parameters for this machine so

31:34

hope you can see know the advantage of

31:37

investing in a a small piece of kit like

31:39

this and it doesn’t matter which piece

31:41

of kit you use because the result would

31:43

be the same this is a very affordable

31:44

tool it takes a little bit more

31:47

inconvenient but it does the job that

31:50

you want to do and of course the other

31:52

great thing about this is which will now

31:55

be able to go around each mirror and

31:58

we’ll be able to check the power in the

Transcript for Measuring Laser Tube Power (Cont…)

32:01

power out of each mirror so we can

32:03

determine that power losses across the

32:05

mirror and the power losses across our

32:07

in now the problem is when you get to my

32:09

age you find that you’ve only got one or

32:12

maybe 1.5 gray cells left and you really

32:16

don’t want to overload them so just as a

32:18

little reminder I’ll put that on there

32:23

so that I don’t forget that’s the

32:26

maximum programmable power that I can

32:29

put into this machine that without

32:30

overdriving the tube 78% that’s the

32:34

magic number I hope you’ve got some very

32:36

quick from today’s session and I was

32:39

just going to say cheerio to you but

32:45

as I stand here I’m looking over at that

32:49

keyboard where I’m using these keys

32:52

I’m very quickly going to start wearing

32:55

off the letters I think I’ve worn

32:59

through these two sheets of plastic on

33:00

quite a few of these and we’re talking

33:03

about only six or eight months well for

33:05

probably about eight months I’ve had

33:06

this machine so I think on the next

33:09

session what we gain has to do is to

33:12

make something that I’ve made for my

33:14

other keyboard and that’s a little clear

33:17

protective screen so you can see that I

33:21

push on the buttons through this

33:25

flexible membrane and the buttons

33:27

themselves do not get damaged

33:33

the problem with this is on this machine

33:36

I haven’t got as much room and as you

33:39

can see this is going to get in the way

33:42

of closing the door so I’m going to have

33:45

to change the design of this in some way

33:47

so that it can be used on this machine

33:51

and we’ve worked out how I’m going to do

33:53

that yet but maybe that’s a project for

33:56

next time

33:58

well this time I’ll try not to get

Transcript for Measuring Laser Tube Power (Cont…)

34:00

distracted so for real this time

34:03

curio and I’ll see you in the next

34:05

session

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