Macbeth densitometer help?

Discussion in 'Black and White' started by juke, Nov 17, 2008.

  1. Hi,
    I recently got a Machbeth TR-944 densitometer.
    I have original operator's manual, but it doesn't tell much about basic usage.
    The things that I wonder are:
    • Should I 'reset' measured value after each measurement by measuring without target (ie. take film strip out and then push measure arm)? Currently I have figured out that it's probably the right way but I am not really sure.
      However, if I take multiple measurements without taking targout out and taking plain reading between each measurements, the result is quite odd.
    • There is probably some fault in my densitometer. The reading does not stay on, it just flashes quicly when I press measure arm down and take measurement. Not a big deal, but it would be nice if measured density value would stay visible until next measurement is done.
      Or is that the way it should work?
    • Otherwise the densitometer works fine, it is calibrated using macbeth's own calibration target and checked againts Stouffer's step wedge.
      Some basic use tips would be nice though :)
     
  2. My only experience is with my old Kodak color densitometer made about 1948.
    Seems to me that as long as you can get a match between the test strip/wedge and
    the negative and get even a brief reading, you should be on the mark.
    Anyone, please correct me if I am wrong.
    Best regards,
    /Clay
     
  3. Wait a minute, just had a thought ! Ha !
    Mine is just a density wedge and a dial that you turn untill the neg and wedge match.
    No electronics that may need calibration. Sorry, had a 'senior moment'.
    Best regards,
    /Clay
     
  4. Usually you are interested in the densities above the film base. Zero the densitometer on an
    unexposed/developed/fixed negative. You can use the end of a roll or the edges if you do not have an unexposed
    frame. Then read the negative densities; they will be densities above film base + development fog. This is asuming
    the denistometer holds the zero and is not drifting. If you read the film base several times and the value is not stable
    (fluctuates more than 1 or 2 units) your other readings will not be valid.

    Paul
     
  5. Jukka, I'm not sure exactly what the 944 model is, but I've used a wide variety of Macbeths over the years. The normal routine is that the machine should first be at operating temperature. Then you would zero each individual color channel, and finally check or reset the higher density test patch (which is labeled as "calibration" patch?).

    Once you have done this, the machine should be stable for hundreds or thousands of readings, with no need to rezero. If reading the same spot multiple times, the readings should be almost exactly the same every time.

    Normally, the reading should stay on the display until your next reading is taken. I wonder if perhaps the switch that triggers the reading is defective or misadjusted. When you press the measurement arm down, you may be able to hear a soft "click"; this would be that switch. If that switch accidentally makes an intermittent contact while not actually reading a sample, the display would probably blank out until a good reading is taken.
     
  6. Bill C, yes the switch could be defective. The readings that I get are fine (tested against various step wedges).<br>
    I have to check whether my densitometer gives stable readings from the same spot. So far I have always taken measured target off (eg. negative). Then I make one measurement without anything between light source and transmission measurement head. After that I make another measurment.
     
  7. Now I think that I understand why I have to take measurement without any target between each measurements. It seems that densitometer will show relative density change to previous reading each time when measurement is done.
    Why is that, I don't know. The manual really does not help :(
    The mode I use is the "Density", other options are "HD-LD" (don't know what this is) and "Offset Null density".
    Here are the pictures of the densitometer:
    [​IMG]
    [​IMG]
     
  8. I've used a 943, which has the same front panel as your photos show. So I can tell you how it SHOULD operate.
    First, regarding the HD-LD: this is from the good old days, when lab techs had to manually draw line graphs to show the process condition. There would be two test patches, called HD (high density) and LD (low density), which the tech would read, then subtract and plot the difference. In HD-Ld mode, the machine would simply prompt for the first reading, then the second reading, and show the difference, saving the trouble of subtracting in one's head. You probably don't have any use for this feature. BTW, I don't think HD-LD works with the "visual response" (that is, with yellow dot forward as in your photos).
    I'm still bothered that your readings just flash, but don't stay on the display. Something is amiss here. I presumed you meant that the display blanked out. However, your photo shows values in the display, even though they are zero. Perhaps you could elaborate on this.
    Here's something you might try, perhaps it will give some clues to the problem. Pres the "offset null" button, it should illuminate, and the display shows "1". Then, while holding the probe down, press "zero", you should get the reading "0.00". At this point, you should be able to take normal readings of anything and get the correct result (which stays on the display until the next reading). You should NOT have to rezero between readings. Does this work? If not, what result do you get?
     
  9. Yes, the zero value will stay at the display. When I press the probe lever down, actual measurement "flashes" quickly and then zero comes back. However there is enough time to see reading, and occasionally reading will stay. This ofcourse gives the idea that one problem is probably defective in the trigger.
    Then the problem with relative readings. I did what you suggested. Offset Null showed "1". Then I zeroed all channels while keeping lever down.
    Now it should be zeroed and ready for measurements. OK, the first reading is fine. It is absolute density, however the next reading is difference to the that first reading.
    For eg. If first reading (absolute value) was +0,14 and absolute value of second measurement is +0,17 then densitometer would show value +0,03.
    If second measurement would give less density, then value is negative. If in the example above, absolute reading for second measurement would be +0,5 then densitometer would show value -0,09.
    There is always either minus or plus sign. But according to the operator's manual, reading should be absolute, not relative. The relative readings should be get only when using memory and comparing values with ones stored to the memory.
    Really weird.
     
  10. hmmm...
    At first, I thought that perhaps something was wrong with the selected (density) mode. However, you have now selected a completely different mode (null) and the same bad behavior follows. So the fault continues to exist in more than one mode.
    Your fuller description now leads me to a different idea: it seems that perhaps the problem is in the "zero" button, such that it remains activated (all or most of the time). I tried this on a 943 machine. I simply held down the "zero" button (continuously), and the instrument responds exactly as you have described.
    Here is another test to try: turn the machine off, then back on so that it is no longer zeroed. It should display the odd characters which show that it has not been zeroed (I forget exactly what they are). Then depress the probe and observe the display. In a proper machine, it will continue to display the odd characters, and it will continue to do until you manually press the "zero" button. Whereupon the display should finally change to "0.00".
    If you find that simply depressing the probe causes your machine immediately displays "0.00", this suggests that the "zero" button is accidentally being operated. If so, I would suspect that you may have defective "Zero" switch, or perhaps a broken or shorted wire.
    Hopefully this will identify the problem!
     
  11. That it is!
    I am really sure that "zero" button's trigger is jammed or broken, so that it behaves like button is always pressed.
    I tested by powering densitometer with "zero" button pressed, it behaved exactly like when I did same but without "zero" button.
    Have to open and check the trigger and wires of the button.
    Thank you really much! :)
     
  12. You're welcome, glad to help!
     

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