Help on changing the sensitivity of a volt meter

Discussion in 'The Projects Forum' started by wineguy26, Apr 26, 2014.

  1. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    Hi,
    My name is Mark and I've joined seeking some assistance with a volt meter (aka pH meter) in my lab. I'm working with a vintage Orion analog pH meter. For the purpose of my tests, I want to change the measurement range of the meter from -400mV >+400mV to -100mV >+100mV.

    It's a pretty basic instrument with a couple of capacitors and resistors. I do have a wiring schematic for the meter. I'm attaching some pics for reference. Any input appreciated. Thank you.
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    You really have to understand all of the circuit to modify it without unbalancing the temperature compensation and offset adjustments. I'd say, measure the resistance of the meter (800 ohms?) and install that resistance in the meter position. Then add an op-amp with a gain of 4 and have that drive the meter. That keeps your fingers out of the gears.

    If I had enough brains left to understand the whole circuit, I could redesign it for an easier fix, but it's been over 40 years since I did Ph meters.

    Now, watch some hot shot kid walk in and embarrass me.:D
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,978
    744
    The gain of the op amp is set by the resistance of R10 + Meter coil value, you could start by halving R10 to say 6K2, or replace it with a 22K preset, and try from there.
     
  4. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    If it helps, the temperature compensation is not needed. I'm only using it as a millivolt meter with a platinum electrode to sense electrical potential in samples.
     
  5. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    Thanks, but I'm lost. If you can tell me to go buy Radioshack part #xyz and splice it into xyz wire, I can do that! I did buy an extra Orion 301 to use as my genie pig. I will open it up and snap some pics tomorrow. Thanks.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,783
    1,103
    You may be able to modify the meter movement itself, by adjusting an internal shunt, to change its display range.
     
  7. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Would it not be better a get a decent low cost digital multimeter?
     
  8. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    How do I do this? A chemist told me that I need to adjust the meter in such a way that a measurement in the sample of 50-100mV corresponds to the meter needle moving to the 300-400 mV range on the display.

    In other words, if my electrode measures 50mv in a sample, I want to magnify the sensitivity of the meter such that the needle moves to 300mV.

    Please let me know if anything I'm saying makes sense. I lack the technical skills to perhaps explain this properly.
     
  9. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    No, it has to be an analog.
     
  10. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    No. Ph electrodes like their impedance in the range of dozens of megohms. If it was that simple, the instrument wouldn't be using the 2N5909.
     
  11. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    OK. I'm getting to some questions now.

    100 mv input shall display 400 mv on the meter. Right?
    Is the meter labeled as 500 mv full scale deflection?

    Do you want only the 50 mv to 100 mv range?
    Is it only in the positive direction?
    Would this be OK if it just did: 100 mv input displays 400 mv on the meter and 50 mv input displays 200 mv on the meter?
    (It's much easier to do a linear scale instead of an offset, especially if you want both positive and negative polarities.)

    I've cleaned up the drawing a bit in case somebody else can see a way to do this,
    Here:
     
  12. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Ah I was not quite clear. I meant inserting the digital multimeter after the input stage at the same point as the meter use. The benfit of this is not to meddle to much with the original layout. It should not do much harm I think to put some extra banana plug sockets on the instrument front panel for this
     
    Last edited: Apr 28, 2014
    #12 likes this.
  13. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    Some DMMs, even low cost ones, do offer very high input impedance on the 200mV range. In these, the input is taken straight into the display ADC (with Gig Ohms Zin) through an LPF. The 10M is normally on the higher ranges due to the dividers.
     
  14. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,978
    744


    Digikey preset resistor, http://www.digikey.com/product-detail/en/1630480-9/A105820-ND/2055809
     
  15. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    Oh...sure...that makes sense now. :)

    ps, I've never met a genie pig, but they sound interesting. Little furry thing that whistles and can make the refrigerator door open just by saying, "Abacadabra"?
     
    Last edited: Apr 28, 2014
  16. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    Yes, "genie pig", lol, dumb auto-correct. I think I need to explain what I'm doing on a chemistry basis to better tie in with the electronics end. I'm using the meter with a platinum ORP electrode to sense an oxidation-reduction reaction, and in this case it's measuring sulphur dioxide in wine. An acidified wine sample is titrated with iodine, which reduces (consumes) the SO2. When all the SO2 has been consumed, the appearance of free iodine causes an immediate and large spike in potential, usually 30-100mV, though the theoretical endpoint should be ~56mV. The wine sample immersed into the electrode measures ~300 mV. As the titration begins, the mV goes down, usually to ~280mV. Then while titrating, the mV slowly rise (.5 mV at a time), until there is a sudden surge in mV, which causes a needle inflection. That's the endpoint.

    In a lab that I worked at long ago, they had the Orion 301. The point of an analog meter is that you are only looking for a needle inflection, as opposed to trying to read an ever changing digital display on a modern pH meter. When we started the titration, we used the calibration knob to set the meter between 0>-60mV (pH 7-8). As the titration started, the needle would rapidly move towards -400mv (many times pegging at -400 mV), then it would slowly move to the left (increasing mV), then all of a sudden the needle would "free fall" usually all the way to +400mV (pH 0).

    So, if I'm right, the needle needs to move from a beginning potential, to a lower potential, then back to a more positive potential. If it was scaled in a manner of say, 0-100mV, the needle should peg full scale right, then full scale left. I'm not sure if a linear scale would work. It should, as I can use the calibration knob to set the start point of the needle where I want it. So how about a 30mV change in the sample causes a 200mV change on the meter, and a 60mV causes a 400mV change.
     
    Last edited: Apr 28, 2014
  17. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    • R10.jpg
      R10.jpg
      File size:
      293.3 KB
      Views:
      35
  18. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    The way I'm using the word, "linear" is not a problem here. I only meant, "Do you want a dead band in the middle?" You don't. You only need the amplification factor increased. Both polarities are relevant.

    Did I get the probe correctly placed on the left side of the drawing? (I know the voltage label is wrong) or is one side of the probe grounded and J2 is a different source? edit: poking around on the internet tells me one side of the ORP probe is grounded. What connects to J2?

    Do you use RP1 as the, "calibration" knob?
    You can dial in an offset with it, and it seems to be quite large compared to your useful range.

    (R10 is brown, red, orange gold)
     
    Last edited: Apr 28, 2014
  19. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,978
    744
    Yes, remove R10 and replace it with the preset, use the middle terminal and one of the end terminals on the preset.
     
  20. wineguy26

    Thread Starter New Member

    Apr 25, 2014
    14
    0
    I don't know how to answer your questions as I'm a complete novice.
     
Loading...