Problem with pressure transducer

Discussion in 'General Electronics Chat' started by Rokeeeez, Feb 8, 2018.

  1. Rokeeeez

    Thread Starter New Member

    Feb 8, 2018
    19
    0
    Hello, my problem is related to pressure transducer. I have a pressure transducer Data Instruments (now Honeywell) AB model 25 psi. Datasheet is here: https://stevenengineering.com/tech_support/PDFs/31DTMAIN.pdf . Problem is, that when I power this transducer from usb connector, transducer didn't work correctly. When there is no pressure, I get the 2.5V DC voltage in my multimeter, but according to datasheet Zero Output is 0 + 5 mV. Also, when I pressure transducer there is no change, anyway 2.5V from transducer. In my opinion power source is good, because voltage between Excitation+ and Excitation- is 5V. So, maybe anything of you know what problem it can be?
     
  2. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,193
    2,024
    You are absolutely sure you have the pin out correct.
    Pin Wire Color Code Code Function
    A Red + Excitation
    B Green + Signal
    C White - Signal
    D Black - Excitation

    The only time I have seen similar problems is when the Excitation and Sig Out were not connected correctly, that or a defective sensor. The fact that you are seeing 2.5 volts is peculiar in that 2.5 volts is the Excitation / 2 and sort of points to incorrect wiring of the bridge.

    Ron
     
  3. bwilliams60

    Senior Member

    Nov 18, 2012
    1,083
    193
    Is it possible your power source does not provide enough amperage to power up the transducer?
     
  4. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,193
    2,024
    I also gave that some thought but from the data sheet:
    Input Impedance 150 ± 50 Ω
    Output Impedance 115 ± 25 Ω

    Not much current as you don't want to self hear the bridge and a USB II should have no problem supporting the bridge. That was my thinking anyway.

    Ron
     
    bwilliams60 likes this.
  5. Rokeeeez

    Thread Starter New Member

    Feb 8, 2018
    19
    0
    now i check the voltages, and the voltage between two data pins (green and white) is 0, when there is no pressure. But in my case, I need to measure voltage from pressures 0-200mmHg (blood pressure monitors pump - 0-4psi), my transducer is absolute pressure and his pressure range is (0-25 psi, output 0-100mV). So, I think output voltage for my pressures could be aproximately 0-20mV. Maybe, there are so options to perform more precise measurements with this transducer?
    [​IMG]
     
  6. Rokeeeez

    Thread Starter New Member

    Feb 8, 2018
    19
    0
    So, now I check again, and I see that voltage between data signals is ~65mV, in datahsheet i saw, that my transducer is absolute pressure. So, when I try to measure pressure from my blood pressure pump, I see voltage 65mV, which equal to 14,7 psi (atmospheric pressure). 25psi - 100mV, 14,7psi ~50mV, i came to conclusion, that transducer works correctly, but now I have a question, how to perform pressure measurement with this transducer? It's there some ways to callibrate them or any onther options? When I apply some pressure on transducer, still I get value of 65mV (atmospheric pressure).
     
  7. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,193
    2,024
    So what you actually have is a transducer with a part number of ABH025PAxxxx where xxxx is the remainder of the part number. This would have gone better had you provided the actual part number rather than a link to a data sheet covering a dozen or more transducers. Blood pressure involves two numbers, normally measured in mm Hg (milli meters Mercury).
    • Systolic blood pressure (the upper number) — indicates how much pressure your blood is exerting against your artery walls when the heart beats.
    • Diastolic blood pressure (the lower number) — indicates how much pressure your blood is exerting against your artery walls while the heart is resting between beats.
    The abbreviation mm Hg means millimeters of mercury. Why mercury? Mercury was used in the first accurate pressure gauges and is still used as the standard unit of measurement for pressure in medicine. There are transducers designed for measuring mm Hg and you would not use an absolute measuring transducer but rather a transducer which measures gauge pressure, here in the US PSIG verse PSIA. Next 14.7 is not always atmosphere. For example 14.7 PSIG = 760.208152 millimeter mercury which is about 29.9295 inches of mercury. That number depends on the barometric pressure which varies on any given day.

    You have the wrong transducer if you want to measure PSIG and not PSIA. That said you can subtract about 14.7 PSI from the PSIA (PSI referenced to atmosphere) from whatever your transducer reads or the mV equivelent from your mV readings. This is normally done in software.

    The best and simple calibration would be to know the barometric pressure (call the weather). Right now my barometric pressure is Barometer 29.83 in. 757.7 mm - Rising Slowly where I am at. Actual calibration would require having a known pressure applied to your transducer from a source about 10X more accurate than the uncertainty of your transducer.

    Ron
     
  8. MisterBill2

    Distinguished Member

    Jan 23, 2018
    3,220
    631
    If the voltage between top and bottom of the bridge is 5 volts then, if the bridge is not damaged, the voltage between each output terminal and the negative supply terminal will be about 2.5 volts. So to read the actual output you will need to read between the two output terminals. That output will have about half tghe supply voltage as a common mode, and the millivolts as the actual output. Normally such a bridge transducer is used with an instrument amplifier as a means to ignore the common mode voltage.
    There is a lot of instrument amplifier information available. Analog devices is one good source.
     
  9. RichardO

    Late Member

    May 4, 2013
    2,273
    894
    I doubt that the air pressure is ever 14.7 psi here. :p

    Here at one mile high it is more like 12 psi. Vail pass is almost 11,000 feet high and is about 10 psi.][/QUOTE]
     
  10. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,193
    2,024
    [/QUOTE]
    There you go. So as a side note at what temperature does water boil? :)

    Ron
     
  11. RichardO

    Late Member

    May 4, 2013
    2,273
    894
    There you go. So as a side note at what temperature does water boil? :)

    Ron[/QUOTE]
    I seem to remember that it was about 200 degF (I'm at about 5300 feet) when I was calibrating a temperature meter using boiling water as a reference.


    p.s. I looked it up on-line and it is about 205 degF at 5000 feet and a little over 190 degF at 11,000 feet.
     
  12. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,193
    2,024
    Thanks. :)

    Ron
     
Loading...