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  1. #1
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    pneumatics question

    Restrictors used in pneumatics. Different sizes and even adjustable ones. Are they just used to limit the volume of airflow to say, a transmitter? or is there other purposes for a restrictor? How do you know what size to use?

  2. #2
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    The amount of air a restrictor allows to pass is matched to the device (transmitter, thermostat,etc) it's feeding's capability to bleed off.
    If a temperature transmitter couldn't bleed down to 3 PSIG (at the low end of it's span) because the restrictor was too large (or the wrong one passing too much air) it could not work.
    Sometimes restrictors are used to slow down or dampen signal fluctuations.
    Always use the restrictor recommended by the manufacturer.
    I have an adjustable restrictor on my truck, but haven't seen it used much.
    I'm sure there are more examples others will give.
    Do a search for low capacity thermostats for a good example.
    Johnson had some good info years ago on this. Maybe someone has it in a PDF.
    Good luck.
    Jogas
    Four wheel therapy, my 1968 Camaro is gone and will be missed

  3. #3
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    Restrictors became necessary with pneumatic controls when the control manufacturers developed cheap control. Restrictors are necessary for sensors where the sensor is a one pipe bleed off type. As stated above, the sensor must be able to bleed off the air so that the controller can receive a proper signal.

    In the old days remote sensors were relay type. That meant the remote sensor received main air, ran the main air through a relay and passed a signal to the controller located somewhere in the building using the signal input port.

    So control companies found that they could make more or less plastic controls ( in place of expensive metal) and that they could regulate the signal pneumatic line to the controller by using a one pipe (bleed off or bleed down) sensor, which required a restrictive device in line with the one pipe bleed off sensor.

    Hope this makes sense. It's really about saving money and making less expensive devices by the control manufacturers.
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

  4. #4
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    Restrictors are also used to slow down the operation of a valve or damper motor. This gives the controlling sensor time to see the change in temperature caused by movement of the damper or valve and can prevent oscillations in your control where the valve or damper will overshoot and then overcompensate again and again.

  5. #5
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    The size required is usually listed in the SPEC / TECH sheet for the sensor .0075 is the most common.

    Another reason that many Pneumatic manufacturers went to the single pipe / restrictor transmitter was to allow for a greater distance between the controller and the sensor.

  6. #6
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    SCIM

    Pneumatic devices are rated in (SCIMs) standard cubic inches per minute. This is how much air a device consumes. The fixed restrictors are based on a particular device and it's application. The ones I'm use to are red (.005), and aqua (.0075) scims. When dealing with pnuematics a really good air dryer and air station are a must. Also an automatic air compressor blowdown is very essential. I also recommend the inline filters that change color to indicate moisture. If moisture makes it to the restrictors, then you'll have some serious issues. We've even replaced all the restrictors in a system if we had any questions.
    Law Of The Thermostat: He who has the thermostat wins!!!!!

  7. #7
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    To control occ/unocc mode...

    Still used today in a building in downtown Dallas. They have a restrictor tee on the damper signal. They have normaly closed boxes. So when they want the boxes on that loop to close no matter what the t-stat is saying, they open the valve and let it bleed off.
    "It's not that I'm smart, it's that I stay with the problem longer”
    Albert Einstein

  8. #8
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    Thanks for the info, clears up some confusion.

  9. #9
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    Quote Originally Posted by xarralu View Post
    Still used today in a building in downtown Dallas. They have a restrictor tee on the damper signal. They have normaly closed boxes. So when they want the boxes on that loop to close no matter what the t-stat is saying, they open the valve and let it bleed off.
    I don't thing that's a function of a resistor..although it could be part of the control circuit. More than likely it's a EP switch energized so the branch air line to the damper motors bleed off.
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

  10. #10
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    Quote Originally Posted by DeltaT View Post
    I don't thing that's a function of a resistor..although it could be part of the control circuit. More than likely it's a EP switch energized so the branch air line to the damper motors bleed off.
    Potato POTATOE. Yea same thing.
    "It's not that I'm smart, it's that I stay with the problem longer”
    Albert Einstein

  11. #11
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    Quote Originally Posted by simux View Post
    Pneumatic devices are rated in (SCIMs) standard cubic inches per minute. This is how much air a device consumes. The fixed restrictors are based on a particular device and it's application. The ones I'm use to are red (.005), and aqua (.0075) scims. When dealing with pnuematics a really good air dryer and air station are a must. Also an automatic air compressor blowdown is very essential. I also recommend the inline filters that change color to indicate moisture. If moisture makes it to the restrictors, then you'll have some serious issues. We've even replaced all the restrictors in a system if we had any questions.
    I remember seeing some old ( '20's ) JCI pneumatic wall stats peeing water and STILL WORKED after drying out!
    Can't do that with todays stuff.
    Jogas
    Four wheel therapy, my 1968 Camaro is gone and will be missed

  12. #12
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    The OLD stuff took a beating , just replaced 82 POWERS D STYLE day / night thermostats , oldest 1962 newest 1966 every one was working , school did a major retrofit ( appearance , no $ for DDC ) and wanted NEW looking stats.

    Out of 82 new thermostats had 3 faulty from the factory , 2 that needed MAJOR recalibration and have had at least 5 damaged by the students.

  13. #13
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    Nov 2001
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    Quote Originally Posted by jogas View Post
    I remember seeing some old ( '20's ) JCI pneumatic wall stats peeing water and STILL WORKED after drying out!
    Can't do that with todays stuff.
    I had a pneumatic control board full of Johnson T800's, T900's & 901's that when I lifted the lid ports a stream of water would shoot out. They were still working.

    I dried the air & lines out, put in some new diaphrams and the things were still working last time I was at this lab. But that was years ago so I am sure the lab has been torn down.

    Funny cause one of the service calls I use to get was water running down the wall under the thermostat.
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

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