Bad Texas Instruments Pressure Sensor

Discussion in 'Off-Topic' started by #12, Jun 7, 2014.

  1. #12

    Thread Starter Expert

    Nov 30, 2010
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    Whine!

    I designed a safety lock out circuit with a Texas Instruments Low Pressure sensor. Closed above 90 PSIG and opens at 50 PSIG. It only had one job to do: Open the circuit if the pressure ever gets below 50 PSIG.

    It failed. Twelve years after the install, it would not open the circuit until the pressure got down to 18 PSIG. That means it never worked, even once.

    Part # HQ1087039TX
    Presently sold at Graingers under part number 6ALA6

    I'd say, "Don't bet a $700 compressor on a Low Pressure switch" but as far as I know, there is no better way to detect a pressure problem.:confused:

    As a back-up to the back-up, I will install a thermal switch set at about 35 F.
     
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  2. Georacer

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    So, you mean that the need for the safety switch came up only after 12 years and that at that one time of need, the switch failed?

    It really is a bummer, but did you expect it to work as if it was new, without inspection or maintenance forever? I might be talking nonsense, so I have to ask. What's the life expectancy for these things? Do they need service?
     
  3. THE_RB

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    I want to know how you deduced it never ever worked from the fact that it was faulty after 12 years in use? Maybe it was 100% functional for the first 11.9 years?

    Or am I missing something?
     
  4. #12

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    There is no maintenance for a hermetically sealed pressure sensor. Period. You can't open it. You can't adjust it. It's supposed to sit there until it's needed. It never cycles until a fault happens, so it doesn't wear out from constant movement. It just waits, hopefully to never be needed. It's only job is to open the circuit when the pressures go out of bounds.

    I deduced that it never worked to detect the fault because it didn't detect the fault even once in its life. It might have been perfectly vigilant until a week ago, but then got drunk the day the fan relay failed. I'll never know. I only know that I designed the lock-out system and it never worked because the sensor failed.

    I suppose the answer is to add another back-up for the back-up pressure sensor and put the machine through intentional failure modes every year to prove the sensors still work. Of course, that would put 12 times the wear on this particular sensor that it suffered under normal use and force the expensive compressor into over-current conditions every year.

    Can't win for losing.:mad:
     
  5. JoeJester

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    #12,

    I look at it like the NFPA's rules for fire suppression systems. You have to test them to see if they work up to the point of discharge ... but not actually discharging the suppressant.

    You can "bet" the squibs were disconnected on the one system we tested annually.
     
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  6. #12

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    To test this system, I will have to attach my gauges, force the machine into a high pressure fault, then a low pressure fault, and see what the pressures are when the pressure sensors do their jobs. I don't like doing that because every time you open an access port, you might create a leak in a hermetically sealed system.

    These machines don't leak an ounce a year. If they did, they would be out of bounds in less than a decade, and I have dozens of machines running for 15 or 20 years with no detectable leaks. I hate to risk that by messing with the seals every year. I am trying not to let the safety back-ups cause more problems than they cure.
     
  7. ErnieM

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    Apr 24, 2011
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    As my favorite President oft said: "Trust, but verify."

    Did you test the sensor before you installed it into a safety lockout system?
     
  8. inwo

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    Electrical specs are kind of scarce. What kind of circuit was it in?

    Will there be an autopsy?
     
  9. #12

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    I guess you guys are curious. Here's the circuit that has been working on this machine (and I forget how many others) for 12 years. All run commands to the high power section have to go through x number of normally closed sensors. If any of the sensors opens for even a few milliseconds, the low current relay grounds its own coil and nothing runs until a manual reset.

    And, yes, I checked the sensor when I bought it....just like the sensor I bought 3 days ago. It's too easy to screw the sensor on the gauges, add pressure, and release pressure, to find out what the pressures are when the sensor works. Besides, do you really think a person that would design and install this circuit, in addition to what came with the machine, wouldn't bother to check the parts???

    PS, no autopsy planned. It's a curved disk in a chamber. It is never subjected to passing particles because it is mounted on a tube at least 2 feet above the flow. Nothing goes up that dead end except pressurized gas. Normal pressures are about 75 PSIG to 150 PSIG. It's a hermetically sealed system. There is no water in there and nothing has changed since the day I built it. This one even has a filter in the system that removes particles and water...in case there ever are any.
     
  10. #12

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    OK. I lied. I couldn't resist sawing the sensor apart.

    The insides are exactly what I would expect, a round disk holding up a plastic pin that pushes contacts against the wires from the outside world. When the pressure goes low, the disk has to, "ping" away from the plastic pin to let the contacts fall apart and break the circuit. It is all shiny and new inside because it has been in an atmosphere with no water and no oxygen all its life.

    ps, no particles were found fouling the action. The return spring apparently just got tired while waiting, and that is my opinion of, "Doesn't have a reason to exist". If it can't just sit there and wait, without moving, until it is needed, what's it good for? In this case, nothing.:mad:

    pss, turning the brightness up on your monitor helps see the insides.
     
    Last edited: Jun 8, 2014
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  11. THE_RB

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    Cool, thanks for sharing. :)

    So the cause of failure diagnosis was that the internal spring had aged?

    If so, this hints at a maximum standby life for the sensor, or could it have been exposed to higher temperatures or some other factor that would decrease the life of a spring?

    You said earlier about the motor drawing over-current? Maybe if the low pressure fault causes motor over-current you could add an over-current sensor? I think that would be very reliable and have a long life, being electronic.
     
  12. #12

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    Actually, it's checking the high pressure limit that causes too much current, but if you're going to check the low pressure safety, you'd be negligent if you didn't check the high pressure safety.

    When the pressures go low, the pipes ice up and liquid refrigerant can get to the intake valves and bust them. They are reed valves, as far as I know, and they get bent. That messes with the efficiency and wrecks the temperature readings as the partially compressed gas (at over 100 F) blows by the intake valves and heats the nearest section of pipe. In this job, heating the intake pipe even a little is a big error.

    For example: The formula for "superheat" on this machine is Temperature ambient /10 plus 1 F. If it's 90 degrees outside, the temperature of the cold pipe should be 10 F above the boiling point of the refrigerant at whatever pressure it is at the moment. If the boiling point is 45 F, the pipe should be 55 F and you can see that even 5 degrees F is a major error. And that's why I never have less than 3 thermometers. I usually carry 5 thermometers; 3 thermocouples with digital readouts and 2 analog, "twisted spring" thermometers.

    The compressor has an internal overheat sensor that opens, but it's best to never use that because sometimes they don't recover and you end up losing a compressor to the exact part that was installed to protect it. High side pressures that are normally below 200 PSIG can get higher than 450 PSIG before the internal overheat sensor trips out. That's why I use a 250 PSIG sensor for the high pressure safety.

    This is a major safety consideration when using a refrigeration compressor for an air compressor. Even a 1/4 HP refrigerator compressor can get to more than 450 PSIG if you don't shut it off.

    This particular low side sensor was installed in the condenser section of an air conditioner and suffered up to 110 F for many months. In my opinion, not the kind of heat that wrecks spring steel. I can only end up at the conclusion that it wasn't good steel. ps, this is not a soldered part. It was only screwed on, as you can see in the photos.

    Carrier used to use an external current sensor board with a current transformer. I never replaced a Carrier compressor, but I made a living fixing their safety circuits.
     
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  13. inwo

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    That brought back a long forgotten memory. Late 1950s

    One of the experiments from a children's book of activities. (not kidding)

    Among the birdhouses and windmills.

    Connect compressor to something and making a big bang.
    The picture suggested a hand air pump, but I was into electric.
    As I recall some refrigerants were poisonous too.

    I do remember doing it in the basement. I think it was a tin can. For the life of me, I don't remember how a kid could make the airtight connection.

    How did we survive the days without bicycle helmets and common sense?:eek:
     
  14. #12

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    Remember rolls of, "caps". They were supposed to go into a toy gun and make a tiny, "snap" noise. I hit a whole roll of caps with a 40 pound cement block. After my hearing returned, I graduated to things that would get me sent to Guantanamo in todays world. :D Home made rockets that set a record at 20 feet up, something like a cherry bomb made with a cardboard coat hanger tube... Now, the toy cap gun can get a 7 year old expelled from school for a year and a home made firecracker is a terrorist weapon.

    http://www.foxnews.com/us/2014/06/0...ling-teacher-accidentally-brought-toy-gun-to/

    ps, the poisonous refrigerants were ammonia and sulfur dioxide.
     
  15. inwo

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    I'm curious about the circuit.

    Is this your own, or something common in industry?

    The reason I ask, is that I don't recall seeing anything quite like it.

    Seems the relay coils must be carefully selected for series operation during the fault to latch transition.
    ie. the run relay must draw enough current in the seated state to pull in the latch relay. Yet be rated at 24vac continuous to remain latched.

    Looks like something I would do and then years later, wonder why I chose that circuit.:D
     
  16. #12

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    It's entirely my own design.
    It works perfectly.
    As soon as any sensor opens, the stimulating voltage tries to go through the small relay to power the large relay. When the wiper breaks the circuit to the heavy relay, that reinforces the action. A SPST would probably work. Most of the lockouts are for plugged up drain pipes. Never had a failure on that end, and the "float" switches are now glass encapsulated so that eliminates the failure mode of dust getting into the old microswitches.

    Another part of the trick is that I only had a 4PDT relay, so the wiper is triple redundant. My opinion of an excellent design. X number of normally closed sensors and a small relay. All sensors must be, "good" or the machine won't run, so it's a primitive sort of self diagnostic.
     
    Last edited: Jun 13, 2014
  17. #12

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    Ps, I just had to put a fan motor in a six year old Trane air conditioner. They replaced a $90 fan motor with a motor that costs $441, wholesale. It has 18 coils around the perimeter and feels like a stepper motor. The brains are encapsulated, so I don't expect to be able to fix it, in spite of the fact that the windings and bearings are still in excellent condition. Yay, microcontrollers! And I have a new motto: If you can remember the TV advertisement, don't buy that brand. It is YOU that will end up paying for the advertising. More expensive in the first place and incredible prices for parts that break when generic parts wouldn't be half used up.
     
  18. Georacer

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    I like your new motto...
     
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  19. #12

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    Thanks. Just part of my lifelong quest against unrepairable machines, the throw-away economy, and the incredible prices needed to sustain massive planned obsolescence.
     
  20. GopherT

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    Nov 23, 2012
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    Motto? Where is that?

    I was looking for it and noticed Number Twelve just hit 9000 posts, Congratulations and thank you.
     
    Last edited: Jun 16, 2014
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