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mbar/s to shock-measurement

Posted on 2010-11-18
Medium Priority
Last Modified: 2012-05-10
We have a cabinet that is rapidly pressurised to 11mbar. In the specification of one of the components in the cabinet, it says that the component can handle
2,940m/s2(300G) 2.0ms 1/2 sine / 980 m/s2 (100G) 11ms 1/2 sine.

What does this mean? And can this be compared to for example 100mbar/s?
Question by:nebb-tgr
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LVL 27

Accepted Solution

d-glitch earned 2000 total points
ID: 34163562
>> 2,940m/s2(300G) 2.0ms 1/2 sine / 980 m/s2 (100G) 11ms 1/2 sine.

These are units of acceleration:

   300g for   2 milliseconds   square pulse
   100g for 11 milliseconds   half sinusoidal pulse

11 mbar is a low pressure equivalent to a very high altitude.

Pressure (force per area)  and Acceleration (force per second squared) are not
equivalent.  They represent very different stressors on equipment and components.

Author Comment

ID: 34163632
What if you say that the pressure is changed by 100mbar per second, would there be some kind of correlation between this value and the shock? Some way to tell how big of a shock that would correspond to?

Would it for example help to measure the area of the component to determine what kind of shock we are looking at?
LVL 27

Expert Comment

ID: 34163688
>>  Acceleration (force per second squared)

That is horribly wrong.
My only excuce is that I just woke up.

Acceleration is  (meters per second squared) or (change in velocity per second)

   F = ma   ==>  acceleration is Force/mass

But in any case....

You get 100 and 300g shocks from dropping equipment onto a fairly hard surface.
These forces are much higher that the forces you get from slow decompression.

1 second is slow compared to 2 and 11 ms.

This is not to say that decompression can not be destructive, but it is a different realm of mechanical stress.
same as mech

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Author Comment

ID: 34163797
You have been of much help, but I have to extend my question even further to understand what we are dealing with here:

You could to a certain degree say that the two forces are comparable? Would you say that 100mbar/s is a very small force compared to the specification? If so, would you say that 1000mbar/s is weak compared to the specification?

I'm just trying to understand if this pressure could be the source of the problems we are having, I understand that you can't give a definite answer, but if you could somehow compare these types of shock that would be great.
LVL 27

Expert Comment

ID: 34164050
What kind of problems are you having?  Shock or decompression?

I'll assume decompression, since that seems to be what the chamber is for.

For decompression forces, you have to look at the trapped volume and wall strength.
For acceleration shock you have to look at mass and structural design.

The details are going to be crucial.

If you put a closed tupperware container in your chamber and decompress it, the
top will pop off.  If you want to avoid this, you would have to put a vent in.
The faster you decompress, the bigger the vent needs to be.

If you can't allow a vent, you have to make the walls and seals stronger.

1 atmosphere on a small volume shouldn't be  a problem.
1 atmosphere on a large volume can be catastrophic.
It's all in the details.

You are not generating anywhere near 100 to 300g shocks with a 1 atm
decompression.  But there are other ways to break things.
any trouble

Author Comment

ID: 34164145
I think I understand what you are saying, but I can try to be a little more specific.

The component we are having trouble with is a hard disk, it has a vent, but we don't know the specification for this vent. We just have to assume that it can cope with a steady pressure. The hard disk is in a computer which is in a pressure chamber. A compressor generates a pressure higher than the outside pressure. I'm thinking that the hard disk can deal with a steady pressure, but that a sudden change in pressure might cause problems.

The scenario would be that the compressor is running at full speed, and the door closes. The pressure in the chamber increases rapidly before it tunes down to the wanted pressure(15-20mbar), causing the hard disk to fail. It is important to note that the hard disk doesn't break, if we release the pressure it works again.

If I understand you correctly, a pressure of 15-20mbar wouldn't make a difference. We encounter these pressure changes in nature, and the hard disks are specced for it.

Bottom line is: could a sudden increase in pressure cause the disks to fail?
LVL 27

Expert Comment

ID: 34164337
Sorry again.  

11 mbar absolute is a pretty good vacuum.  I work with vacuum environments a lot.

11 mbar gauge is a slight overpressure.

Sudden changes in air pressure can be a big problems or hard disks.
The spacing between the heads and the platters is critical and
the heads ride on a cushion of air.  If this is the problem you might
want to block the vent.

Pressure changes can also flex the case and disrupt internal operation.
If this is the problem, you might want to make the vent larger.

In any case, these failure modes have nothing to do with shock ratings.

LVL 27

Expert Comment

ID: 34164505
>>  Pressure changes can also flex the case and disrupt internal operation.
      If this is the problem, you might want to make the vent larger.

You can calculate the peak forces squeezing the case.

What is the area of the drive in square inches?

The equivalent force would be:

     Area*(11 mbar/1013 mbar)*14psi

This number is probably going to be pretty small, a fraction of a pound.
Do you see problems if you squeeze the drive in you hand?

Author Comment

ID: 34188563
Sorry for taking so long to get back to you.
The disc is about 6cm X 10cm. I don't think flexing of the case is the problem here anymore. It might not even be the harddisks. We are seeing symptons that leads us to believe the harddisk is the root cause, but that might not be the case.

Thank you very much for your help though. I think I understand better what kind of forces we are looking at here. We might even try blocking the vent to see if that will help.
LVL 27

Expert Comment

ID: 34189172
You also should make sure that the mechanical coupling between the chamber and the compressor
isn't transmitting any significant shock or vibration.  

Good luck.

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