Pressure – Absolute, Gauge, and Units of Both

Compressed air is a common utility used throughout industrial facilities and it has to be measured like any other utility in order to know just how much a facility is using. When dealing with compressed air a common unit of measurement that readily comes up is psi, pound-force per square inch. This unit of measure is one of the most basic units used to measure pressure in the compressed air industry. There are other means to measure this though, so let’s discover the difference.

Again, the pressure is a force distributed over an area, the Earth’s atmosphere has pressure, if it didn’t we would all balloon up like the Violet from Willy Wonka, just without eating some prototype gum causing internal pressure. PSIA is a unit of measure that is relative to a full vacuum. It is pounds per square inch absolute (PSIA). The absolute pressure is calculated as the sum of the gauge pressure plus the atmospheric pressure. If you were to travel into space, the atmospheric pressure would be absolute zero which is actually a vacuum. There is nothing pushing from the outside in so the inside pushes out, hence the ballooning.

The atmospheric pressure on earth is based on sea level. This is 14.7 pounds per square inch absolute pressure. This pressure will change along with the weather and the altitude at which the measurement is taken.

So how do we get to the pressure that is displayed on a pressure gauge?  When shown open to room air, my pressure gauge reads zero psi. Well, that is zero psi gauge, this already has the atmosphere showing. It is not showing the Absolute pressure, it is showing the pressure relative to atmospheric conditions. This is going back to the fact that gauge pressure is the summation of absolute pressure and atmospheric conditions, for sea level on earth that is 14.7 psia. So how do we increase this and get the gauge to read higher levels?

We compress the air the gauge is measuring, whether it is using a screw compressor, dual-stage piston compressor, single-cylinder, or any other type of compressor, it is compressing the ambient, atmospheric air. Some materials do not like being compressed. Air, however, reacts well to being compressed and turns into a form of stored energy that gets used throughout industrial facilities.  By compressing the air, we effectively take the air from atmospheric conditions and squeeze it down into a storage tank or piping where it is stored until it is used. Because the air is being compressed you can fit larger volumes (cubic feet or cubic meters) into a smaller area. This is the stored energy, that air that is compressed always wants to expand back out to ambient conditions. Perhaps this video below will help, it shows the GREAT Julius Sumner Miller explaining atmospheric pressure, lack of it, and when you add to it.

Lastly, no matter where you are, there is a scientific unit that can express atmospheric pressure, compressed air pressure, or even lack of pressure which are vacuum levels. To convert between these scientific units, some math calculations are needed. While the video below is no Julius Sumner Miller, it does a great job walking through many of the units we deal with daily here at EXAIR.

 

If you want to discuss pressures, atmospheric pressure, how fast the air expands from your engineered nozzle to atmospheric, why all the moisture in the air compresses with it, and how to keep it out of your process, contact an application engineer and we will be glad to walk through the applications and explanations with you.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

1 – Willy Wonka & the Chocolate Factory – Violet Blows Up Like a Blueberry Scene (7/10) | Movieclips, Movieclips, retrieved from https://youtu.be/8Yqw_f26SvM

2 – Lesson 10 – Atmospheric Pressure – Properties of Gases – Demonstrations in Physics,  Julius Sumner Miller, Retrieved from https://www.youtube.com/watch?v=P3qcAZrNC18

3 – Pressure Units and Pressure Unit Conversion Explained, Chem Academy, retrieve from https://www.youtube.com/watch?v=2rNs0VMiHNw

 

Sound: What Is It … More Importantly, Weighted Scales of Frequencies

We’ve blogged about sound and what exactly it is before, see the link. Understanding that sound is vibration traveling through the air which it is utilizing as an elastic medium.  Well, rather than me continue to write this out, I found a great video to share that is written in song to better recap how sound is created.

Now that we have that recap and understand better what sound is let’s dig a little deeper to better understand why some sounds may appear louder to a person when they may not appear different on a sound scale that is shown by something like a Digital Sound Level Meter.

Loudness is how a person perceives sound and this is correlated to the sound pressure of the frequency of the sound in question.  The loudness is broken into three different weighing scales that are internationally standardized. Each of these scales, A, C, and Z apply a weight to different frequency levels.

  1. The most commonly observed scale here in the USA is the A scale. A is the OSHA selected scale for industrial environments and discriminates against low frequencies greatly.
  2. Z is the zero weighting scale to keep all frequencies equal, this scale was introduced in 2003 as the international standard.
  3. C scale does not attenuate these lower frequencies as they are carrying the ability to cause vibrations within structures or buildings and carry their own set of risks.

To further the explanation on the A-weighted scale, the range of frequencies correlates to the common human hearing spectrum which is 20 Hz to 20kHz. This is the range of frequencies that are most harmful to a person’s hearing and thus were adopted by OSHA. The OSHA standard, 29 CFR 191.95(a), that corresponds to noise level exposure permissible can be read about here on our blog as well.

When using a handy tool such as the Digital Sound Level Meter to measure sound levels you will select whether to use the dBA or dBC scale.  This is the decibel reading according to the scale selected. Again, for here in the USA you would want to focus your measurements on the dBA scale. It is suggested to use this tool at a 3′ distance or at the known distance an operator’s ears would be from the noise generation point.

Many of EXAIR’s engineered compressed air products have the ability to decrease sound levels in your plant. If you would like to discuss how to best reduce sound levels being produced within your facility, please contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

1 – Fun Science: Sound – @charlieissocoollike – https://youtu.be/xH8mT2IQz7Y

 

Not All Suppliers Are The Same

Just last week I received a package on my front porch.  This was a replacement part for a tool which I purchased used, knowing it needed the part.  The trick is, I needed it a few months ago.  I ordered the part from the manufacturer directly on July 2, 2015.  This part was said to be in stock and would ship to my home from their location which was also in the United States.  I heard good things about the company and I bought the tool with confidence I could have the part and the tool fixed quickly. I thought, no big deal I will get it within a week, fix it, and then use it.

After not receiving a shipping notification and nothing showing up at my door for two weeks I decided to call the company.  I finally got hold of a customer service representative after I had to wade through the automated phone attendant.  The person explained that they had in fact received my order and they would try to ship it out the next day so watch my email.   Well, the next day came and when it was nearing the end of the day I decided to call in again since I had still yet to receive a shipping confirmation.  This time I got in touch with a different customer service rep who explained there had been a fire in their warehouse and that nothing was shipping that day or even that week.   The fire didn’t happen that day, it had happened over 2 weeks prior to that.   Instead of notifying me when I placed my order, or even when I called in the first time I was simply told incorrect information.   I gave them the benefit of the doubt and after discussing the issue the customer rep. told me they are doing their best to get items lined up and out as quickly as they can.  It should only be a few more weeks.

I accepted the explanation and began the waiting period.  a few weeks came and I received a back-order notification in the mail, still no notification of any sort stating they are not shipping any products out.  Few more weeks and another post card.   After the third post card I had almost forgotten about it.  Finally I received an e mail, my item had shipped.  Two days later it was on my porch and packed like any other shipment.  No explanation for the delay, no apologies, and as if it was just normal business for them.

I ordered the part on July 2nd,   I received the part on October 13th.  Needless to say, the quality is good but the customer service communication is fairly lacking.

I began to think about what we do at EXAIR, and came to the realization that if something like this had happened here we would have sent out an E-News, a simple e-mail, tell customers who call in, and other forms of communication to every last customer that had an order in and we would be notifying every customer that was placing new orders.  We would be up front with the information and we would not hesitate to apologize for the inconvenience.   We have had disruptive incidents in the past which we handled this way, this is just good business etiquette.  This goes hand in hand with the fact you speak to a human when you call in to our office, all stock products (and we stock it ALL) ship same day on orders received by 3 PM ET when shipping in the US, and we will give you updates via e-mail or phone however you prefer.   Then to top it all off, we will give you a 30 day guarantee and a 5 year built to last warranty on pneumatic parts.

30 Day Guarantee
30 Day Guarantee

So if you want to be informed, treated right, get the products you need in a timely manner, and get your problem solved, you have zero reason to go with anyone else.

Brian Farno
Application Engineer Manager
BrianFarno@EXAIR.com
@EXAIR_BF