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Day 6 – Bubble Charts Delta

May 22, 2012 By Leave a Comment

Bubble Chart Showing Rack Top - Rack Bottom

In the bubble chart yesterday, we looked at rack top temperatures over the course of a week.  I mentioned that with bubble charts, it’s not easy to compare two parameters such as rack top and rack bottom.  However, what you can do is look at the delta temperature (Rack Top Temp – Rack Bottom Temp) for each location.  The bubble chart above is showing this difference.

The story that this data is telling us is incredible and extremely useful in determining airflow inefficiencies.

Here is some background. This facility has an under floor air distribution system with cold aisle/hot aisle layout.  Cool air is delivered through perforated tiles to the rack inlet.  Generally, the difference in air temperature between the bottom and top of the rack should be around 8°F due to the heating as the air rises and the typical mixing that occurs.

Takeaways

  • Rack 6, 8, 9, and 29 have greater than a 10°F delta T.  This may indicate areas where lots of mixing is occurring.  Most likely air from the hot aisle is sneaking over the top or through the racks affecting temperatures.  Blanking panels may help.
  • There are many racks with a negative delta T, meaning that the top of the rack is colder than the bottom of the rack.  Potential causes are  hot air from the hot aisle  infiltrating the cold aisle low to the ground due to missing blanking panels, through holes in the floor, or through the IT equipment itself.  The negative delta T may also indicate that IT equipment is installed backwards exhausting hot air into the cold aisles.
  • Most of the racks have a delta T that changes over the course of the week as indicated by multiple bubbles on a single vertical line.  However racks 12, 16, 17, and 18 show only one large bubble –  meaning that they are operating at exactly the same delta T for the entire time.  How could this be ?

Issues

  • The real value of looking at delta T in a bubble graph is to identify airflow issues..  This graph would most likely be used when hot or cold spots are identified to determine the root cause.

Previous Posts
About the Data
Day 1 – From 10,000 Feet
Day 2 – Rack by Rack
Day 3 – Learn to Love Graphs
Day 4 – Wordle
Day 5 – Bubble Charts
Today – Bubble Charts Delta
Tomorrow – A Day of Rest

Filed Under: Data Centers Tagged With: , ,
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Day 5 – Bubble Charts

May 21, 2012 By 1 Comment

Rack Top Temp Bubble Chart

Bubble charts visualization takes accuracy to the next level because they can show three variables at one time.  In this case the parameters are rack, top temperature, and time. Each vertical line is a specific rack.  The graph above is displaying the top temperatures.  The bubbles on that vertical line represent the temperature of the rack during the time period (the temperatures are rounded to the closest 2°F increment).  The size of the bubble indicates how long the rack was at that temperature.  For example: the top temperatures at rack 6 were roughly 68°F, 70°F, 72°F, 74°F, and 76°F.  The larger bubbles at 72°F and 74°F indicates that rack is operating at those temperatures the majority of the time.

Takeaways

  • It is instantly obvious that racks 12, 17, and 20 have persistent hot spots.
  • There appears to be multiple racks that share the same temperature profile.  Racks 1 – 4 are all operating in the same temperature range.  Racks 5 – 10 are also similar.
  • Rack 15 seems to be an outlier operating cooler than its surrounding sensors.  Possibly an airflow issue?
  • Racks 31 – 34 are the coldest of the bunch.  There may be an excessive amount of floor tiles in this region.

Issues

  • Bubble charts can only show one measured parameter at a time. In this case, it represents rack top temperatures .Bubble charts are not good for comparing rack top and rack bottom temperatures
  • Also, bubble charts should be reviewed over a specific time duration.   The chart above displays data over the course of a week.  You could certainly look at data over the course of a month or longer, but it becomes more  difficult if you would like to compare two time periods such as one week versus another week.

Previous Posts
About the Data
Day 1 – From 10,000 Feet
Day 2 – Rack by Rack
Day 3 – Learn to Love Graphs
Day 4 – Wordle
Today – Bubble Charts
Tomorrow – Bubble Charts Delta

Filed Under: Data Centers Tagged With: , , , ,