Welcome to Keep Your Cool - a blog tackling simple cooling optimization strategies for the busy data center operators.

Last week, we posed the question” how hot is too hot” when it comes to the people working in the hot aisles. As temperatures rise within the data center environment, the risk of heat-related illnesses for technicians and other staff working in the hot aisle increases significantly.  Today we’ll do a deep dive into how you can the Heat Stress Index, or Heat Index, to influence operator safety. 

What is Heat Stress Index (HSI)?

The Heat Stress Index, also known as the heat index, is a metric that combines temperature and humidity to assess how hot it feels to the human body. Unlike ambient temperature alone, HSI considers the body's ability to cool itself through sweating, which is less effective in high humidity. Thus, high humidity can make it feel hotter than the actual air temperature. Heat index is best used within any building, athletic center, environment that is not climate controlled and is out of direct sunlight. For Data Centers, the hot aisle is ideal for HSI being used as the metric to set MOPS/SOPS for technicians working there.

How do you determine HSI? 

While there are different formulas used to calculate HSI, a commonly used one is the Heat Index formula developed by the National Weather Service (NWS) in the United States. The NWS Heat Index formula takes into account both air temperature and relative humidity to determine how hot it feels to the human body. The formula typically involves complex mathematical equations and constants to calculate an apparent temperature, which reflects the combined impact of temperature and humidity on perceived heat. This apparent temperature, expressed in degrees Fahrenheit or Celsius, is then used to categorize heat stress levels and provide guidance on safety precautions for outdoor activities (Figure 1). 

How to Measure HSI in the Data Center?

To measure HSI in the data center, you would need to collect temperature and relative humidity data throughout your aisles. 

Let’s take for example a hot aisle that we measured (Figure 2). First, we measured the dry bulb temperature, which is the actual temp of the air without considering any moisture content/humidity.  As you can see, the temperature ranges from 91.5°F to 96°F.  

At the same time, we collected relative humidity measurements (Figure 3). The range is from 14.0% to 15.8%

 Finally, we used the NWS Heat Index formula to calculate the Heat Stress Index (Figure 4). The top row is “extreme caution.” Note the extreme low humidity in the hot aisle is lowering the heat stress indicator temperature to be lower than the dry bulb temperature.  Suggested guidelines for working in this hot aisle would be frequent breaks (at least 15 minutes every hour) and access to water to drink.

Importance of Multiple Measurements

It is not sufficient to just take one measurement in the Hot Aisle to assume the Heat Stress level in the hot aisle. As is apparent from Figure 4, the values may be different at different elevations and at different parts of the hot aisle. Measurements should be made at different elevations throughout the hot aisle, to gather a complete picture of the heat index profile for that aisle.

Conclusion:

As you can see, measuring the HSI in your hot aisle can be as simple as collecting temperature and humidity data in your hot aisles. This is a simple way to assess the level of heat stress within the workspace and take necessary precautions. The challenge truly is to conduct regular audits of your environment. Of course, Purkay Labs can always help conduct the audits for you – and provide comprehensive reports on the HSI– if you lack the time or personnel. Ideally, the data center/mission critical industry will create their own standard based on scientific studies on heat stress in a data center specific environment. In the meantime, measuring HSI is a way to create MOPs/SOPs for anyone working in the hot aisle.