A location of humidity and temperature sensors is not regulated by the GOST (governmental standards).
The only recommendation about sensors occurs in CH512-78, and it is formulated the next way: "It is recommended to provide at least two temperature sensors that will allow to ensure an additional control over temperature conditions in a server room. One sensor is output to a console of a dispatcher of building engineering systems while the second sensor must be installed at the console of computer information network administrators.
Meanwhile, a proper location of temperature sensors is crucially important for a correct work of conditioning systems.
For example, on the slide above we see that an air conditioner does not cool the air and maintains 26,5 degrees of Celsius because of an incorrectly located sensor.
According to the regulations for locating sensors provided by the Commercial Building standards for Telecommunications Pathways and Spaces EIA/TIA569: "Average temperature and humidity values must be measured during the work of all active equipment on the height of 1.5 meters above the floor in any central point of the aisle between rows of active equipment".
That's how the same is described by the ASHRAE TC9.9 Mission Critical Facilities from 2004: "a separate humidity sensor must be allocated for every cold corridor that has to be located in the front in the middle of a row of server cabinets. Temperature sensors must be installed on every third cabinet at the bottom, in the middle and on the top of a cabinet. In a case of turbulent air flow, a recommendation is different, particularly one sensor is needed per 1000 square feet".
Uptime Institute offers to measure the temperature on the average height (1,5 meters probably) in the end of each row of racks and in the upper part of a rack in the center of each row.
One more viewpoint is provided by the specialists of the OPENXTRA Ltd., which is engaged to air-conditioning: "You should locate sensors in a distance of 10 to 25 meters between them to avoid the impact of turbulent flows. Locate sensors on the height of 0,6m and 1,5m above a level of the floor to measure the difference in temperatures".
A slightly different approach is demonstrated by the Cisco company: "measure temperature at the top of a cabinet, behind a cabinet, at the bottom near the raised floor and at the entrances to cooling modules".
As far as you can see, there are no unified recommendations. A scheme that includes installing tree temperature sensors for each (or each third) server cabinet seems redundant to me.
I would advise to use the recommendations from the Uptime Institute and OpenXtra, combined with the TIA569 recommendations as follows:
- locate two temperature sensors on the height of 0,6 meters and 1,5 meters in a server cabinet;
- choose cabinets in the end of each row of racks and in the center of each row but no closer than 10 meters between cabinets;
- combine a temperature sensor and a humidity sensor when installing them in the top position in a center of a row.
1) CH512-78. Technical requirements to buildings and rooms for installing computer equipment (retrieved from here).
2) EIA/TIA569 (retrieved from here).
3) ASHRAE TC9.9 Mission Critical Facilities, Thermal Guidelines for Data Processing Environments, 2004 (retrieved from here).
4) The article from the website of the company OPENXTRA Ltd (retrieved from here).
5) Uptime Institute. Implementing data center cooling best practices (retrieved from here).
6) Cisco Unified Computing System Site Planning Guide: Data Center Power and Cooling Guide (retrieved from here).