The Home of High-Precision Metrology
Illustration by Minna Kuusela
Building maintenance technology with a focus on airconditioning
About 30 % of the total building area is devoted to building maintenance technology, mainly ventilation equipment, required for creating laboratory conditions. Potential sources of electromagnetic interference, such as compressors and the emergency power generator, are located as far away from the laboratories as possible. There are 36 laboratories that are divided in 20 sections, each of which has its own dedicated ventilation system used for the fine tuning of temperature.
Expansion joints and double walls
The propagation of internal vibrations caused by building maintenance technology and the movement of people has been inhibited by dividing the building into several separate sections using expansion joints. In the surface laboratories, the basic idea is an onion-like structure, in which the laboratories at the core are protected from external temperature variations by a corridor, an office room and outer protective shutters. To optimise vibration performance, the walls, floor and ceiling of each laboratory are joined together by steel rods and poured concrete in such a way that each laboratory behaves as a rigid body.
Isolation of heat sources
Because of the strict room temperature requirements, the air heated by, for example laboratory light fixtures and hot furnaces, is drawn directly away from the laboratory and returned into circulation. The temperature, humidity and pressure of the air in laboratory facilities, as well as the mechanical vibrations between floors are constantly monitored.
Instruments requiring high temperature stability are mounted in enclosed equipment cabinets supplied with air whose temperature is regulated to an accuracy of ±0.01 degrees, or liquid baths that can achieve accuracies of a few millidegrees. For vibration-sensitive equipment, the laboratories use granite tables with natural rubber isolators.
Extremely vibration-sensitive length and mass metrology equipment is mounted on either air-spring supported 80–120 ton concrete slabs of gravel/insulation carpeting/solid slab structures. These provide attenuation of bedrock vibrations at 1 Hz and higher frequencies. The building is founded on a competent bedrock.
Electrical interference and electromagnetically shielded laboratories
Steel reinforcements within the concrete frame of the building are welded together at 1.2 m intervals to provide a continous mesh. This type of a "Faraday Cage" protects the laboratories from external electrical interference up to the radio frequency range (fcut-off~200 MHz). The thick concrete walls and the underground placement of the laboratories also reduce interference. For extremely sensitive electrical measurements, the building has 12 electromagnetically shielded "rooms". For external fields, their minimum shielding attenuation is 100 dB (10 kHz to 20 GHz). In order to minimise power supply interference, these rooms are equipped with optical fibre lighting and symmetrical filtered power supplies (2 x 115 V).
Ventilation and diffusion technology
The laboratories with the strictest temperature stability requirements have been constructed in naturally constant temperature underground floors based on the "room within a room" principle. The actual measurement room is placed within a slightly larger room so that the air drawn from the measurement space air washes the intermediate space as it is returned into circulation. This solution enables the maintenance of a set temperature of 20 °C to an accuracy of 0.01 degrees. Most of the laboratories are constructed using the floor diffusion principle, with the exeption of mass metrology laboratories, which require cleaner conditions, where ceiling diffusion is used, and the humidity laboratory, where wall diffusion is used. A room where the temperature can be set at (20±5) °C makes studies of the temperature dependence of devices and meters possible.
The atomic clocks are continuously compared against UTC using a GPS satellite receiver. An antenna tower provides optimum satellite visibility in the southern sky.