EN 15758:2010 – Conservation of Cultural Property – Procedures and instruments for measuring temperatures of the air and the surfaces of objects

4 Recommendations relating to measuring methods
4.1 Measurement of air temperature Measurements should only be taken to answer questions which will help to solve environmental or conservation problems and they should always form part of an overall plan for environmental improvement. Air temperature is monitored to establish a cause-and-effect relationship between atmospheric variables and the response of cultural heritage objects.
When measuring air temperature precautions should be taken to reduce the effect of thermal radiation and inertia of the probe. Particular care should be taken to shield the air temperature sensors from radiation sources at different temperature levels, e.g.: direct solar radiation, incandescent lamps or radiant heaters. Screens should be made from reflecting materials and should have adequate natural or forced ventilation as described in EN ISO 7726:2001, Annex A. A thermometer placed in a given environment does not indicate the air temperature instantaneously but needs time to reach equilibrium. A measurement should not be made before a period has elapsed equal to at least 1,5 times the time constant of the probe and the output has reached 90 % of the difference between the initial and the equilibrium values. See EN ISO 7726:2001, Annex A for further details.
4.2 Measurement of the effective temperature including radiant contribution
4.2.1 Black-globe thermometer The effective temperature measured by the black-globe thermometer results from a thermal balance including air temperature, radiation from other bodies, object absorbance and convection. The measurement is important to characterize environments affected by intense visible light (from sun, or spot lights), or infrared radiation (from hot surfaces or heaters). The black-globe should absorb both visible and IR parts of the spectrum. The black-globe-thermometer and the measurement of the effective temperature should conform to the requirements of EN ISO 7726. The black-globe thermometer has a spherical shape because it was originally introduced for measuring the amount of radiant heat received by the human body. The shape is not appropriate for measurements for objects of very different geometries, for example flat paintings on canvas, wooden panels or tapestry. The response time is generally long, from 20 min to 30 min, depending on the globe and the ambient conditions.
4.2.2 Measuring temperature with a blackbody strip target
The surface temperature an object will have when hit by radiation shall be simulated and measured as described in the next clauses. This clause is concerned with diagnostics for preventive conservation and is relevant for the measurement of not only air temperature, but also the potential synergism of air temperature, visible and infrared radiation and convective air motions on an object (e.g. paintings on canvas, wooden panels or tapestry).
It is particularly useful to know in advance the surface temperature which an object will reach when the object is due to be relocated for temporary or permanent exhibitions, or the environment should be changed for the installation of spot lamps or radiant heaters. The measurement is taken with a blackbody strip target located in the position planned for the object, or placed close to the object without contact with it.
The blackbody strip target will simulate what happens to the object without making tests with the object itself. This simulation is representative of most of the materials used for cultural property, except for polished metals that have a very low emissivity and reflect most of the incoming radiation. The method is also useful to assess the level of the thermal comfort for visitors to museums, galleries, churches, historical buildings, etc., where heating should be reduced to a minimum acceptable level and/or to restricted areas.
EN 15758:2010 – Conservation of Cultural Property – Procedures and instruments for measuring temperatures of the air and the surfaces of objects