The Conditions Are the Survey
Most survey enquiries open with a date. The building is free on the fourteenth, the site is quiet that week, the contractor is there anyway, so can the flight happen then. It is a fair question, and the honest answer is usually a window rather than a day.
An aerial thermal survey does not photograph a fault. It records a temperature difference across a surface, and that difference has to exist at the moment the image is captured. Where conditions do not produce it, the camera returns a flat, uniform surface and the survey finds nothing, at full cost. The equipment is available every day of the year. The asset is not.
What makes this harder to plan than it first looks is that different assets want opposite weather. A photovoltaic array needs bright, stable sunshine, because the array has to be working hard before a failing part of it runs hot enough to separate from its neighbours. A building envelope needs the reverse, a cold night well after the fabric has given up the heat the sun put into it during the day. Book both for the same afternoon and one of them is wasted.
What follows is what those conditions are actually doing, where they come from, and what a window means for booking a survey, standing one down, and judging afterwards whether it was worth flying.
Blog Content TL;DR...
Conditions decide whether an aerial thermal survey produces an answer or an expensive photograph.
- A thermal survey measures a difference across a surface, so that difference has to be present at the moment of capture.
- A flat image is not a clean bill of health, and reporting it as one leaves the client confident about an asset nobody has assessed.
- Photovoltaic inspection under IEC 62446-3:2017 asks for at least 600 W/m² in the plane of the module, wind no more than 4 Beaufort, and no more than 2 okta of cumulus.
- A building envelope wants the opposite, a cold night flown well after dark once the fabric has released the day’s stored solar heat.
- Surveys are therefore booked against a window with a go or no-go call near the day, and standing down is a legitimate outcome.
The camera is available all year. The conditions that make its output mean something are not.
The Camera Is Ready, the Asset Is Not
A survey measures difference, and the difference has to be there
A thermal camera reports the apparent temperature of the surfaces in its field of view. It has no concept of a fault. What it can show is that one part of a surface is behaving differently from the part beside it, and the entire value of a survey rests on that contrast being present, and on it being caused by the thing under investigation rather than by something incidental.
This is why conditions are not a comfort item or a matter of pilot preference. They are the precondition for the measurement meaning anything at all. A flat image is not a clean bill of health, and reporting it as one is the most expensive error available on a thermal survey, because the client then believes an asset has been checked when it has not.
Finding nothing, and there being nothing to find
The distinction matters commercially as much as technically. A survey flown outside its window has two possible outcomes and both are poor. Either nothing shows, and the client is handed false confidence in an asset nobody has genuinely assessed, or something shows and cannot be quantified against anything, so the finding cannot be graded or prioritised against the rest of the portfolio. Neither result supports a decision, which is the only reason the survey was commissioned. Drone Media Imaging will postpone a survey rather than fly one that can only produce pictures, and that judgement is made against the conditions the asset in question requires.
when to fly a thermal survey ~ aerial thermal survey weather conditions ~ IEC 62446-3 inspection conditionswhen to fly a thermal survey ~ aerial thermal survey weather conditions ~ IEC 62446-3 inspection conditions
Two Assets, Two Opposite Windows
A photovoltaic array wants the brightest part of the day
IEC 62446-3:2017 is specific about this, and the reasoning behind each figure is worth having. Module inspection calls for a minimum of 600 W/m² of irradiance measured in the plane of the module. Wind is capped at 4 on the Beaufort scale, roughly 28 km/h. Cloud cover is limited to no more than 2 okta of cumulus. Where irradiance or load shifts by more than 10 per cent in a minute, a settling period of about fifteen minutes is recommended before measurement continues.
Each of those exists for a physical reason. A cell that has stopped conducting properly turns the energy falling on it into heat instead of current, so the array has to be working hard before a defect will separate itself from the modules around it. Wind carries that heat away and flattens the difference the survey is looking for. Cumulus drifts across the sky and throws moving reflections onto module glass that read as anomalies and are nothing of the kind.
A building envelope wants the coldest, darkest part of it
The requirement inverts completely. Heat has to be flowing outward through the fabric, and to have been doing so steadily for some hours, which in practice means the heating on and the survey flown well after dark, once the structure has released the solar energy it absorbed during the day. Wind is unhelpful here too, and wet fabric muddles the picture badly. EN 13187:1999 provides the methodology for qualitative detection of thermal irregularities in building envelopes. The temperature difference a particular building needs is judged for that survey and that structure, rather than lifted from a universal number.
Which is why one site can be two surveys
An industrial unit carrying a rooftop array above a heated warehouse is a single address holding two assets whose windows do not overlap and cannot be made to. Treating that as one visit is where a great many disappointing surveys begin.
Planning Around a Window
Booking, standby, and the decision to stand down
Once the window governs the date, the sensible way to book is a provisional slot with a genuine go or no-go call made close to the day against the forecast. That is less tidy than a fixed appointment and it is considerably cheaper than the alternative, because a wasted mobilisation costs a day whereas a survey flown in the wrong conditions costs the survey, the report and whatever decision was waiting on it. Standing down is a legitimate outcome rather than a failure, and it is written into how Drone Media Imaging plans this work. The seasons matter as much as the forecast, since a building envelope survey belongs to the colder months and a photovoltaic inspection to the brighter ones.
The conditions are part of the deliverable
Recording them is not administrative tidiness. IEC 62446-3:2017 requires the ambient conditions to be captured alongside the imagery, irradiance in the plane of the module, wind speed and direction, cloud cover in okta and type, and ambient air temperature. That record is what allows a finding to be defended if it is challenged, and what allows this year’s survey to be compared honestly with the last one rather than merely placed beside it.
Findings are reported as areas warranting further investigation by a suitably qualified contractor. Survey, analysis and reporting is the service, and the conditions under which the imagery was gathered are what make the analysis worth reading.
Standards and Governance
- Surveys conducted by Steve Fisher, ITC Level III Certified Master Thermographer (Infrared Training Centre; Certification #205722059).
- Practising in accordance with the frameworks of ISO 18436-7:2014, ASNT SNT-TC-1A and ANSI/ASNT CP-105, with BINDT CMGEN Appendix B as UK training governance.
- Methodology aligned with IEC 62446-3:2017 for photovoltaic thermographic inspection and EN 13187:1999 for building envelopes, as applicable to the asset.
- Ambient conditions are recorded alongside the imagery and reported with the findings, so the basis of every observation is auditable.
- Solar electrical work is limited to string-level testing.
- Survey, analysis and reporting only. Findings are areas warranting further investigation by a suitably qualified contractor, and no remedial or corrective work is offered or implied.
Tell Us the Asset and We Will Tell You the Window
Describe the site and what you need to decide, and we will tell you when the conditions are likely to allow a survey worth reporting on. Drone Media Imaging works across Sussex, Hampshire, Kent and Surrey, travels throughout the UK, Ireland and Europe, and undertakes thermography worldwide.
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