The Mid-Afternoon Calibration Drift and Two Suspicious Readings
You are the certified inspector on a 1948 single-family home. Your XRF is checked against a 1.0 mg/cm² calibration standard with a manufacturer tolerance of ±0.1 mg/cm². The morning check read 0.95. After roughly 40 component readings, a mid-afternoon check reads 1.13. In the dining room, two readings on painted plaster walls were 2.4 and 2.6 mg/cm². Where a section of that same wall has been stripped to bare plaster, the reading is 1.2 mg/cm². In the basement, a hollow steel door reads 3.1 mg/cm²; a wood window sash upstairs reads 0.8 mg/cm². The client wants a positive/negative determination for every component before you leave today.
1. The 1.13 mg/cm² check is out of tolerance. What must you do before testing continues, and what happens to the data you collected since the last valid check?
Read the worked answer
Stop testing immediately. The performance characteristic sheet governs: a check outside ±0.1 mg/cm² means the instrument is not producing defensible numbers.
Sequence:
- Follow the manufacturer's correction procedure (recalibrate/restandardize; check source condition and detector window; verify the standard itself is not damaged or dirty).
- Re-run the calibration check. If it returns within tolerance, resume. If it cannot be brought into tolerance, tag the unit out of service and arrange repair - do not 'test and hope.'
- Any component tested after the last acceptable check is of unknown validity. Either re-test those components once the instrument is verified, or flag them as inconclusive and confirm by laboratory paint-chip analysis.
Reasoning to document: the readings were not random noise - the drift was small (1.13 vs. 1.0), so most component calls probably survive. But near-threshold results collected during the drift window cannot be defended, and a result like 0.9 or 1.1 collected during drift could flip. Record the failed check, the time window, the components affected, the corrective action, and the re-test results in the inspection report. Silent re-testing without documenting the failure is a data-integrity problem.
2. How do you report the two dining room plaster wall readings of 2.4 and 2.6 mg/cm², given the 1.2 mg/cm² reading on the stripped area?
Read the worked answer
This is a substrate-correction situation. The XRF measures lead in everything in its beam path, not just the paint film, so a lead-bearing substrate inflates every reading over it.
Check the premise first. The stripped spot must be genuinely bare - same substrate, thick enough to be effectively infinite, and free of residual paint in pores or pinholes. If paint residue remains, the 1.2 is paint, not substrate, and correction is invalid; in that case take a paint chip for laboratory analysis.
If the bare area is valid:
- 2.4 − 1.2 = 1.2 mg/cm² → positive (≥1.0).
- 2.6 − 1.2 = 1.4 mg/cm² → positive (≥1.0).
Judgment points:
- Correction is applied only when the substrate is the same material and thickness as the one under the paint being corrected. Do not borrow a substrate value from a different wall assembly.
- A corrected value that lands below 1.0 is reported as negative; you may not report a negative number.
- Because these corrected values sit only slightly above the threshold, confirm at least one with a laboratory paint chip. A chip result is reported as percent lead by weight against the 0.5% criterion - a different metric, so state both the XRF and the chip result and the basis for each.
- Note the correction method in the report so the next inspector understands why 2.4 became 1.2.
3. The basement steel door reads 3.1 mg/cm², but a laboratory chip from the same door's paint was lead-free last year. What do you report?
Read the worked answer
Report the door as inconclusive pending laboratory confirmation - not positive, and certainly not negative. Do not average the two results or pick the one you like.
Why the conflict is predictable: dense or high-atomic-number substrates (steel, some masonry, and thick metal assemblies) can inflate XRF readings or produce false positives. Substrate correction only works if you can obtain a reading of the same bare substrate in the same geometry; on an installed hollow steel door you generally cannot, so correction is not available here.
What to do:
- Collect a paint chip from the door (all layers, to substrate), document the location, and submit it for laboratory analysis. The chip gives a percent-by-weight answer against the 0.5% criterion, which resolves the ambiguity.
- Report the XRF reading with a clear note that it conflicts with a prior laboratory result and that a confirmatory chip is pending.
- If the laboratory confirms non-lead paint on the door itself, note that a false positive occurred and that readings on similar metal components in this house should be treated cautiously.
The exam-tested judgment is that a single instrument result never overrides a conflicting laboratory result on the same component - and that neither one is allowed to disappear from the record.