# Blank thresholds

> supplier limit, then your limit, then an opt-in DL floor

Source: https://geodb.io/guide/topic/qaqc_blank_thresholds/ · Category: QAQC
When does a blank "fail"? The common conventions, in rough order of use:

- **10× detection limit** as the single failure line (the most common);
- **5× DL warning, 10× DL failure** as a two-tier compromise;
- **≥90% of blanks below 5× DL** as an aggregate form;
- CIM sets **no numeric threshold at all**.

Two important refinements beat any DL multiple:

- **A certified blank is not a zero.** CDN-BL-10S certifies Au **<0.01
  g/t** — a bound, with a method. OREAS 21e is a "blank" with real
  certified values for Cu, Fe and Li; OREAS 24c carries 138 ppm Ni. For a
  certified blank, **use the supplier's prescribed limit**, not a DL
  multiple.
- **Economic significance can override.** For a 0.5% Cu deposit, failing a
  blank at 10 ppm Cu is pointless — 10 ppm is economically meaningless
  there; ~200 ppm would be a sensible limit (Bloom). Mass ratio matters
  too: carry-over into a 500 g blank reads ~10× stronger than the same
  contamination into a 5 kg sample.

## In geoDB

geoDB resolves a blank's threshold through a **ladder**, stopping at the
first rung that answers:

1. **The certified blank's supplier limit** — if the QC sample is linked to
   a certified blank whose certificate states a `<` bound for that element,
   that bound is used. This is the industry default above, applied
   automatically.
2. **A warning limit you configured** on the project for that element
   (optionally scoped to one method). Element casing no longer matters — an
   `au` limit row covers an `Au` reading.
3. **A detection-limit multiple**, *if* you have set one on the project's
   QAQC protocol (`blank_dl_multiple`). This is **empty by default** and
   staying empty is a legitimate choice.
4. **Nothing** — the reading is reported as **"unknown"**: shown on the
   page, excluded from the pass rate, never counted as a pass.

The page tells you **which rung produced the number** in the *Limit source*
column, because the three rungs are different claims — a supplier bound is
the CRM vendor's, a warning limit is yours, and a DL multiple is a
convention.

**Set a DL multiple deliberately, not reflexively.** It is a floor, not a
recommendation. On a multi-element ICP scan it is meaningless for the
rock-forming elements a barren blank legitimately contains — a silica or
carbonate blank really does carry percent-level Na, Ca, Fe and Mg, which
sit thousands of times above their detection limits without any
contamination at all. Configure limits for the elements your blanks are
actually there to police.

If your blanks page shows a lot of "unknown", that is the honest reading:
no limit exists for those elements yet. Warning limits are managed from the
QC pages.