When you collect breath alcohol test results into files or standardize the units in a report, %BAC and ‰ can get mixed. %BAC is a percentage unit used to express blood alcohol concentration, and ‰ is a per mille (parts per thousand) unit. When the same concentration is shown in different units, the numbers look different, but once you check the units they may point to the same value. The problem arises when the originally recorded number and unit are overwritten during the unit change.
DATECH's official product information for the AF-40BT gives its display range as 0.000–0.500 %BAC, 0.00–5.00 ‰, and 0.00–2.50 mg/L. The fact that one device can display several units is convenient for comparison and management, but it does not mean you can judge the unit of past files from the current screen setting alone. The purpose of a unit change is not to replace the original result with a new number but to add a comparison value while keeping the original.
First confirm that %BAC and ‰ express the same thing
% is a percentage and ‰ is per mille. So if the values express the same concentration basis, 0.100 %BAC equals 1.00 ‰. To convert a %BAC value to ‰, multiply the number by 10. Conversely, to convert a ‰ value to %BAC, divide by 10.
However, you should not apply a conversion rule based on the unit symbol alone. The AF-40BT also lists mg/L as a separate display unit alongside %BAC and ‰. mg/L is not something to handle by shifting decimal places the way you would between % and ‰, so first distinguish the unit written in the record from the conditions under which the result was produced. If you change only the numbers without confirming that the values being converted express the same concentration, you end up mixing different systems of expression in the same column.
Keep the original number and unit together as one set
Preserving the original value does not end with copying the number before conversion. You need to keep the number and unit as first saved or received, a save time or result identifier that distinguishes the result from other rows, and the display setting that could be confirmed at the time. Only then can you tell, when a value later looks different, whether it is a calculation problem, a difference in screen settings, or a comparison between different results.
It is better to manage the creation of values in a standardized unit for comparison and aggregation as a separate task. Keep the display unit selected in a report separate from the original unit as well. This prevents a choice made to make the screen easier to read from being recorded as if it were the unit of the original test result.
| What to keep | What to note with it | Why it must be kept separate |
|---|---|---|
| Original result | The number and unit as first received | You can re-check the original expression |
| Information to locate the result | Save time and information that identifies the result | You can link which result was converted |
| Converted value for comparison | Converted number, unit, and calculation rule applied | You can compare without overwriting the original |
| Report display value | Display unit and rounding digits | You can explain the difference between calculated and displayed numbers |
Separate calculated values from rounded display values
After a unit conversion, values can look different because of the number of display digits. As a hypothetical example, if the original record is 0.105 %BAC, the value calculated in the same concentration expression is 1.05 ‰. If the report is set to show only one decimal place, the screen may show 1.1 ‰.
What to observe here is the number of decimal places in the original and the display digits in the report. 0.105 %BAC, 1.05 ‰, and 1.1 ‰ are, respectively, the original value, the converted calculated value, and the rounded display value. A possible action is not to save the three values by swapping them into a single field, but to keep the conversion rule and rounding method along with them. The completion criterion is that someone else can recalculate 1.05 ‰ from the original 0.105 %BAC and re-check how 1.1 ‰ came out under the set display rule.
In particular, you must not convert a rounded display value back and save it as the original. Decimal places lost in display cannot be restored by back-calculation, and afterward it becomes hard to tell the precision of the original test record from the precision of the display number.
Leave past files without units as unconfirmed
If a past file has only numbers and no units, it is hard to settle the unit from the current device's unit setting or the file name alone. You may consider cross-checking, in order, materials tied to when the file was created, such as the export template used at the time, files created in the same period that do state units, device setting records, and product documents.
It is better to keep rows for which you could not find evidence separately, marked as unit unconfirmed. If those numbers have already been included in statistics or reports, you can first find which period and which documents they were used in, and then re-aggregate based on data whose units are confirmed. If you fill empty unit fields in bulk with a default unit, both the fact that the unit was missing from the start and the scope that needs review disappear.
After a setting change, look at the paths results travel through
Checking only the new test screen after changing the unit setting does not tell you how past records are handled. You need to look separately at how numbers and units appear on each path through which results are actually delivered, such as newly saved results, the view screen for previously saved results, file exports, and smartphone integration. The AF-40BT offers storage for 10,000 test results, Bluetooth smartphone integration, and standalone and app modes, so it is important not to assume the device and the connected app screen behave the same way.
The observation here is to confirm whether past results are recalculated and shown in the new unit, keep the expression from when they were created, or change only the label on screen. Depending on what you find, you can decide to keep exported data that retains the original units or to add the calculation rule to the conversion report. The completion criterion is that whatever screen or file you look at, you can find the original number and unit, and each number in the standardized unit can be traced to the original result it came from.
Standardizing units makes results easier to compare, but it is not a reason to delete the original. If you separate the original number and unit, the converted calculated value, and the rounded display value, you can explain both the meaning of past test results and the conversion process even when you switch the display between %BAC and ‰.
