30 CFR §57.22228

1. (a)Preshift examinations shall be conducted within three hours prior to the start of the shift for which the examination is being made.
2. (b)Prior to the beginning of a shift following an idle shift, a competent person shall test the mine atmosphere for methane at all work places before persons other than examiners enter the mine.
3. (c)When one shift immediately follows another, a competent person shall test the mine atmosphere at each active working face for methane before work is started on that shift.
4. (d)A competent person shall test the mine atmosphere at each face blasted before work is started.
5. (e)Except in Subcategory I-C or Category III mines, vehicles used for transportation when examining the mine shall be approved by MSHA under the applicable requirements of 30 CFR parts 18 through 36.