Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at

May 29, 1975

Mr. J. L. Wentz,
General Supervisor - Safety
Midwest Steel Division of
National Steel Corporation
Portage, Indiana 46368

Dear Mr. Wentz:

Your letter of May 5, 1975 requests an interpretation of the requirements of Section [1910.124(j)(1) and (j)(2)]. The following information was secured from the Standards Division:

"Initially, during installation, provisions should have been made for a Pitot Tube traverse of the duct. At present, since some of the ducts are lined, air flow measurements can be made with a calibrated velometer, or heated thermoanemometer across the hood entry and edges of the tank. Exhaust ventilation rates must meet the requirements of Table G-15."

[This document was edited on 06/01/99 to strike information that no longer reflects current OSHA policy.]

You are reminded that the air flows are for undisturbed locations. Higher airflows may be required to compensate for excessive cross drafts. Air sampling for a specific contaminant is one means of evaluating effectiveness of the air exhaust system.

A modified Pitot Tube does exist which consists of two tubes joined laterally (approx. 1/2" D x 1" wide). This permits straight entry into a duct. A 2" threaded sleeve welded onto the duct is the permanent entry (when not in use - a threaded cap closes the opening).

If you have additional questions pertaining to the matter above, please feel free to contact this office.

Very truly yours,

Associate Assistant
Regional Director for
Technical Support