Answer
Per 1,000 US gallons of 3% or 5% (by weight) HCl: not less than 1 gallon at 150 °F, 2 gallons at 175 °F, 3 gallons at 200 °F. (5% w/w HCl is 14.1% v/v of 20° Bé acid.)
Detailed question
How much inhibitor for a hydrochloric acid clean?
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