There is nothing wrong with selling equipment that measures the levels of alcohol in the body
Question

                What is the ruling on selling equipment for measuring alcohol levels, as has been imposed on drivers recently, whether they are sold in places where alcohol is drunk, or in public places, or to individuals? Is there a difference between selling them in a Muslim country or in a non-Muslim country?
Praise be to Allah.
      
      Firstly: 

    There 
    is nothing wrong with selling equipment for measuring levels of alcohol in 
    the body, whether that is in a Muslim country or a non-Muslim country, and 
    whether the purchaser is a Muslim or a non-Muslim. That is because the basic 
    principle concerning all things is that they are permissible, as Allah, may 
    He be exalted, says (interpretation of the meaning): “Allah has permitted 
    trading” [al-Baqarah 2:275]. 

    
    Moreover, equipment for measuring alcohol levels does not help people to 
    drink alcohol, and it is not a direct or indirect cause of this sin. The 
    prophetic texts only forbid, with regard to alcohol, anything that may help 
    people to drink it, manufacture it and deliver it to those who consume it. 
    The Prophet (blessings and peace of Allah be upon him) said: “Allah has 
    cursed khamr and the one who drinks it, the one who pours it, the one who 
    sells it, the one who buys it, the one who squeezes it, the one for whom it 
    is squeezed, the one who carries it and the one to whom it is carried.”

    
    Narrated by Abu Dawood in as-Sunan, 3674; classed as saheeh by al-Albaani. 

    As 
    this equipment does not come under any of these headings and cannot be 
    likened to any of them, the basic principle that it is permissible remains 
    in effect. 

    Even 
    if it is used by a driver who has drunk alcohol, his use of it cannot be a 
    means to help him to drink, and his not buying it can never be a deterrent 
    to drinking; rather the equipment will be a means of checking himself, so he 
    does not add to the evil of drinking the evil of driving in an intoxicated 
    state which may lead to harm and damage to people and property. 

    We 
    asked Shaykh ‘Abd ar-Rahmaan al-Barraak about selling this equipment, and he 
    said: What appears to be the case is that it will serve a purpose, so it is 
    permissible. 

    
    Secondly: 

    It 
    may be said that it is Islamically prescribed to manufacture and sell this 
    kind of equipment, which protects society from the harm caused by those 
    sinners in traffic accidents and the like, and may be used by police forces 
    to deter drinkers so that they will not commit further crimes when they are 
    intoxicated. So use of this equipment is usually in the interests of 
    society, and we do not see any harm in it or anything that would suggest 
    that it is haraam. 

    
    Thirdly: 

    With 
    regard to selling the equipment in places where alcohol is drunk, we do not 
    think that a Muslim should go into places where these evil deeds and sins 
    are committed, so that it will not be a source of temptation for him and so 
    that he will not be recorded with Allah, may He be exalted, as being among 
    those who approved of or kept quiet about evil, or who condoned disobedience 
    towards Allah on earth, unless he goes there to advise and warn, without 
    causing any trouble or doing anything that would lead to a greater evil. 

    And 
    Allah knows best.