What is the tolerance level for the intoximeter ec ir II?

The Intox EC/IR II (made by the Intoximeter Corporation) is a breath testing device used by law enforcement and scientists worldwide. The margin of error (also known as the margin of acceptability or tolerance) is +/- 10% of the target value (which differs depending on whether you use a dry gas standard or wet bath solution for calibration checks).
To make this clear assume that a sample of vapour containing alcohol is introduced into the instrument. If we know the true value of the gas is 100mg% alcohol (EtoH) then the instrument is capable of producing accurate readings of between 90-110mg%. This range (the difference between the measured and the true value) represents the tolerance of the instrument. So long as it falls within this range the instrument is working properly. Note that all instruments have margins of error. There is no such thing as perfect measurement.

One additional note of interest. The above is correct if you are asking about the margin of error of the instruments ability to measure alcohol in a vapour. It is not however true if you are asking the margin of error of the instruments ability to measure alcohol found in the blood. This will seem odd (since the whole point of breath testing is to determine blood concentrations) until we realize that while the instrument's margin of error is +/-10% this only applies to its direct measurement of the vapour. In order to produce a blood alcohol concentration the instrument must convert that breath measure into a blood measure. This conversion skews the margin of error dramatically (and in criminal matters to the benefit of the accused). Factors that account for this include use of a 2100:1 partition co-efficient (when the actual is closer to 2400:1) the number of samples taken (and use of the lowest), delay in taking samples (thus allowing for elimination of alcohol), truncation (rounding down of readings) and the quality of the samples provided (in criminal matters by uncooperative subjects which results in lower than true values). Depending on where you live (and thus which factors are present in your situation) the true margin of error between breath and blood is likely to be a situation where the instrument systemically underestimates blood by around 10%.