Q.When measuring the atmosphere in the tank since the explosimeter has 2 sensor for HC, one by LEL & by Vol. Does it automatically come to % vol after reaching 100% LEL? Say for propane its 2.3%, after its reached 2.3 which is equal to 100%, will the explosimeter show 2.4, 2.5 and so on? Do we need to set anything for a particular gas say pentane has 1.5%?

So we want to measure hydrocarbons as a percentage of LEL and percentage of Volume. 

I have seen HC meters that measures %LEL and those that measure %Volume of HC. And I have seen explosimeters that measure hydrocarbons as both %LEL and %Volume.

These do not switch automatically from LEL to volume and vice versa. There is a knob which needs to be turned to the desired measurement unit.

Though I have never come across an explosimeter that would automatically switch to % volume when the LEL reaches close to 100%, I won't be surprised if there is one.

Now coming to the 2nd part of the question. How can the explosimeter identify what percentage of Volume equals to 100% of the LEL?

Well, the equipment can not identify this. We have to check the manual of the gas meter to check what gas it is supposed to measure or is tested with. This is the gas that we need to calibrate the gas meter with.

So if the gas meter is calibrated with iso-butane and the 100% of LEL for isobutane is equal to 1.8% of the volume, then the gas meter will correlate to these values when measuring. 

But what if the measured gas is different from the gas with which the equipment is calibrated.

In this case we need to apply the appropriate correction factor to the readings we get with the gas meter.

Now we must be wondering why we never apply these correction factors on oil tankers even when the measured gas may be different from the calibration gas?

Most of the gas meters are calibrated with iso-butane and LEL value of this is close to 1% of the volume (1.8% volume). The LEL value of other HC gases is either close to this or more than this value.

So even if we do not apply the correction, we are keeping ourself on the safer side.

I hope that clarifies. Any other related question, do not hesitate to ask.