I think the explosion limits are a function of temperature.The limits you have mentioned are valid at room temperature. I am not sure of the pressure effect.

Thank you. I understand that temp is a factor, but pressure is also. As pressure increases the explosive triangle (as shown on a Coward diagram)increases in size, but I don't know how to calculate the increase in size. Do you know how to calculate HEL/LEL as temperature rises?

Temperature Effect :

LFL is decreased by approx 8% for every 100oC rise in temp and the UFL is increased by approx 8% for every 100oC rise in temp.

LFL = LFL @ 25*[ 1-0.75(T-25)/Hc]
UFL = UFL@25*[ 1+0.75(T-25)/Hc]

Hc = heat of combustion in kcal/mole

Pressure :

Pressure does not have any significant effect on LFL but it has on UFL.

UFL  = UFL@25+20.6( log P + 1 )

P is in mPa.

Hope the above helps.

Have you tried "Flammability Characteristics of Combustible Gases and Vapors", U.S. Bureau of Mines publ no 627.
It doesn't have everything you could ever wish for but it's an excellent starting point and has a heap of reference documents too.
I have a couple of pdf files with graphs of natural gas LEL/UEL vs temp and pressure if you send me your email address to flareman_xs@netzero.net.  The graphs, however are no substitute for a clearer understanding which you can get from the full book because the basis isn't just a single easy formula.

Please note, from Gkandy's answer, that pressure has none or very little effect on the LFL. At the atmospheric pressure = 0.1 megaPa (abs.) the log sub 10 turns out = -1, cancelling any addition to the UFL. The addition of an inert gas such as nitrogen, CO2 or even non-condensing steam, would reduce the oxygen content below a safe minimum MOC.