Dim Wannabee needs ISA help
 

I understand that temp an be stated as ISA (or indeed JSA) plus or minus a figure

eg: Temperature is ISA + 7.

What I'm really stuck with is the nature of the calculation. Does it require one to apply the correct mathematical technique as in:

ISA = - 57 so ISA + 7 would be - 50 and ISA = -57 and so ISA -7 would be -57 - (-7) = -57 + 7 = -50

Both can't be right.

Many thanks for any help

The easiest way is to use words and common sense not maths.

ISA +X means X degrees warmer than the relevant ISA temp

and

ISA -X means X degrees colder than the relevant ISA temp

Which of course is your first case

Thank you for the reply which if I understand correctly means my examples now read:

ISA = - 57 so ISA + 7 would be - 50 (warmer)

ISA = -57 so ISA -7 would be -64 (colder)

Absolutely right Cron.