# air con super heat calculations

Discussion in 'All Things Boats & Boating' started by Frosty, May 22, 2012.

1. ### FrostyPrevious Member

I have a -5 degree C on the low pressure line and 22 degrees gas temp taken near the pressure reading.

That is 20 subtract -5

How do you subtract -5 from 20? Some people say its 15 some say its 25.

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### StumbleSenior Member

Subtracting a negative means add the integers.

3. ### FrostyPrevious Member

Can you elaborate on that. I assume you are falling on the 25 side.

20 - 5 is 15 --how can 20 subtract -5 be more than 20?

I think Ive just cured the Greek debt crisis.

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### StumbleSenior Member

Lol. Subtraction can be written as (+6)-(+3)=3. When subtracting a negative.
(+6)-(-3)=
Two negatives equals a positive so...
(+6)+3=9

For you the answer is 25.

It may be easier to think of it as the difference between +20 degrees on one end and -5 degrees on the other. The change in temprature is 25 degrees.

5. ### FrostyPrevious Member

Well yes the difference is 25 but that is the range or difference. I need to subtract -5 from 20. How the hell can it be more.

If I have 20 dollars credit and 5 dollars debit I have 15 dollars. The difference in my credit and debit is 25 but I dont have 25 I have 15

I just dont get this,---yes ok I must be a bit thick but I still dont get it.

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### StumbleSenior Member

Another way of saying subtract is 'difference'. So in this case what is the difference between 20 and -5.

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### CDKretired engineer

Yes, it isn't easy Frosty.
Why don't you add 273 to these values, that gives you absolute temperature (Kelvin) which is always positive.

8. ### FrostyPrevious Member

Whats 273+ -5 =268

9. ### CatBuilderPrevious Member

Yes. That's correct.

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### philSweetSenior Member

They want the temp of the gas inside the pipe, not room temp, Frosty.

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### daiquiriEngineering and Design

That's how Greek got into debt crisis.

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### daiquiriEngineering and Design

Hey Frosty, I think that a visual example might help you understand how it works. Observe the length of lines in three subtractions shown:

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