# What should I do if didn't find the given pressure?

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1. Mar 24, 2015

### Amr719

I want to know what should I do if I don't find the given pressure in a problem in the super heated tables ?
I have P=1.1MPa and I want to get the " h " but I don't find this pressure in the tables

2. Mar 24, 2015

### Staff: Mentor

Linearly interpolate.

Chet

3. Mar 24, 2015

### Amr719

Isn't there another way ? My teacher told me another way to do it that takes less time in the exam , but I can't remeber it

4. Mar 24, 2015

### Staff: Mentor

How long does it take to linearly interpolate?

Chet

5. Mar 24, 2015

### Amr719

I don't know exactly but if there is a faster way , I should go with it

6. Mar 24, 2015

### Staff: Mentor

I can't think of a faster way. I bet I could do the linear interpolation in less than a minute. Interpolation is the standard way of working with tables.

Chet

7. Mar 24, 2015

### Amr719

Could you tell me how to do the interpolation for this : I have P=1.1MPa T=250°C and I want to get "h"
Sorry I am not familiar with this type of solution so I need your help :)

8. Mar 24, 2015

### Staff: Mentor

At what values of the pressure does the table give values of h? (on either side of 1.1 MPa) What are those values of h at 250 C?

Chet

9. Mar 24, 2015

### Amr719

I can't understand . what I know that it is at the superheated vapour Tables

10. Mar 24, 2015

### Staff: Mentor

Write down some of the numbers from your table. For example in the steam tables I have,

P = 10 bars, T = 240 C, h = 2920
P = 10 bars, T = 280 C, h = 3008
P = 15 bars, T = 240 C, h = 2899
P = 15 bars, T = 280 C, h = 2993

Chet

11. Mar 24, 2015

### Amr719

That's the question. Which table ? I don't have a table for the pressure 1.1MPa . I have for 1.0 MPa and for 1.5 MPa

12. Mar 24, 2015

### Staff: Mentor

I'm acutely aware of that. What does your table give for h at 1.0 MPa and 1.5 MPa? After you provide those values, I will show you how to get the value at 1.1 MPa.

Chet

13. Mar 24, 2015

### Staff: Mentor

Amr719, it doesn't seem like you know what it means to "interpolate". Did you look that up after Chestermiller said it is what is needed?

14. Mar 25, 2015

### Staff: Mentor

Shocking, huh? Don't they teach interpolation in high school algebra any more?

Chet

15. Mar 25, 2015

### SteamKing

Staff Emeritus
The problem is, "interpolate" is too big to fit on a calculator key.

16. Mar 28, 2015

### Amr719

No I know it but like I said before I'm not familiar with it because my teacher doesn't use this method. That's all

17. Mar 28, 2015

### Amr719

Table for 1.0MPa : Table for 1.5MPa:
T=240,h=2920.4 T=240,h=2899.3
T=280,h=3008.2 T=280,h=2991.7

18. Mar 28, 2015

### Staff: Mentor

And, what, you never noticed that these are the same values I gave you from my table in post #10?

I am going to show you how to get the value of h at 250 C and 1.0 MPa. Then you are going to show me how you apply the same interpolation approach to get the value of h at 250 C and 1.5 MPa.

$$h(250 C,1 MPa) = 2920.4 + \frac{(250 - 240)}{(280-240)}(3008.2-2920.4)=2942.4$$

Now I want you to apply this same algorithm to get the value of h at 250 C and 1.5 MPa. Do you think you can do that?

Please don't tell me at this point that you need to have the value of h at 1.1 MPa. I know that. There will be another step after you complete this step.

Chet

19. Mar 28, 2015

### Amr719

h(250C,1.5MPa)=2899.3+((250-240)/(280-240))*(2991.7-2899.3)=2922.4

20. Mar 28, 2015

### Staff: Mentor

Looks good. Now, do you think you can take these results for h at 250 C for 1.0 MPa and 1.5 MPa and, by using this same kind of interpolation algorithm for pressure, find the value of h at 250 C and 1.1 MPa?

Chet