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

In summary, the conversation revolved around finding the value of "h" at 1.1 MPa pressure in the superheated tables. The solution suggested was to use linear interpolation, which was unfamiliar to the person asking the question. They were then given examples and asked to apply the same algorithm to find the value of "h" at 1.5 MPa pressure. The conversation also included discussions about the use of interpolation in exams and the lack of knowledge about it in high school algebra.
  • #1
Amr719
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0
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
 
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  • #2
Linearly interpolate.

Chet
 
  • #3
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
Amr719 said:
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
How long does it take to linearly interpolate?

Chet
 
  • #5
I don't know exactly but if there is a faster way , I should go with it
 
  • #6
Amr719 said:
I don't know exactly but if there is a faster way , I should go with it
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
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
Amr719 said:
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 :)
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
I can't understand . what I know that it is at the superheated vapour Tables
 
  • #10
Amr719 said:
I can't understand . what I know that it is at the superheated vapour Tables
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

Your turn.

Chet
 
  • #11
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
Amr719 said:
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
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
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
russ_watters said:
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?
Shocking, huh? Don't they teach interpolation in high school algebra any more?

Chet
 
  • #15
Chestermiller said:
Shocking, huh? Don't they teach interpolation in high school algebra any more?

Chet
The problem is, "interpolate" is too big to fit on a calculator key. :oops:
 
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  • #16
russ_watters said:
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?
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
Chestermiller said:
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
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
Amr719 said:
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
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
Chestermiller said:
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
h(250C,1.5MPa)=2899.3+((250-240)/(280-240))*(2991.7-2899.3)=2922.4
 
  • #20
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
 
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  • #21
Amr719 said:
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
Sometimes teachers show you an easier exmple, then give you a slightly harder example than what they showed you. An important part of being an engineer is being able to apply separate pieces of your knowledge to a problem you haven't seen before in order to solve it. You must learn to think beyond just learning the exact thing the teacher taught.

So -- now that you know interpolation works here, can you tell me why?
 
  • #22
russ_watters said:
Sometimes teachers show you an easier exmple, then give you a slightly harder example than what they showed you. An important part of being an engineer is being able to apply separate pieces of your knowledge to a problem you haven't seen before in order to solve it. You must learn to think beyond just learning the exact thing the teacher taught.

So -- now that you know interpolation works here, can you tell me why?

Why what ? Why it works here ?
 
  • #23
Amr719 said:
Why what ? Why it works here ?
Russ's question meant do now you understand why and how the linear interpolation formula that you used gives a good approximation to the enthalpy at 250 and 1.1 MPa, or are you just satisfied with applying it blindly and moving on? Be aware that understanding how and why it works is more important than getting the answer to a specific homework question.

Chet
 
  • #24
now I know if I didn't find a table for a specific pressure I should interpolate .Do I understand correctly ?
 
  • #25
Amr719 said:
now I know if I didn't find a table for a specific pressure I should interpolate .Do I understand correctly ?
Yes
 
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What should I do if I didn't find the given pressure?

If you are conducting an experiment or conducting research and you did not find the given pressure, there could be a few reasons for this. One possibility is that there was an error in your measurement or calculations. Double-check your equipment and methods to ensure they are accurate. Another possibility is that the pressure may be outside of the range you were expecting. Consider adjusting your experiment to accommodate a wider range of pressures.

Am I using the correct units for pressure?

It is important to use the correct units for pressure to ensure accurate measurements. The most commonly used units for pressure are Pascals (Pa) and atmospheres (atm). Make sure you are using the same units as the given pressure in your experiment. If not, use a conversion factor to convert to the correct units.

What other factors could affect the pressure in my experiment?

There are several factors that could affect the pressure in your experiment. These include temperature, humidity, altitude, and surrounding air pressure. Make sure to control for these variables and consider their potential impact on your results.

What should I do if the given pressure is too high or too low?

If the given pressure is significantly higher or lower than expected, it could indicate an error in your experiment or a malfunction in your equipment. Double-check your methods and equipment to ensure they are accurate. Adjust your experiment accordingly to accommodate the given pressure.

Is there a way to calculate pressure if it was not given?

Yes, there are several ways to calculate pressure. One method is to use the ideal gas law, which relates pressure, volume, temperature, and number of moles of a gas. Another method is to use a pressure transducer or pressure gauge to measure the pressure in real-time. However, it is important to note that these methods may not be accurate in all situations and it is best to have a given pressure to work with in an experiment or research setting.

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