# Pressure gauge

1. Feb 14, 2016

### y2j

When we take pressure readings by pressure gauge (Bourbon pressure gauge or pressure transducer) in the laboratory for any fluid system ,in this case, is the atmospheric pressure value (1 bar) must be added to this readings when we use them in any calculations later or them must be used alone without adding atmospheric pressure value
Thanks to everyone who takes a look at the subject

2. Feb 15, 2016

### gleem

If you application involve working with or knowing the absolute amount of gas in a vessel then you must add the current atmospheric pressure to the reading for example in using the ideal gas law. However if you are interested in say the rupture pressure of a vessel then you only use the gauge pressure since the atmosphere is providing a counter pressure from the outside. so you only need in the pressure above atmospheric This would also apply to fluids except if the vessel is in a vacuum (outer space) in which case you need to add one atmosphere to the reading.

3. Feb 15, 2016

It depends on what type of gauge you have. Some gauges measure differential pressure (and atmospheric pressure must be added) while others measure absolute pressure (so they already account for the atmospheric pressure).

4. Feb 15, 2016

### y2j

I am working on Rankine cycle
and in some component there is a liquid and in the other there is a vapor then,
when I want to obtain enthalpy from tables at these pressure (in addition to temperature ) ,can I use only gauge pressure?

5. Feb 15, 2016

### gleem

If you have a vessel with a gaseous and a liquid phase and you need to consider the total amount of gas, what do you think?

6. Feb 15, 2016

### y2j

I am using pressure transducer with 4-20 mA signal output to measure the pressure in specified point and not measure differential pressure

7. Feb 15, 2016

That's not really helpful in terms of determining whether the transducer is differential or absolute. Do you know its manufacturer and model number? Just look that up and take a peek in the manual to make sure (or on the specifications page online).

8. Feb 15, 2016

### y2j

Thanks
I will try

9. Feb 16, 2016

### billy_joule

I did the exact same thing in an internship. Think about the precision you need. Will an average local atmospheric pressure value work? Or does it need to be more accurate and in real time?
Compare the price tag and range of absolute vs gauge transducers.

Tuning a small rankine cycle plant (organic, i assume?) is hard. Not knowing what your gauge transducers are referencing to would make it a lot harder.

10. Feb 16, 2016

### y2j

Exactly , I am working on small organic rankine cycle, But I did not understand exactly what you mean .
Manufacturer of these pressure transducers tell me that are relative pressure transmitter and not absolute .

11. Feb 16, 2016

### billy_joule

What exactly don't you understand?
Have you done a course in basic fluid mechanics?