What role does the temperature change in the soil play that is transferred to the medium in the pipe (air)?
The formula for the isochoric change of state of ideal gases (law of Amontons) is used. It states that temperature and pressure are proportional to each other (p1/p2 = T1/T2), if the volume does not change. At an absolute test pressure of 8 bar (7 bar test pressure), one obtains the pressure differences (dp) from the above table.
In the example 20 cm soil depth:
The temperature of the air in the pipe increases by 1.36 °C, resulting in a pressure increase of 39.2 mbar.
It is also very obvious that with increasing depth each additional cm of coverage becomes “less important”. The relationship is not linear, but exponential. This has the following effect, for example: Between 20 cm and 40 cm depth there is a difference of 17 mbar, between 60 and 80 cm depth there is only a difference of 2.9 mbar!
A thought play at the end: How do you actually assess the case when the earth spike does not reach all the way down to the pipe (pipe apex), e.g. when the temperature is measured 40 cm above the pipe? Is this a benefit for the on-site inspector or not?
Answer: Probably not, because the basic principle is that temperature fluctuations should be avoided. This is proven by measuring the soil temperature at a suitable location, i.e. near the pipeline or by measuring at points of the pipe particularly exposed to the temperature influence, namely the exposed ends of the pipe with a contact sensor. The example calculations have shown why we should avoid or at least minimize temperature fluctuations in the pipe. If, however, the maximum pressure drop is slightly exceeded during a pressure test, correct temperature measurement can help to assess the situation. The better the measurement can reflect the change in the pipeline, i.e. it is measured close to the pipeline, the easier it is to assess whether the changes in temperature can be the cause of a pressure drop or pressure increase.
With a calculation of the effect of the measured temperature changes on the pressure, it is clear whether I can put a pipeline into operation or the test has to be repeated.
Our recommendation: Due to the complexity of the interrelationships, an evaluation of the temperature influence should only be carried out by specially trained and experienced personnel.