© 2019 Department of Petroleum Resources. All rights reserved Department of Petroleum Resources (DPR) PROCEDURE GUIDE

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80 | P a g e 
 
 
ID =  
Inside diameter of the probe in mm 
S =  
Strouhal number, dependent on the Reynolds number and shape of the cylinder, but 
can be taken as 0.4 for worst case or 0.2 as suggested by API MPMS Chapter 8 
V =  
velocity of the fluid in m/sec 
E =  
modulus of elasticity in kg/cm2 
ρ =  
density of probe material in kg/cm3 
 
1.6 Pressure measurement  
Differential Pressure (ΔP) is the static pressure difference measured between the upstream 
and downstream flange taps. The static pressure shall be the absolute pressure measured 
at the flange tap holes 
1.7 Flow measurement 
The Orifice Flow Rate is the mass flow rate (qm) or volumetric flow rate (qv) through an 
orifice meter per unit of time. (AGA 3.1.1990) 
𝑞𝑚= 𝐶𝑑𝐸𝑣𝑌(𝜋
4)𝑑2√2𝑔𝑐𝜌𝑡,𝑝∆𝑃 
Where: 
 
Cd  
is the orifice plate coefficient of discharge 
Ev  
is the velocity of approach factor 
Y  
is the expansion factor 
π  
is the numerical constant 3.142 
d  
is the orifice bore diameter calculated at flowing temperature, Tf 
gc  
is the dimensional conversion constant 
ρt,p  
is the density of the fluid at flowing conditions Tf, Pf 
Tf 
is the temperature of the flowing fluid 
Pf 
is the pressure of the flowing fluid 
ΔP  
is the orifice differential pressure 
qm 
is the mass flow rate 
 
 
And qv, the volumetric flowrate is given as 
𝑞𝑣= 𝑞𝑚
𝜌𝑡,𝑝