Sampling Guide for Air Contaminants in the Workplace - Irsst

36 IRSST – Sampling Guide for Air Contaminants in the Workplace
reference pressure and temperature (variable depending on the source). The flow must be corrected in relation
to the pressure and temperature on the sampling site using the following equation:
Equation 17: Flow correction in relation to the pressure and temperature (rotameter)
Qs
= Q
flowm
∗
Q s : real sampling rate
P s : absolute ambient pressure (mm of Hg)
Pflowm
∗Ts
T s : absolute ambient temperature (°K)
Q flowm : flow indicated on the flow meter (Q read )
Ps
∗T
flowm
P flowm : flow meter calibration pressure (mm of Hg)
T flowm: flow meter calibration temperature (°K)
Table 7- Reference pressures and temperatures used by manufacturers for certain rotameters
Company
Matheson
Allegro
Gilmont
Reference
pressure
mm of Hg
760
Manufacturer's
reference
temperature
294°K
(21°C)
IRSST's reference
temperature
298°K
(25°C)
2.7.2.3 Mass flow meter
A hot-wire flow meter uses the phenomenon of heat transfer between a hot body and a gas to quantify a flow
rate. When a gas circulates above a hot surface, heat is transferred between the hot surface and the gas. The
rate of heat transfer will depend on the mass of product circulating and the thermal properties of the gas and
the surfaces. This rate of heat transfer will be proportional to the mass flow rate if the specific heat of the gas
and ∆T remain constant.
Hot-wire flow meters are designed to display the same response for the same mass flow rate, even if the
temperature and pressure conditions are different. They therefore have the advantage of displaying a flow rate
at the instrument's standard conditions, regardless of the temperature and pressure conditions.
The flow rate displayed by a mass flow meter (Q flowm ) therefore does not always correspond to the actual flow
circulating in it (Q s ). The latter can be calculated using the flow meter equation which depends on its
calibration conditions described in the following table and the ambient conditions of use. For a given gas, the
flow meter equation is simply that of mass flow conservation:
Equation 18: Flow correction in relation to the pressure and temperature (mass flow meter)
Q s : sampling rate
P s : absolute ambient pressure (mm of Hg)
⎛ Pflowm
⎞ ⎛ T ⎞
s
Q
T s : absolute ambient temperature (°K)
s
= ⎜ ⎟
⎜
Q
flowm
P
⎟ ∗ ∗
Q flowm : flow indicated on the flow meter (Q read )
s
T
⎝ ⎠ ⎝ flowm ⎠
P flowm : flow meter calibration pressure (mm of Hg)
T flowm : flow meter calibration temperature (°K)
Table 8- Reference pressures and temperatures used by manufacturers for some mass flow meters
Company Reference pressure
Reference temperature
mm of Hg
Hasting 760 293°K (20°C)
Kurz 760 298°K (25°C)
2.7.2.4 Corrections for variations in temperature and/or pressure
Depending on the situation, corrections can be applied to take into account variations in volume as a
function of ambient temperature and pressure. A volume or flow rate evaluated under certain conditions of
temperature and pressure can be converted to other conditions of temperature and pressure. These
corrections are based on the conservation of the number of molecules in the sampled volume: