AN07 Application Note – Gas corrections & Mass flow


The VPFlowScope thermal insertion probe and in-line flow meters use a thermal mass flow principle. This isgreat, because you don’t need an external pressure or temperature sensor to compensate the output signal. But it also means that the output depends on the density, the thermal conductivity of the gas, the heat capacity and the viscosity. For common technical gases, we have verified the conversion factors experimentally. In this application note, we provide more background information on how conversion factors can be used.

1. Gas calibration

Real gas calibration vs theoretical factor
We offer real gas calibration for a selected group of safe, non-corrosive, non-oxidizing, non-combustible and non-toxic gases. Due to safety regulations of our calibration lab, we cannot test with toxic gases, nor can we calibrate with strong oxidizing gases. Real gas calibration results in a higher accuracy of the instrument compared to conversion factors, as this eliminates any on-linear effects.

As oxygen is a strong oxidizing gas, which can cause fire under some circumstances, we cannot calibrate with pure oxygen in our lab. Therefore, the conversion factor is a calculated value. Oxygen, Nitrogen and air have quite similar thermal properties, resulting in a conversion factor of 1.0.

When the gas properties differ significantly, the conversion factor will also differ significantly. The most dramatic effect is seen with helium gas. This gas has a very low density and a high thermal conductivity. This results in a non-linear conversion factor, so we always offer a real gas calibration for this gas.

Conversion factors table
The conversion factor table is shown below. The factor applies to most in-line flow meters. Note that the factor may vary when applied to insertion probes due to viscosity effects. Our standard gas is compressed air, which has a reference factor of 1,0. When subjected to another gas, the flow meter output needs to be multiplied with a conversion show the correct measurement value. The factor is applied over the full range of the instrument, and does not cancel out any non-linear effect.

VPFlowScope DeltaP
For differential pressure based instruments, the conversion factor cannot be applied in the same way. In general, a differential pressure meter signal depends mainly on the density of the gas. Please consult us when you intend to apply the VPflowScope DeltaP for gases other than Compressed Air, Nitrogen or Oxygen.