What is a Representative Thermal Fluid Sample?

Thermal fluid is collected at Operating Temperature.

In their document entitled ‘Monitoring Heat-Transfer Fluids: The Sampling ‘Bomb’’, (ref: MP 623 International), dated December 1980, BP Oil states,

‘A truly representative sample of the complete charge can be taken only when the fluid is hot and circulating.’

The document goes on to describe use of ‘the bomb’ – a closed sampling device designed to capture the volatile light ends that would otherwise be “boiled off” if the sample was taken when open to atmosphere.


Systems design varies widely from site to site: pipe diameter, fluid velocity and pipe layout must be taken into consideration. In addition, aged oil can be significantly more viscous, therefore;

  • If the sample is taken with the system running and at normal operating temperature it is far more likely that turbulent flow is occurring thus ensuring that a homogenous mix of fractions – within the bulk fluid – is sampled (please see example below)
  • Any insoluble contaminants will, for the same reason, be more likely to be suspended within the bulk fluid
  • Both high and low molecular weight ranges of fractions will be detected


For typical ISO 32 heat transfer mineral oil

At fluid velocity V(m/s) x  pipe diameter d (mm) = 10<

Reynolds number = 2,000 for fluid @ 100°C (212°F) but
Reynolds number = almost 20,000 for fluid @ 325°C (617°F)

At ambient temperatures turbulent flow cannot be guaranteed

  • There is a substantial difference in viscosity between a sample at operating temperature and an ambient sample. This will also affect the way the fuel-like light fractions mix

It can only be representative when at temperature – if there was an event it would be from a system operating at normal temperature.

  • If there was an event and a subsequent investigation, it would be vital to know that your flash point values and any corresponding advice given were correct – the only way to ensure this is by having hot, closed & circulating samples