Page 18 of 24
Orion 796 Iss.08 – 01/14
CHOICE OF LIQUIDS
The choice of liquid is important. If it is too viscous at the operating temperature then the bath may not perform
correctly. Other criteria also need to be met, such as safety, operating range and fume emission for example.
Isotech is able to supply a number of fluids for various temperature ranges, refer to Solutions for Primary &
Secondary Laboratories, Volume 1.
Unfortunately, there is not one fluid which can be used over the whole operating range of the bath.
Beware of oils that are claimed to work from below ambient to over 100°C. Whilst we do not supply or
recommend such liquids, others do. It is our experience that such oils absorb moisture from the air when used
below the dew point. Then at temperatures around 100°C this moisture boils and can lead to the oil spilling from
the top of the bath. Even without the boiling risk, the oil is soon spoilt by the ingressions of moisture and the oil
has to be replaced regularly, which can be an expensive task. It will be seen that when changing fluids from say
water to oil that it is important that ALL the water has been removed before adding the oil.
As oils approach their upper operating limit fumes become significant and fume extraction becomes essential.
Isothermal Technology manufactures a Fluidised Alumina Bath, the 875 Bath which overcomes the limitations of
oils and may be used to 700°C without fuming problems.
Below ambient we use a mixture of methanol and water. Methanol is poisonous and has a low flash point. It needs
to be used very carefully by those suitably trained and aware of the dangers if handled inappropriately. As local
safety requirements vary not only from country to country but also from organisation to organisation the safety
aspects of these fluids should be determined with the user’s safety officer.
LIQUID LEVEL
All liquids expand as they are heated and contract when they cool.
The bath has been designed to allow for varying liquid volumes and also features an overflow system which
prevents the liquid from spilling from the top of the bath.
The overflow will be seen on the top right of calibration tube. Ensure at the operating temperature the level is
within 10-70mm of the overflow opening.
Ensure at all time that the liquid level is kept at the correct level and that the liquid is circulating around the bath. If
the level falls excessively the flow will stop with a consequence of large temperature gradients. If a liquid is being
used close to its flash point a hazard could then occur.
