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Hydra 798 Iss.10 – 04/18
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 Volume 1 and 2 of the Solutions
Catalogues.
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 are hygroscopic. 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 ingress 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 Ethanol and water or pure Ethanol. 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 optimum liquid level with the motor off at the operating temperature is for the liquid to be approximately 25mm
from the top of the calibration tube.
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.
