Everything you need to know about stability and uniformity
A thermostat controls the temperature of, for example, a water
bath by measuring the temperature of the water and comparing it
with the set temperature. It then adjusts the amount of heat put
into the bath to make the measured temperature equal to the set
temperature. Because there is a time delay between measuring the
temperature and putting in the heat there will be a fluctuation in
the temperature of the bath.
The heat is distributed in an unstirred bath by convection and
conduction, and in a stirred bath by convection, conduction and the
stirring action. The heat losses from the surface of the liquid and
through the sides can also cause a change in temperature.
Due to the losses and distribution of heat there are small
fluctuations in temperature across the bath.
The temperature fluctuation at any one point is called the
stability, and the largest temperature difference between any two
points in the bath is called the uniformity.
The temperature at any point varies regularly between two limits
but occasionally a larger variation is observed. The stability as
stated in DIN 58966 is the temperature difference between the
maximum and minimum level over 100 cycles after removing the effect
of the largest 25% of readings.
The stability is determined by measuring the temperature in the
centre of the working volume of the bath and is stated as plus or
minus one half of the measured value.
The uniformity is determined by measuring the temperature in the
centre and corners of the bath and is the greatest difference
between the mean temperatures at any of these points. It is stated
as plus or minus half this value.
What is Intelligent Control Optimisation™ "ICO" ?
Intelligent Control Optimisation™ (ICO) is a control algorithm
developed by Grant Instruments for use in the Optima™ range of
immersion thermostats to automatically optimise the performance for
liquid type and tank volume.
Most thermostatic controllers utilise proportional PI or PID
temperature control, which is set up in the factory and fixed. ICO
uses a heuristic real-time adaptive PID control; in this case the
proportional control parameters are adjusted as the controller
heats up and stabilises. Based on the heat-up rate, the load and
volume of liquid are calculated and an assessment made of liquid
type; the control algorithm is then adjusted accordingly. The
benefit to the user is that the controller applies the optimum
control algorithm for any given situation. This is a feature not
available on competitive products, as the fixed control is always a
ICO offers additional safety for users working with oil, as
excessive localised heating at the heater element is prevented, and
cracking avoided; cracking oil can produce toxic fumes and may
present a potential fire hazard.
With Labwise™ software it is possible to see the control
adapting in real time on the status display, as the controller
heats up and locks onto the desired temperature.
Which water should you use in your bath?
For the long-term reliability of water baths it is important to
use oxygenated water that is free from ions and minerals that can
cause corrosion of stainless steel. We recommend the use of
distilled water and de-ionised water from modern ion exchange
systems that do not use salt back flushing to regenerate the
Stainless steel is protected from corrosion by a layer of
chromium oxide. If the layer is damaged oxygen present in water can
reform the oxide layer. If the water is still or de-oxygenated and
the oxide layer is damaged, ions can corrode the stainless steel
tank. If a water bath has been unused for some time, or water
boiled, we recommend changing for fresh distilled water or correct
Water normally contains calcium or magnesium ions. De-ionised
water has most ions removed as indicated by its conductivity level;
the purer the water the lower the conductivity. It is important to
use only de-ionised water from an ion exchange system with
replaceable cartridges. Do not use de-ionised water generated from
an ion-exchange system that incorporates a salt back-flush system
to regenerate the ion-exchange resin as this can leave sodium ions
that are very corrosive to stainless steel.
How to prevent rust in water baths
Most Grant tanks, as well as immersed parts, are made from type
304 stainless steel, an extremely versatile general purpose grade
of stainless steel. It is the excellent forming characteristic that
has made this grade dominant in the manufacture of laboratory and
industrial water baths, as well as domestic sinks and saucepans.
Type 304 stainless steel is highly suitable for applications where
hygiene is important; it exhibits good heat resistance and
excellent resistance to corrosion.
However, despite resistance to general surface corrosion,
stainless steel is susceptible to specific types of corrosion, in
particular pitting (small pin hole style corrosion) and stress
corrosion cracking. It can also undergo general corrosion in
specific environments, such as one containing hydrochloric or
Stainless steel is protected by its high content of alloying
elements, primarily chromium and nickel. Chromium is the most
important with respect to corrosion resistance, although the nickel
assists in allowing the chromium to do its job. The chromium forms
an oxide layer on the surface of the steel, which inhibits further
oxidation. This layer adheres extremely well to the metal
substrate, but it is essential that it remains intact, and must be
protected from various forms of damage.
If the surface chromium oxide layer becomes damaged, oxygen
present in water can partially reform the oxide layer, so it is
advisable to ensure that water is always fresh and well oxygenated.
Baths that will be out of use for an extended period should be
emptied, and all moisture should be wiped from the bottom of the
In some cases a brown layer may appear on the surface of a
stainless steel tank. In most of these cases this is not rust, but
it may be a surface deposit of minerals from the local water
supply, or ferrous particles or salts that have fallen into the
tank. These surface deposits can often be removed by using a
household cleaner such as Duraglit or Silvo metal polish.
How to prevent algae and bacteria?
Water baths provide the ideal environment for the growth of
micro-organisms. If left uncontrolled the growth of these organisms
can result in a range of serious problems and health risks from
The growth of algae on the surface of parts will cause
biofouling which will reduce performance.
Micro-organisms that produce acidic metabolic by-products can
cause bio-corrosion by depolarisation of metal surfaces.
There are a number of biocides available on the market.
How to clean your stainless steel tank, accessories and heater elements?
The cleaning of the stainless steel is important to maintain a good corrosion free finish. Stainless steel is easy to clean, washing with soap or mild detergent and warm water followed by a clear water rinse typically being adequate. Where stainless steel has become extremely dirty with signs of surface discolouration (perhaps following a period of neglect or misuse) then the following alternative methods of cleaning can be used.
Routine cleaning of light soiling
Soap, detergent or dilute (1%) ammonia solution in warm water. Apply with a clean sponge, soft cloth or soft-fibre brush then rinse in clean water and dry.
Satisfactory on most surfaces
Detergent and warm water, alternatively, hydrocarbon solvent
Proprietary spray-applied polishes available to clean and minimise remarking.
Oil and grease marks
Hydrocarbon solvents (methylated spirit, isopropyl alcohol or acetone)
Alkaline formulations are also available with surfactant e.g. 'D7' Polish.
Stubborn spots, stains & light discolouration. Water marking. Light rust staining
mild, non-scratching creams and polishes. Apply with soft cloth or soft sponge and rinse off residues with clean water and dry
Avoid cleaning pastes with abrasive additions. Suitable cream cleansers are available with soft calcium carbonate additions. Do not use chloride solutions.
Localised rust stains caused by carbon steel contamination
Proprietary gels, or 10% phosphoric acid solution (followed by ammonia and water rinses), or oxalic solution (followed by water rinse).
Small areas may be treated with a rubbing block comprising fine abrasive in a hard rubber or plastic filler. Carbon steel wool should not be used, nor should pads that have previously been used on carbon steel.
Adherent hard water scales
10-15 volume % solution of prosphoric acid. Use warm, neutralise with dilute ammonia solution, rinse with clean water and dry. Alternatively soak in a 25% vinegar solution and use a nylon brush to remove deposits.
Proprietary formulations available with surfactant additions. Take special care when using hydrochloric acid based mortar removers.
a) Non-scratching cream or polish
b) Nylon-type pad
a) Creams are suitable for most finishes, but only use on bright polished surfaces. Some slight scratching can be left.
b) Use on brushed and polished finishes along the grain.
Badly neglected surfaces with accumulated grime deposits
A fine, abrasive paste as used for car body refinishing, rinsed clean to remove all paste material and dried.
May brighten dull finishes. To avoid a patchy appearance, the whole surface may need to be treated.
Overtemperature cut-out operation
Many Grant temperature control products feature an adjustable
overtemperature cut-out. This safety feature may be valuable to the
user in a variety of ways:
- protection of delicate or expensive samples, where too high a
temperature would cause irreparable damage
- protection in the use of hazardous chemicals, by preventing the
flashpoint from being exceeded
- protection in the use of a remote probe - e.g. you have a tank
of water heated by heat-exchange coil, controlled at 50°C by a
remote probe; if the remote probe should become dislodged, the
cut-out set at 60°C would prevent the temperature controller
continuing to heat ad infinitum
The overtemperature cut-out cannot be set to a specific
temperature. It is a manual device, separate from the temperature
control electronics, in accordance with IEC61010. There are two
methods of setting the overtem-perature cut out:
Option 1 (quick method)
Set the temperature to the required value and leave the bath to
stabilise for a least 5 minutes after the set point has been
reached. Turn the control slowly anticlockwise, using a
screwdriver, until the alarm lamp comes on. Press the reset and at
the same time turn the control slowly clockwise until the alarm
lamp goes out. This gives an overtemperature trip point of
approximately 10 to 30 °C above set temperature.
Option 2 (precision method)
Set the temperature to the cut-out level required and leave the
bath to stabilise for a least 5 minutes after the set point has
been reached. Turn the control slowly anti-clockwise, using a
screwdriver, until the alarm lamp comes on. Allow the bath to cool,
then press the reset button and the unit will start. (The bath will
have to cool by 15 to 30º before the reset will work). This gives
an overtemperature trip point of the set value. Now decrease the
set temperature to the working set point.
Please note that products are dispatched from Grant with the
cut-out set at minimum in order to prevent any accidents before the
equipment has been properly set up. Depending on the operating
temperature required, the cut-out should initially be adjusted to
mid way, or even maximum, to allow the set temperature to
How to calculate the heat up time in a Grant water bath?
The heat up time for any Grant water bath can be determined from
the volume of liquid, the heater power, the temperature difference
and the heat capacity of the circulation systems. A simple formula
is used to calculate the heating time in the system.
t = V x Δ T x K
60 x W where:
t = Heating time (minutes)
V = Total liquid volume (litres L)
ΔT = Temperature difference (ºC)
K = Liquid heat capacity (J/L/ºC)
For water: K = 4200
For silicone oil: K = 1680
W = Heating power (Watt)
Example: A 12 litre bath containing water with a thermostat of
1400 W heating capacity can raise the temperature from +20ºC to
+56ºC in 21.6 mins.
t = 12 x (56 -20) x 4200 = 21.6 mins
60 x 1400
How to calculate the cool down time in a Grant low temperature circulator?
The cool down time can be calculated by using the same formula
t = V x ΔT x K
60 x W
However, as the temperature decreases the cooling power of the
circulator reduces, it is therefore necessary to estimate the
average cooling power across the temperature range to be
Cooling power Watts = Cooling power at lower temperature + Cooling
power at upper temperature/2
An example: For a 6 litre cooling bath with a cooling power of
1000W at +20°C and 500W at 5°C using water, the cool down time from
+20°C to 5°C will be:
t = 6 x 15 x 4200 = 8.4 mins
60 x (1000+500)/2
Everything you need to know about Grant Pumps Part 1
In Grant water baths and circulators the pump is used only to
circulate liquid to an external device, not for stirring within the
tank. An independent stirrer is used for circulation within the
tank, in order to achieve optimum temperature uniformity throughout
the working area.
Pump performance is specified in terms of flow rate and
pressure. The flow is normally quoted in litres/min. Pressure can
be quoted in mbar, metres (of water) head and psi (pounds per
The relationship between pressures is:-
One atmosphere is 1010 mbar, 10.3
metres of water or 14.6 psi
In accordance with DIN58966 part 1, maximum flow is measured
into an open vessel, through a horizontal pipe attached to the pump
outlet; the pipe must be at least 0.5 m in length.
The maximum pressure is determined from the maximum height to
which water can be pushed in a vertical tube connected to the pump
output. The head is measured in metres of water.
The measurements described above are maximum flow, which is at
zero head, and maximum pressure which is at zero flow. Neither of
these conditions is likely to be met in practice. In practical
terms, when a pump is connected to an external device, the actual
flow rate is determined by the bore (diameter) of the pipe, and any
friction in the pipes. Any kink or change in diameter in the pipe
will cause a reduction in flow rate.
To achieve maximum flow rate:
- use large bore (diameter) pipes
- avoid changes in bore size within the circuit
- ensure all interconnecting joints are smooth
- position pipes such that curves are smooth
The flow rate through the circuit can be adjusted by, for
example, installing an in-line tap. If the flow rate is too low,
- there are no foreign objects obstructing the flow through the
- the bore is as large as possible
- the pipes are as short as possible
- there are no kinks or tight bends
Everything you need to know about Grant pumps Part 2
The Grant product range incorporates a variety of pumps with differing specifications of both flow and pressure. These specifications, which will often be application specific, should therefore be carefully considered along with the additional parameters of, for example, temperature range, and heating or cooling power.
The table below indicates pump performance for the current range of equipment. Grant will however be pleased to discuss any requirements for pump specifications outside of the standard range.
|Bath type circulators:
||-201 to 150
||-471 to 150
||-471 to 200
|VTP1 accessory pump
||-472 to 150
|VTP2 accessory pump
||-472 to 150
||-201 to 100
||-301 to 100
|HQ pumps 3
||50 to 260
|RC350G, RC1400G & RC3000G
||-10 to 60
||-10 to 60
||-101 to 80
Grant equipment may be utilised in both open and closed circulation systems although each may necessitate different procedural practices.
When, for example, operating with an open load (open tank) it is generally more appropriate to utilise a closed circulator which prevents any overflow from the open tank.
In addition, if the circulator can be located at a lower level than the load, priming of the circulation pump is substantially easier (Note that a priming reservoir is available for Grant RC systems if required).
When using a closed load (for example a jacketed vessel) it is often more convenient to use an open circulator (bath system) where there is no need to prime the pump at all.
In all circumstances, as noted above, the specific application needs should be taken into account.
Can I use my Optima with Seawater?
Grant recommends using oxygenated water that is free from ions and minerals water with Optima heated circulators and circulating baths. Whilst sea water can be used with these products, you will find that the stainless steel will rust quicker, its use will invalidate the warranty.
At higher temperatures there will be a general increase in the rate of corrosion, if the submersed parts are rinsed with fresh water and stored dry when not in use this would help prolong the life.