Scientific FAQs

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 compromise.

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 ion-exchange cartridges.

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 de-ionised water.

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 sulphuric acids.

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 tank.

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 pathogenic bacteria.

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.

Requirement Suggested Method Comments

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

Fingerprints

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.

Heavy discolouration

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 stabilise.

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 as above.

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 cooled.

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 square inch).

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, check that:

  • there are no foreign objects obstructing the flow through the pipe
  • 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.

 Temperature
range
°C
Pump performance
Max. pressure
mbar
Max. flow
Litres/minute
Bath type circulators:
TC120 -201 to 150 210 16
TX150 -471 to 150 310 18
TXF200 -471 to 200 530 22 adjustable
VTP1 accessory pump -472 to 150 1000 9
VTP2 accessory pump -472 to 150 1650 12
LTC2 -201 to 100 210 16
LTC4 -301 to 100 310 18
HQ pumps 3 50 to 260 155 6
Closed circulators:
RC350G, RC1400G & RC3000G -10 to 60 1600 (@1L/min) 15
RC400G -10 to 60 620 (@1L/min) 12
FH16D -101 to 80 215 19

All pump performance data is for water @ 20ºC unless stated otherwise.
1 with accessory cooling
2
VTP pumps will transfer additional heat to the baths, so the minimal temperature achievable will be increased
3 using silicone oil
To convert mbar to PSI multiply 14.5 x 10-3
To convert mbar to metres H2O multiply 10.2 x 10-3

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.