A guide to selecting the right float switch for your application
When selecting a float switch for a liquid level sensing application, a number of factors need to be considered, including float switch and gasket materials, physical arrangement, electrical ratings and cable type, says Simon Dear of Cynergy3 Components.
Many industrial processes require devices that are able to sense the level of liquid stored within various types of tank or chamber. The signals from these devices may be used to control the production process or to provide indication of the status.
One of the most reliable, well-proven technologies for liquid level sensing is a float switch. This type of switch comprises a magnet contained within a float, as well as a magnetic reed switch contained within a fixed housing. The movement of the float, due to the changing liquid level, will cause the reed switch to operate (i.e. close or open) at a particular level. This tried and trusted technology is based on a relatively simple design that offers long term reliability without the need for the user to calibrate the switch.
Whilst there are different methods available for selecting the right float switch for a particular application, the main factors to consider include the following:
Physical arrangement and style
The choice of styles that may be suitable for an application will depend on the physical arrangement of the tank, the available mounting positions and whether access is available to the inside of the tank. The main styles are horizontal/side mounting and vertical mounting. The horizontal/side mounting type normally has a fixed housing, which passes through the sidewall of a tank, with a hinged float attached to the fixed housing. Vertical mounting types normally have a fixed vertical stem, which is installed through the top or bottom of a tank, along which slides a cylindrical float.
Another important consideration is whether a build up of deposits from the liquid on the float body is likely to occur. These deposits can, over a period of time, accumulate to such an extent that the float switch can fail to operate. Whilst particular types of float switch have been developed to limit the effects of this build up, the experience and knowledge of the switch manufacturer is also critical here.
It is critical to select a float switch that is constructed from the right materials that are compatible with the liquids and temperatures of the particular application. Component damage as a result of incorrect materials selection can ultimately cause failure of a float switch, which may have severe consequences. Typical float switch materials include:
Nylon: suitable for many oils, diesel, organic chemicals and MEK-based printing inks.
Polypropylene (PP): suitable for many acids and alkali, detergents, inorganic and organic chemicals, oils and water.
Polyphenylene sulphide (PPS): suitable for many of the more aggressive chemicals and higher process temperatures, up to 120ºC.
Buna/Nitrophyl: suitable for many oils, diesel, petrol and water (non-potable applications).
D300 foam (PVC): suitable for most hydraulic oils and many chemical solutions.
Stainless steel: suitable for most medical and food applications, chemicals, hydraulic fluids, fuel oils and applications with process temperatures up to 135ºC.
Selection of the most suitable materials for both float switch and gasket can be made by referring to a ‘Chemical Compatibility’ table. These tables provide a good indication of the suitability of the various float switch materials in a wide range of liquids. For some process liquids, it may be necessary to obtain a sample float switch in order to test the compatibility.
It is important to fully understand the nature of the load that needs to be switched and to ensure that the float switch is capable of handling this load. The electrical ratings, which most manufacturers provide in their float switch specifications, are for purely resistive loads. Any loads that have either inductive or capacitive components should have the appropriate contact protection measure applied.
In applications where aggressive liquids may spill onto external wiring, it is important to specify particular materials for the cables used to connect to the float switches. There are standard, UL-approved cable types for the various float switches, as well as high temperature, low smoke zero halogen (LSZH) and other specialised materials.
Custom engineered float switches
The operating environment is critical to the choice of float switch. A water tank for an industrial process may only require a simple plastic float switch. However, if the application is in a hazardous area, for example, a petrochemical storage tank where flammable gases, vapours or dust are present, a stainless steel, explosion-proof float switch will be required.
Over the years, Cynergy3 has custom engineered many variants of its float switches to match particular customer requirements in a wide range if industry sectors including oil, gas & petrochemicals; food and beverage; chemicals and pharmaceuticals; water and wastewater; and process manufacturing. These include float switches for all types of environment, including industrial process control, safe areas, intrinsically safe and hazardous areas (ATEX-certified), as well as WRAS-approved switches for drinking water applications.
The principal function of a high voltage reed relay is to isolate as high a voltage as possible, Cynergy3 achieve this through the use of evacuated reed switches. These are available with Tungsten or Rhodium contacts, depending on the switching requirements of the applications. These relays are intended for use in DC or AC (50Hz to 60Hz) applications.
RF Reed Relays
Cynergy3's range of RF reed relays are carefully designed to provide minimal RF signal loss and low contact resistance. This is achieved through the use of copper plated reed switches, with Rhodium contacts, packaged in carefully designed coil assemblies. These relays are designed for use in the HF band and are typically used in HF amplifiers and aerial tuning units (ATU's). The standard performance measure of an RF reed relay, adopted by Cynergy3, is Effective Series Resistance (ESR); this is because these relays are very often used in high “Q” circuits, which experience high currents throughout the frequency range, as found in certain ATU circuits.
The following are some general guidelines, which should be considered when working and designing with Cynergy3's range of reed relays. It should be noted that all data is presented, throughout, at an assumed ambient temperature of 20°C, unless otherwise stated.
Coil Temperature Compensation Coil resistance, as well as 'Must Operate' & 'Must Release' voltages, is quoted at 20°C, unless otherwise stated. It should be noted that these parameters will change at a rate of 0.4% per °C change in temperature.
It is recommended that all relay coils be suppressed, by fitting a reverse biased diode across the relay coil; this is essential for latching relay types. Operate and release times for the relays are quoted with coil suppression fitted (unless otherwise stated).
Coil: RF Screening
The benefit of RF screening is that it reduces the RF through losses (ESR) of the relay and extends the relay performance, at high frequencies and high ambient temperatures.
The RF reed relays offered by Cynergy3 have various screening options:
Unscreened relays maybe used in RF circuits where the highest currents occur at the lower frequency bands e.g. 2MHz.
Partially screened relays will offer extended current carry capability at elevated temperatures and frequencies up to 30MHz, over unscreened variants, due to lower ESR.
Fully screened relays offer the ultimate in low loss performance, resulting in the lowest ESR figures, significantly reducing the effects of RF coil heating, and maximising current carry performance at elevated temperatures and frequencies.
Figure 2: Effects of screening on ESR
Coil: Magnetic Screening
A magnetic field is produced, when power is applied to the coil of a reed relay. Ferromagnetic screening can be used to contain this magnetic field, so allowing closer placement of reed relays on a PCB. It should be noted that many of the RF relays do not employ magnetic screening, as this can adversely affect the ESR; in this case refer to figure 3, for a suitable layout for RF relays without magnetic screening.
Figure 3: Mounting Configurations to Minimise Magnetic Interference (Note Orientation)
Figure 4 shows a suitable arrangement for 'D' & 'S' type high voltage relays.
Note that the 'D' Series has magnetic screening fitted as standard.
Figure 4: Recommended Mounting for "S" and "D" Series High Voltage Reed Relays
SPNO (FORM A): Normally open - energise to close contacts SPNC (FORM B): Normally closed – energise to open contacts Latching: Contact is bistable – energise “SET” coil to close contacts; energise “RESET” coil to open contacts.
Contact Isolation Voltage
The isolation voltages quoted in the data are for DC or AC peak. The two may be considered directly equivalent, at mains frequency (i.e. 50 or 60Hz).
High voltage breakdown mechanisms at RF differ from those at DC. In general breakdown across the contacts occurs at a higher RF peak voltage than DC. Conversely, breakdown outside the switch (i.e. switch to coil or screen), caused by surface tracking, can occur more readily at high frequency than at DC. Verification of any particular voltage or frequency combination, within the HF band (1–30MHz), can be undertaken upon request.
Contact Switching Power
The switching power, when quoted, will be for a resistive load. It should be noted that any combination of voltage and current can be switched, provided they do not exceed the stated switching maximums, for either parameter or the power rating of the contact. It should be noted that relays, used in a power switching application, will experience reduced operating life; it is recommended that sales be contacted for advice on specific applications.
Contact Material – Rhodium vs Tungsten
Rhodium offers superior low contact resistance, which, coupled with Cynergy3's copper plated reed switch technology, enables Cynergy3 to produce very low loss RF reed relays, with exceptional current carry performance. Rhodium contacts are offered, in our 'D' series range, for high voltage applications, where low contact resistance and good current carry performance are required, provided the switching voltage is below 1000 volts DC or AC peak.
Tungsten contacts are used exclusively for our high voltage 'D' series range, where they are offered as high voltage switching contact able to switch voltages up to 10kV DC or AC peak at very low current. Tungsten is a good general purpose switching contact material but the higher contact resistance means it is not well suited for RF applications.
PROCESSING & HANDLING
Cynergy3's reed relays are high performance products and the materials and methods of construction are significant factors in achieving performance specifications. The following guidelines should therefore be followed when adopting assembly, soldering and cleaning processes.
Many of the RF reed relay designs are of open frame construction to achieve optimum RF performance, these designs need to be handled with due care to avoid damaging the exposed coil, contact and screen connections.
Cynergy3 recommend the following wave flow soldering profile: 250oC ±10oC, immersion time 3 seconds.
Alternatively, for manual soldering iron operations: 350oC ±10oC, application time 3 seconds. Maximum exposure time 10 seconds.
Cynergy3 propose that either low residue fluxes are used in the soldering process (to eliminate the need for cleaning), or that the reed relays are fitted onto the PCB after the cleaning process has taken place.
FRD12000/13000 Series: Electrical connections to the relay contacts on the FRD12000/13000 Series relays are made directly onto the reed switch leadout. Care must be taken when hand soldering to the contact terminals, as physical and thermal shocks can damage the glass to metal seals. It is recommended that a thermal shunt (Aluminium Clamp) be clamped to the reed switch lead out adjacent to the glass seal; this will reduce the chance of breakages due to thermal shocks. Alternatively relays may be warmed prior to soldering.
Cynergy3 relays are not hermetically sealed (unless stated otherwise) and as such are not suitable to aqueous cleaning solutions or processes. If after the soldering process the customer wishes to clean the PCB's containing Cynergy3 relays then the use of post operative cleaners such as IPA or HCFC based solvents with low pressure brush applicators is recommended. Please contact sales for further information on the suitability of various cleaning solutions.
Many of the designs manufactured by Cynergy3 Components Ltd are used in Military and Defence systems therefore many of the relay designs have been tested for Bump, Shock and Vibration endurance to the following standards:-
Please contact sales for any particular requirements. Alternatively Cynergy3 can arrange product testing to other MIL, DEFSTAN or IEC standards with various third party test houses for specific applications.
All chemical compatibility ratings are for reference only, and are based on the data available. Trials should always be carried out in any cases of doubt, using conditions which closely match the actual application.
Commercial and proprietary fluids may contain additives to improve end use characteristics.
Whilst the material may be compatible with the base chemical, the additives can sometimes have an adverse effect.
Trials should be conducted in cases of doubt.
A to E are cold compatibility ratings.
A is best, B is likely to be compatible, C & D should be tested, E is incompatible. 1 to 5 are hot compatibility ratings.
1 is best, 2 is likely to be compatible, 3 & 4 should be tested, 5 is incompatible.
"-" means no data available - refer to Cynergy3 Components Engineering or Quality Department.
Red denotes float body materials, blue denotes gasket materials.
Chemical compatibility data assumes that no fluid comes into contact with either the locking nut (where applicable), the wire, or the potting material used to secure the switch element.
Gaskets can sometimes be used even where their rating is poor, if they are not in permanent contact with the fluid. Consideration should be given to the effect of any vapour on the material. Trials should be conducted in cases of doubt.
Reed Switch contacts, although rugged, can require protection from certain loads
Reed switches can exhibit a high degree of reliability. Their contacts are sealed in an ultra clean environment at a predetermined pressure or vacuum, specifically chosen for their intended areas of application. The contact material, the size of the switch and the mechanical form may also be developed with some particular application in mind. Reed switches can be used at low level in circuits passing pico and micro amps, with associated switch insulation resistance in the order of 10E14 if required, through to high voltage versions capable of switching 15kV or power versions that can handle several amps. Careful attention should be paid to the manufacturers’ published technical literature, to ensure the correct switch type and sensitivity range within that switch type is selected. It is assumed, in this application note, that the most suitable switch has been selected for the intended application and that the remaining problems are associated with the external factors that influence switch life, particularly external load applications.
Capacitive loads: Long wires
Capacitance across a reed switch can dramatically reduce life and even cause early contact sticking. This can occur even with low values of capacitance associated with circuit wiring. Damaging current surges occur at make or closure, in capacitive circuits, that will exceed the rating of the contact. Measures should be taken to reduce the inrush current to a minimum. The most common solution is to fit a resistor in series with the switch.
It is important to note, with lamp loads, that cold filaments have a resistance approximately 10 times smaller than already glowing filaments. This means that, when turned on, the lamp filament draws a current 10 times greater than when hot. This high inrush current can be reduced to an acceptable level, through the use of a current limiting resistor in series with the switch.
Inductive Loads: Relays, Solenoids and Motors
Many applications involve interfacing the main load through a relay system, in order to handle larger values of voltage or current. Coil operated relays or solenoid control valves possess considerable inductive values. The significant factor to guard against is the release of energy, temporarily stored in the coil, at the time the reed switch opens. The collapsing magnetic field, when an inductive circuit is broken, causes a voltage transient or back EMF. This will eventually lead to complete welding of the switch.
There is a choice of devices that can be used for contact suppression that includes diodes, RC snubber circuit, metal oxide varistors or transient voltage suppression diode. The two most common methods of suppression are diode and RC (resistor capacitor) suppression.
Diode suppression (DC circuits only)
The diode is excellent for removing the inductive voltage spike, as the back EMF is directed through the diode. Contact erosion is reduced to a minimum but, when placed across a relay coil, release time of the relay contacts will be increased by several milliseconds while the stored energy is being dissipated. The diode chosen must have a forward current rating equal to, or greater than, the steady state current of the circuit and the diode must be connected cathode to positive.
RC suppression can be used of AC or DC circuits. The associated release time of the RC suppressed relay contacts is faster, although not so efficient in removing the inductive transient. The calculated value of R must be checked, to ensure that the capacitive surge is reduced to a safe value for the type of switch in question. A resistor and capacitor, connected in parallel with the switch, as shown below, is recommended. Transient voltage suppressors or varistors may also be used to dissipate the transient and protect the contacts.
The circuit can appear as a high inductance and low resistance, at the moment of switch closure. Surge current during starting could be some 5 to 15 times the steady value and, depending on the type of motor and starting characteristics, could last for several seconds in the worst case.
The nomagraph below can be used to calculate the RC values requied to achieve the necessary contact protection.
This diagram is for the circuit to empty a tank, using two normally open float switches and a two pole changeover relay.
The bottom switch will be closed provided the liquid is above that switch point. The liquid rises until the top float switch closes and energises the relay. One set of relay contacts connects the pump to the supply and the other maintains the relay on-state, while the level falls towards the bottom switch.
The relay will be held in the energised state until the bottom switch opens, so releasing the relay and cutting off the supply to the pump.
It is advisable to fit a 375V bi-directional Transil across the relay coil terminals, if the control voltage is 240V ac. A 1A Silicon Diode (IN4006) should be connected across the coil terminals, if the control voltage is DC (see diagram below). These measures are to avoid back EMF, induced over voltage transients, which can cause damage to the reedswitch contacts.
This circuit can also be used for controlling the emptying of a tank, by using normally open float switches and swapping the connections to the two float switches on the diagram above.