LIQUID LEVEL SENSING – USING REED SWITCH TECHNOLOGY
COTO CT 10 and Coto CT 05
When measuring discrete levels of liquids,
such as automobile brake fluid, reed switch
technology is simple, inexpensive, and
reliable. A magnet mounted on a float closes
an adjacent magnetic reed switch as the
magnet approaches. Typically, the reed switch
is mounted in a plastic or non-magnetic metal
tube, and a ring magnet mounted on a float rides up and down the
tube on the liquid surface (Fig. 1). A ring magnet is a good solution
since as the magnet sweeps by the reed switch, only one closure
occurs as the plane of the magnet lines up with the contact gap of the
switch. Other types of magnets can cause two or even three separate
closures, causing potential problems in the software used to monitor
the level sensor. Also note that if the wires to the sensor are severed,
the system registers low. It is therefore fail-safe.
This method is frequently used in automobiles for monitoring brake
fluid levels. It can be modified to register multiple levels by incorporating
a chain of reed switches. By using three reed switches instead of
one, for example, the reed switch level sensor can register a RED alert –
“stop and check brake fluid immediately,” ORANGE – “check brake fluid level soon,” and GREEN – “brake fluid level OK.”
Discrete voltage levels corresponding to each switch closure can be
developed using a resistor ladder. A typical configuration for a
4-switch system is shown in Figure 2. The resistor network can be
extended to any number of levels depending on the depth of the tank
and the switch separation. Thus deep tanks can be monitored with
many discrete levels. Schemes can also be devised to “tune” the
resistor values to suit tanks with spherical or other varying cross
sections.
Choice of reed switch
Since bare glass reed switches are fragile, a better choice is a plastic
encapsulated reed switch such as Coto Technology’s CT05 or CT10
series. They consist of a high-quality reed switch encapsulated in a
hard epoxy resin, which effectively “splints” the reed switch, protecting
it from shock, vibration and environmental damage. As such, they
are ideal for under-hood automobile operation. The CT05 series
switches are surface mount devices 6.4 mm long (9.55 mm including
leads), and are also available in a slightly shorter J-bend format. The
CT10 series is 12.8 mm long (16.4 mm with leads).
Apart from the advantages of increased ruggedness, the surface
mount format makes it simple to attach the switches to printed circuit
boards or flex circuits using pick-and-place equipment. While
designing the CT05 or CT10
switches into a sensor, note that every switch that leaves the Coto factory is tested for
operate and release sensitivity AFTER lead cropping and
encapsulation, ensuring that these sensitivities are known
accurately. In contrast, other reed switch manufacturers test the operate and release
sensitivity before lead cropping and molding, and can only
make a rough estimate of the sensitivity of the shipped product.
Relationship between Amp-Turn (AT) operate and release
sensitivity, and required operate and release field
The CT05 and CT10 series reed switches are specified in terms of the
range of operate AT. For example, the CT10-1030-G1 switch has a
range from 10 to 30 AT. In other words, Coto warrants that a switch
randomly selected from a batch of CT10-1030-G1 switches will operate
in a field of between 10 amp-turns and 30 amp-turns measured in
Coto’s factory test coil. (The specification for this coil is available on
request.) However, most CT series switch users expect to use a
permanent magnet to operate the switch. What magnetic field
strength is needed to operate a 10 AT switch? There is a simple rule of
thumb to figure this. It is: 4 AT = 1mT. In other words, divide the
switch AT by 4 to get the required field strength in milliTeslas (mT). So
the 10 AT switch will operate when immersed in a field of approximately
10/4 = 2.5 mT (25 Gauss in the older CGS units.) Correspondingly,
a 30 AT switch will close at 7.5 mT.
For a deeper look into this topic and for additional level sensor design considerations,
please see Coto’s full Application Note AN1602-1 entitled “Liquid Level Sensing Using
Reed Switch Technology”.
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COTO TECHNOLOGY, INC | www.cotorelay.com | OPPLIQLEV0316®
Data sheet CT 05 / Data sheet CT10 / Application Note Complete Article
