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2. The second piston that is available is a PTFE Encapsulated one. The piston is a magnet that has PTFE moulded around it and
then machined to the appropriate configuration. These pistons are primarily used in PTFE flow switches and also in other flow
switch bodies ( typically 316SS and Acrylic bodies ) where customer prefer a piston that does not have epoxy in the fluid path;
as well as a piston that is impervious to aggressive fluids and gases. This piston is highly recommended for medical applications.
Hysteresis on these pistons does tend to be slightly higher (10 to 15% ) than metal piston due to frictional effects, weight,
and surface adhesion considerations. Prior to selecting this piston, fluid temperatures, and fluid compatibility with PTFE must
be taken into account because certain aggressive chemicals at specific temperatures tend to swell PTFE causing the piston to
change shape resulting in failure of the product. Stainless or PTFE retaining rings can be used with this piston.
3. The third piston that is available is a Special All-Metal piston with no epoxy ( only available in 316SS ). This piston is fabricated
in a proprietary process with only one weld seam ( leak tested ) which presents an all 316SS surface to the fluid path.
This piston is recommended for those applications where the piston could experience a lot of cycling wear. This piston has
been tested to 250,000 cycles at 125 psi. Stainless Steel retaining rings are recommended for this piston type for low pressure
applications and an orifice disc ( See Universal Mounting Diagram ) is recommended for high pressure ( 125 psi ) applications.
SPST vs SPDT
The standard unit is provided with a
SPST N.O.
(N.O = Normally open at rest) dry reed switch. Increasing flow past point will close
(activate) the switch. Decrease flow will open (de-activate) the switch).
SPST NC.
(N.C = Normally close at rest) models are closed at rest. Increasing flow pass set point opens the switch and decreasing
flow to set point will close the switch( Typically used to detect loss of flow condition in overall systems)
SPDT
(Single Pole Double Throw) switches have both N.O. & N.C. configurations by using a common pole along with two
additional poles to achieve this dual capability.
Conversion from N.O. (Normally Open) to N.C. (Normally closed)
1. Loosen allen set screws (4 - 40) on face of body. These are the set screws holding the reed switch in place.
2. Move the reed switch downwards until it shows a closed contact on the multi-meter.
3. Tighten (4 - 40) allen set screws on face of the body.
4. Cycle until on and off by turning upside down to simulate flow. This may necessitate some refinement of the exact location
of the reed switch as determined in step 2.
Precautions during installation
1. Avoid any foreign particles during installation.
2. Large metallic and/or magnetic fields may affect the principle of operation of these units. if disturbance is detected,
magnetic shielding may be necessary to ensure correct operation of the flow switch.
3. During installation of explosion proof versions carefully screw in the conduit connector for the electrical leads piping so
as to avoid damaging the reed switch leads or reed switch capsule which may lead to shorting the switch installation. 6 (Six
threads) have been provided for installation.
4. Electrical ratings etc. have been discussed later in this bulletin. it is advisable to keep within those ratings as excessive
power will FRY the switch.
Electrical Protection/Ratings
The electrical Reed Switch is rated for several million cycles and should rarely require replacement. Most switch failures are
the result of overloading. DO NOT EXCEED RATED CAPACITY, as this will cook the REED SWITCH, Catalogue ratings are based
upon D.C. resistive loads, however A.C. loads are also commonly switched.
Inductive load switching- Unless appropriate contact protection is provided with either a diode or R.C. network, inductive load
switching will cause the contacts to stick.
Capacitive load switching - The static capacitance in cables may cause permanent or intermitter contact sticking. Contacts may
be protected by a series surge protector of 0.5 to 5 mAH, depending on the load. in some instances, a 10-500 ohm resistor can
replace the surge suppressor.
Cleaning
Cleaning the flow switch is easily accomplished without removing from the line. By unscrewing the clean out plug, the
magnetic piston may be removed and the flow passage flushed or cleaned. Care should be taken to thoroughly clean the
piston before replacing ( replace the piston as shown in see page 3 figure 2. if the piston is replaced incorrectly, incorrect
operation will result).
1. A magnet may be used to remove the magnetic piston.
2. PTFE end plugs (on PTFE models) should not be over tightened.