The Spirax Sarco PA20 preamplifier is used with a Spirax Sarco capacitance probe to amplify a measured capacitance and convert it to a voltage output proportional to a liquid level. It consists of a tubular austenitic stainless steel body which screws onto the top of the probe, and has either a DIN 43650 connector with a Pg 11 cable gland, or, for the UL listed version, a ½" NPT conduit thread adaptor with four flying leads.

It is compatible with all Spirax Sarco voltage input controllers/transmitters and features three sensitivity settings selected to suit different probe lengths by wiring variations. The PA20 is a compact, rigidly mounted unit that can be removed or replaced without disturbing the probe, and requires no special maintenance.

Limiting Conditions

Parameter	Value
Maximum ambient temperature	70°C (158°F)
Minimum ambient temperature	0°C (32°F)
Maximum cable length	100 m

WARNING

Do not install the PA20 outdoors without additional weather protection. The preamplifier is intended for use in a Pollution Degree 3 environment.

Technical Data

Parameter	Value
Supply voltage	15 - 36 Vdc
Maximum power requirement	10 mA
Output volts range	1 - 6 Vdc
Output impedance	100 Ω
Weight	Approximately 0.25 kg

Materials

No	Part	Material
1	Body	Austenitic stainless steel ASTM A582 303
2	Cable connector	Polyamide (Glass filled)
3	Connector gasket	Silicone rubber
4	'O' ring	Nitrile rubber

Installation

The preamplifier may be fitted to the capacitance probe before or after installation in the boiler or tank. Always allow the PA20 to stabilise at its normal operating temperature for at least 15 minutes before commissioning the controller/transmitter.

• Fit the 'O' ring supplied with the unit to the base of the male thread on the capacitance probe. (Note: Both the probe and the PA20 are provided with an 'O' ring - fit only one of these).
• Fit the preamplifier to the probe and hand tighten only.

WARNING

Over-tightening by hand or use of a wrench will cause damage to the 'O' ring and may damage the preamplifier.

Wiring

Cabling should be installed in accordance with BS 6739 or local equivalent. The PA20 needs a supply of 15-35 Vdc at 10 mA maximum. Wiring should be carried out in 3-core, 1 mm² (18-16 AWG), high temperature screened cable.

WARNING

Do not install signal cables near high voltage cables or switchgear. Probe cabling must not use the same conduit / wiring trays as power cables. Suitable anti-static precautions must be observed.

Screen Connection

An earth current loop is created if a wire or screen is connected between two earth points at different potentials. The preamplifier and controller screen will only be connected to earth at one end.

• Ensure that the screen is connected to the earth terminal of the PA20 and to the common terminal of the controller.
• Ensure the common terminal of the controller is not internally earthed.
• The common terminal of the controller must only be earthed via the PA20.

WARNING

Do not connect the common terminal to an earth local to the controller. To do so may induce an earth current loop, which may reduce the performance or damage the product. Ensure resistance from probe body to pipework/boiler shell is less than 1 Ω.

Wiring Connections

Terminal	Colour	Function
Terminal 1	Brown	Supply
Terminal 2	Red	Output
Terminal 3	Orange	Sensitivity
Earth terminal	Black	Common

Sensitivity Settings

Terminal 3 provides three different sensitivity range settings depending on the way it is wired:

• Medium Sensitivity: (Up to 500 mm / 20" immersed length) Link 1 + 3 (Including all probes up to 550 mm length)
• Low Sensitivity: (Up to 1050 mm / 41" immersed length) No link
• Low Sensitivity (Longer): (Longer immersed lengths) Link 3 + Earth

Voltage Check

Before commissioning the controller, carry out the following voltage check:

1. Switch the controller on, then raise the water level to the highest level that it is required to measure.
2. Check that the dc voltage between terminal 2 (red) and the earth/common (black) terminal of the preamplifier is between 3 and 6 volts dc.
3. If it is more than 6 volts, select the next lowest sensitivity range by changing terminal 3 wiring.
4. If it is less than 3 volts, change to next highest sensitivity range, where available.
5. Check that the voltage is now between 3 and 6 volts.
6. If it is still more than 6 volts, select the lowest sensitivity range.

Ensure that high sensitivity is selected when probes of between 370 and 550 mm are fitted. Selecting medium or low sensitivity may cause the 'out of range' function to go to alarm.

Maintenance

No special maintenance is required. For probe body cleaning, use a cloth dampened with tap/de-ionised water or isopropyl alcohol. Use of other cleaning materials could damage the product and invalidate the warranty. Boiler water level controls require regular testing and inspection.