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Reduce the possibility of a drowning in your swimming pool. If someone falls in, an excruciatingly loud siren sounds. Build this SWIMMING POOL ALARM by JOHN CLARKE S WIMMING POOLS are dangerous places, especially for toddlers – as the table above right chillingly shows. And the pool in your own back yard is certainly not exempt; in fact, statistics show that’s where more than half of all toddler drownings occur. Even while taking the photographs for this article, with mother millimetres out of shot and grandfather (Ross) in front taking the picture, 14-month-old Keira (who cannot swim) needed no prompting to attempt to get in the pool – not once but again and again. While swimming pools these days must be fenced off, there is always the possibility that a toddler will find a way in. That can be as simple as a gate not latching properly or a determined youngster climbing the fence. So while fences may appear to make a pool secure, they 14  Silicon Chip are never foolproof. A secondary defence, one that warns if someone falls into the pool, can literally be the difference between life and death. A way to add secondary safety is with a pool alarm. The type of pool alarm described here monitors the amount of pool water movement and sounds an alarm when this exceeds a preset level. Of course, wind can also create movement in the pool water – after all, that’s what makes waves in the ocean. The last thing you want is false alarms – remember the boy who cried “Wolf!”? The SILICON CHIP Pool Alarm can be set to a level which ignores typical wind movement but screams its head off when that level is exceeded – ie, someone falls in. siliconchip.com.au Here’s why your pool ne – some sobering facts ab eds this swimming pool alarm out toddler (0-5yrs) drowni ngs*: 41% occur in swimmi ng pools (virtually all in backyard pools) 60% occur in the toddle r’s own home 70% occur in metropolit an areas 40% occur during school hours (38% 3-6pm and 20% 6-9pm) 66% are boys 60% are either one or tw o years old * From NSW Water Safet y Task Force Report, 2002 FEATURES • Monitors wave height caused by any disturbance in the pool • Adjustable quiescent and alarm wave levels • Adjustable alarm period • Pushbutton switch for Hold/Monitor modes    – Hold mode gives visual but silent alarm (for testing and attended pool use)    – Monitor mode for visual and audible alarm (for unoccupied pool use) • Automatic return to Monitor mode after pool water settles • Adjustable return to Monitor period • Optional Set-to-Hold mode with pool turbulence preventing false alarms • Indications of Hold, Status and Alarm conditions • Weatherproof housing • Can drive two alarm sirens • Plugpack-powered • Suits all pools where the top water level is below the pool edge siliconchip.com.au siliconchip.com.au January 2008  15 Fig.1: the Pool Alarm in block diagram form. Pressure variations due to changes in the water level are detected by Sensor 1. Its weak output is amplified and then processed by the PIC microcontroller which controls the alarms and drives the status LEDs. Fig.2: this cross-section diagram shows the internal structure of the MPX-2010DP pressure sensor. The strain gauge varies its resistance according to the applied load. P1 & P2 are the two port openings. OK, let’s see how it works. Fig.1 shows the block diagram of the Pool Alarm. It uses a pressure sensor to detect sudden increases in water depth, as happens when an object falls into the pool creating waves. The unit is built in two sections, each in a weatherproof box. One houses the sensor while a second, which we have dubbed the Pool Alarm box, houses the PIC-controlled alarm circuit. The two are connected via a 4-way cable. While our photo shows the alarm box on the side of the pool, this would not be a typical installation. Rather, the Pool Alarm box would normally be located close to the filter box (where mains power is available) or more likely in the house, if the pool is reasonably close. The cable can be run underground across to the pool sensor box. Inside the sensor box is a pressure sensor. This measures the water pressure variations in the pool due to 16  Silicon Chip waves and sets off an alarm if these variations reach a preset level. The sensor box has a thin tube emerging from it. The box is placed so that the probe tip is about 60-90mm under water. This sensor box can be secured to a pool ladder or fixed to the side of the pool, as we have shown in our photos. The pool alarm is plugpack-powered so it needs to be located near to the mains. Complete safety from the mains power is provided firstly by the isolation given by the plugpack and secondly by the fact that there is no electrical contact with the water itself. Additional features Our Pool Alarm has several features worth noting. Most prominent on the main Pool Alarm box is a weatherproof pushbutton “Hold” switch. This is used to set the operating mode of the alarm. When powered up, the alarm is initially set to its normal monitor mode where it checks for pool wave movement. It takes about 10 seconds after power up to begin monitoring and during this time, the green “Hold” LED remains lit. After the 10 seconds, the LED flashes briefly every 1.5 seconds, indicating that the alarm is in the monitor mode. If the Pool Alarm senses that the pool wave movement is sufficient, it will sound the alarm. The alarm period can be varied from between zero and five minutes, with typical settings around the 30s to 3-minute range. During the alarm period, an Alarm LED flashes on and off at five times per second. The alarm siren can be stopped at any time by pressing the Hold switch. This will also stop the Alarm LED flashing. The Hold LED will also stop flashing but unlike the Alarm LED, it will remain constantly on. The Pool Alarm is now in the Hold mode where the alarm will not sound. The Alarm LED, however, will flash whenever wave movement is above the alarm threshold. The hold mode is used when the pool is in use. The degree of wave movement required to set off the alarm can be calibrated to suit your pool. This is done by dropping a weighted bucket into the pool (simulating a small child falling into the water) and pressing the alarm level switch (on the PC board). The Pool Alarm will monitor the wave movement over a 10s period and set up the level required for the alarm. During this calibration period, a “Status” LED will be lit. A second quiescent level can also be calibrated into the Pool Alarm. This level is the wave movement within the pool when no-one is in it but with a light breeze blowing and perhaps the filter running (normal filter running should not trigger the Pool Alarm). In practice, the level is calibrated under these conditions (ie, when a reasonable wind is blowing) by pressing the Quiescent Level calibration switch. The Pool Alarm then monitors wave movement for 10 seconds and stores the level. During this calibration period, the Status LED is lit. Quiescent level calibration allows the Pool Alarm to provide extra features. First, it allows the mode to return from the Hold to the monitor mode automatically. So when the pool is being used, the Hold switch is pressed to set the Pool Alarm to the siliconchip.com.au Here are the three main elements of the Pool Alarm. At left, actually shown upside-down, is the sensor with the open-ended tube emerging from a gland. Centre is the alarm proper, housed in a waterproof box so it can be mounted outside near the pool if you wish. At right is a commercial strobe/siren which is triggered when a large enough wave occurs in the pool, ie, when someone falls in! Hold mode so that the alarm will not sound. However, during this time, the Pool Alarm continues to monitor the wave movement. Typically, during pool use, the wave movement will continue to be over the quiescent level and the Pool Alarm will remain in the Hold mode. When the pool is not in use, wave movement within the pool will settle to below the quiescent level. In this case, the Pool Alarm will change from Hold mode to Monitor mode, after a preset period of “no pool” activity. The period of inactivity can be adjusted to allow for the way the pool is used. If the pool is often vacant for a short time before it is used again, the period can be made sufficiently long to prevent the return to Monitor happening in that time period. The adjustment range is from 1.25 - 75 minutes. One setting prevents the monitor return function. The change from Hold to Monitor and from Monitor to Hold can also be toggled with the Hold pushbutton switch. The Hold LED then flashes for Monitor and is continuously lit for the Hold mode. During the monitoring mode, windy siliconchip.com.au conditions may cause wave movement which could exceed the quiescent level but may be below the alarm level. The Pool Alarm has an option that can return it to the Hold mode if the quiescent level is exceeded for 30 seconds without the alarm level being exceeded. This feature is included to prevent false alarms from the siren in windy weather. The Pool Alarm will then return to the monitoring mode after the wave movement has reduced to below the quiescent level. Should the alarm sound and time out before the Hold switch is pressed, the alarm will return to Hold after the alarm period expires. The “return to hold” option can be enabled or disabled with a jumper pin selection. Just which option you select depends on your pool and whether it is subject to windy conditions. Protected pools may not need the “return to Hold” feature. This is a compromise between preventing false alarms and providing continuous pool protection. The sensor An air-pressure sensor, the MPX­ 2010DP manufactured by Freescale Semiconductor, is used to measure wave movement. Its internal arrangement is shown in Fig.2. The sensor comprises a strain gauge that provides a resistance variation with applied load. In this case, the load is the air pressure exerted on the gauge due to a tube inserted into the pool. The sensor is called a differential type because it measures the difference in pressure between one port and the other. For our application we use port 1, which has a silicone gel protective layer to prevent moisture affecting the strain gauge element. Port 2 is left disconnected and is vented to the inside of the enclosure. By the way, this is the same pressure sensor as used in the Water Tank Level Meter, currently described in this and past issues. Circuit description The circuit of the Pool Alarm is shown in Fig.2 and comprises the pressure sensor, an instrumentation amplifier and a PIC microcontroller, plus associated switches, LEDs and other components. Sensor 1 has differential outputs at pins 2 & 4. With the same pressure at January 2008  17 The Jaycar Cat. LA-5308 (left) and LA5256 (right) piezo sirens are ideal for use with the Pool Alarm. The LA-5308 includes a strobe as well. both ports, pins 2 & 4 are nominally at the same voltage; ie, 2.5V. If the pressure at port 1 increases compared to port 2, pin 2 rises and pin 4 falls. The change in voltage is quite small – around 1mV for a 1kPa pressure difference. However, the actual voltage change with typical wave movement is only around 200mV so we need to amplify this signal using instrumentation amplifier IC1. Since we are concerned with wave movements (ie, pressure variations) rather than the absolute pressure levels, the output from the sensor is AC-coupled via 1mF non-polarised capacitors to op amps IC1a & IC1b. The non-inverting inputs of IC1a & IC1b (pins 3 & 5 respectively) are biased via 470kW resistors to a +2.5V reference derived using two 2.2kW resistors and a 100mF capacitor. IC1a & IC1b are set up as non-inverting amplifiers with 39kW feedback resistors and a single 10W resistor between their inverting inputs. A 470pF capacitor across the 39kW resistors rolls off signal above about 8.7kHz and this prevents possible oscillation. The gains of IC1a & IC1b are each 1 + 39kW/10W, or close enough to 3900. The outputs of IC1a & IC1b are summed in differential amplifier IC1c which effectively adds the two outputs together. IC1c’s gain is 2 x 27kW/22kW or 2.45 (for the two outputs), so the overall gain is 3900 x 2.45 or 9555. Rain filtering IC1c’s output is filtered using a 2.2kW resistor and 10mF capacitor to remove high-frequency signals above 7.2Hz. This prevents detection of rain 18  Silicon Chip falling on the pool. IC1c also shifts the DC level of the output signal. This is done by feeding it with an offset voltage from IC1d, via the 27kW resistor from pin 14. IC1d obtains its reference voltage from a pulse width modulated (PWM) signal from PIC micro IC2. This signal swings from 0-5V at a frequency of 490Hz and has a duty cycle of about 50%. The PWM signal is filtered using a 220kW resistor and 10mF capacitor and fed to pin 12 of IC1d. The PWM signal is adjusted automatically during calibration so that IC1c’s output is at 2.5V when there is no signal from Sensor 1. Microcontroller functions IC2, the PIC16F88-I/P microcontroller, processes the signal from IC1c and drives the alarm and the Hold, Status and Alarm LEDs. IC2 also monitors inputs at RB1, RB2 and RB3 for the switches, the linking options at RA2, the RB4-RB7 inputs for BCD1 and the voltage at the wiper of trimpot VR1. Output RA7 drives the flashing Alarm LED while output RA6 drives transistors Q1 & Q2 which are the siren drivers. Trimpot VR1 is monitored by the AN4 input and its wiper voltage converted to a digital value from 0-255 for its 0-5V range, to give a timeout period in minutes. This value is placed in a counter that is decremented every 1.18s until it reaches zero and the alarm goes off. Hold switch S1 connects to the RB3 input which is normally held high (+5V) via an internal pull-up resistor. When S1 closes, IC2 responds by altering the mode from Hold to Monitor or from Monitor to Hold. Output RA1 drives the Hold LED via a 1kW resistor. Output RA0 drives Status LED 2 via a 1kW resistor. LED2 lights during the quiescent set and Alarm set procedures. If LED2 is flashing, it indicates levels that are over the quiescent setting. Switches S2 (Quiescent Set) and S3 (Alarm Set) are monitored by the RB1 and RB2 inputs. Pressing S2 or S3 starts the program in IC2. This monitors the AN3 input and calculates the voltage range encountered for a period of 10s. It does this by monitoring the AN3 input every 100ms and storing the level in memory. After sampling for 10s, it finds the minimum and maximum values and subtracts the minimum from the maximum to derive the span range. This value is then multiplied by 95% for the Alarm level and 105% for the Quiescent level. The lower alarm level provides for a small amount of leeway in pool movement to sound the alarm. The higher quiescent setting of 105% is so that the quiescent level for the pool will normally be less than this. The resulting values are then used to check for quiescent or alarm levels at the AN3 input. Whether to return to Hold from monitoring or not is selected with the linking at input RA2. RA2 is pulled high with the link in LK2 and low with the link in LK1. Rotary switch BCD1 selects the monitor return period. When BCD1 is in position 0, all the switches are open and the RB4-RB7 inputs are pulled high via internal pull-up resistors. This setting is for a “no-return to monitoring” from hold. Other settings of the BCD switch will pull at least one of the RB4-RB7 lines to ground via its common pin and select a time period as shown in Table 1. As already noted, the CCP1 output at pin 6 produces the PWM signal. It is initially preset so that the output of IC1c is nominally at +2.5V. However, because of manufacturing tolerances in IC1, the output could vary and so there is a set-up procedure (to set the output to 2.5V). Pressing switch S2 before power is applied to the circuit runs this procedure. The program within IC2 then adjusts the PWM percentage so that siliconchip.com.au siliconchip.com.au January 2008  19 Fig.3: the circuit uses Sensor 1 to sense pressure variations due to waves in the pool. The differential outputs from the sensor (pins 2 & 4) are then amplified by op amps IC1a-IC1c and fed to PIC microcontroller IC2. IC2 then processes the data and drives the sirens (via transistors Q1 & Q2) and status LEDs. value. Better still, use a digital multimeter to check each resistor before installing it. That done, install the PC stakes for test points TP1-TP3 and for the connections to S1, then fit the 3-way header for links LK1 and LK2. Next, install diodes D1-D3 and zener diode ZD1. IC1 can then be mounted but just insert and solder in the socket for IC2 at this stage. Both the IC and socket must be oriented correctly. The capacitors can go in next. Note that the electrolytic types must be oriented with the correct polarity, as shown. Now install transistors Q1, Q2 and regulator REG1, taking care not to mix them up, then install trimpot VR1 and the BCD switch (BCD1). The correct orientation for BCD1 is with the dot to the lower right. Switches S2 & S3 can be inserted next. These will only fit easily on the PC board with the correct orientation. Finally, the screw terminals can be inserted. Note that the 6-way terminals are made up of three 2-way terminals that are interconnected using the moulded dovetails that attach the pieces together. The 4-way terminals are made using two 2-way terminals. Fig.4: the parts layout for the Pool Alarm. Note the jumper pins (top centre) which must be set as per your requirements – see text. the reading at AN3 is at +2.5V. This process takes about 60s. The new PWM value is then stored and used every time the pool alarm is powered up. IC2 operates at 500kHz using an internal oscillator and is run from a 5V supply derived from regulator REG1. Construction The Pool Alarm is built on a PC board coded 03101081 and measuring 102 x 77mm. This is housed in an IP65 Table 1: Capacitor Code Value mF Code IEC Code EIA Code 100nF 0.1mF 100n 104 470pF   n/a 470p 471 sealed polycarbonate enclosure with a clear lid (115 x 90 x 55mm). Similarly, the pressure sensor is housed in an IP65 sealed ABS case measuring 64 x 58 x 35mm. The wiring details for the PC board are shown in Fig.4. Start the assembly by checking the PC board for any defects such as shorted tracks and breaks in the copper. You should also check the hole sizes. The holes for the corner mounting screws need to be 3mm in diameter, while the holes for the screw terminals need to be 1.2mm. Check also that the PC board will fit into the box. Install the single wire link and the resistors first. Use the resistor colour code table as a guide to finding each Pool alarm box Work can now be done on the main Pool Alarm Box. First, drill a hole in the lid for S1, plus holes in the box for the cable glands for the sensor and siren wiring. You will also need a hole for the DC panel socket. That done, place the PC board in the box and secure it with four M3 x 6mm screws. You can now attach the panel label to the lid, install switch S1 and insert the neoprene seal that is pressed into the lid surround. Note: a front-panel label can be downloaded from the SILICON CHIP website if necessary. Next, wire up the DC socket to the screw terminals and wire switch S1 Table 2: Resistor Colour Codes o o o o o o o o o No. 2 1 2 2 2 3 6 2 20  Silicon Chip Value 470kW 220kW 39kW 27kW 22kW 2.2kW 1kW 10W 4-Band Code (1%) yellow violet yellow brown red red yellow brown orange white orange brown red violet orange brown red red orange brown red red red brown brown black red brown brown black black brown 5-Band Code (1%) yellow violet black orange brown red red black orange brown orange white black red brown red violet black red brown red red black red brown red red black brown brown brown black black brown brown brown black black gold brown siliconchip.com.au to the two terminals on the PC board. That done, connect a 12V DC plugpack to the DC socket and apply power. Check that there is +5V between pins 11 & 4 on IC1 and at pins 5 & 14 on IC2’s socket. If the voltage is within the range of +4.75V to +5.25V, then power can be disconnected and IC2 installed in its socket. Apply power again and measure the voltage between TP1 (GND) and TP2. This should be about 2.5V but if this differs by 0.25V, you will need to run the set-up to adjust TP2 to sit at 2.5V. This needs to be done at a later stage when the pressure sensor is connected. This is the view inside the completed unit. Note the orientation of the BCD switch on the PC board. Sensor box assembly The full assembly details for the sensor box are shown in Fig.5. First, a baseplate is made up using sheet aluminium measuring 31 x 26mm. This is then fitted with two M3 x 20mm screws and M3 nuts for the sensor and attached to two central mounting posts in the box using M3 x 6mm screws. That done, the sensor can be slipped onto its mounting screws (notched pin to the left) and secured using two more M3 nuts. Note that the sensor is oriented so that port 1 is the one that is connected to the tubing. The wiring can now be connected to the four sensor pins, with the cable exiting through the adjacent end of the box via a cable gland. Take care with this wiring and make a note of the wire colour used to make each connection. If you are using flat 4-way cable, it will not form a watertight seal within the gland. Applying a small amount of silicone sealant around the wire where it passes through the gland can provide this waterproofing. The port 1 connection to the sensor consists of a 3mm PVC tube that’s covered with a 145mm length of metal tubing. This assembly is passed through the cable gland and clamped in place. The metal tube maintains an even temperature inside the vinyl tube, keeping it at the same temperature as the pool water. The metal tube also keeps the vinyl tubing straight and holds it in place at a fixed depth in the water. If you need to run the TP2 set-up, this can be done now. With power off, temporarily connect the sensor to the siliconchip.com.au alarm PC board terminals, taking care that everything is correct. Now press switch S2 and re-apply power. The Status LED should light and the TP2 voltage will be seen to vary and finally settle at about 2.5V after 60 seconds. The sensor box can now be mounted at the pool, with the probe tip immersed by about 60-90mm. The box can be attached to the side of the pool using brackets to the ladder or secured to the side of the pool using an underwater-curing epoxy such as Bostik Titan Bond Plus. Note that when using the box mounting holes, it has two mounting screw points that are effectively located outside of the box enclosure but are accessed with the lid off. The sensor box must be located so that it does not receive the force of the filter pump outlet. In addition, the filter outlet nozzle should be adjusted so that it does not cause turbulence at the top of the water. The wiring between the sensor box and Pool Alarm needs to be protected from damage by using conduit in areas where it is exposed. This conduit can be placed underground. You can use one or two sirens with the alarm. These can be located in different parts of your property to provide full sound coverage. It is best to have these disconnected until the Pool Alarm is calibrated. Calibration The calibration is carried out by using on-board switches S2 & S3 to January 2008  21 Fig.5 (left): this diagram shows the construction details for the sensor unit. Note that the unit is offset to the left inside the case, so that port P1 of the pressure sensor lines up with the adjacent cable gland. Take care with the wiring – pin 1 of the pressure sensor is the lead with a notch in it. The photo at right shows the completed unit. The PVC tubing is held straight by the thin metal tube. This is slid over the tube and through the cable gland right up to port1, before the gland is tightened down. 22  Silicon Chip siliconchip.com.au Table 3: Setting The Alarm Period VR1 Setting (measured between TP1 & TP3) Alarm Period 0.5V 30 seconds 1.0V 1 minute 1.5V 1.5 minutes 2.0V 2 minutes 2.5V 2.5 minutes 3.0V 3 minutes 3.5V 3.5 minutes 4.0V 4 minutes 5.0V 5 minutes Table 4: Monitor Return Settings BCD Setting Return Period 0 No return 1 1.25 minute 2 2 minutes 3 3 minutes 4 4 minutes 5 5 minutes 6 6 minutes 7 7 minutes 8 8 minutes 9 9 minutes A 10 minutes B 20 minutes C 30 minutes D 45 minutes E 60 minutes F 75 minutes set the water movement levels that correspond to your pool. For the alarm level, you need to simulate pool water movement when a small child falls into the water. To do this, fill a 10-12 litre bucket with water about one-third full and drop the bucket from about 30mm above the pool water into the pool. Press S3 (Alarm Set) to record the movement. The status LED will light during this procedure. Note that the calibration may not be successful if the wave from the bucket does not reach the sensor during the 10s calibration period. If it doesn’t calibrate, try again (after the pool water has settled) and wait until the siliconchip.com.au Parts List – Pool Alarm 1 PC board, code 03101081, 102 x 77mm 1 IP65 sealed polycarbonate enclosure with clear lid, 115 x 90 x 55mm (Jaycar HB-6246 or equivalent) 1 IP65 sealed ABS case, 64 x 58 x 35mm 1 sheet of 18g aluminium, 26 x 31mm 1 12V 400mA DC adaptor 1 piezo siren (Jaycar Cat. LA5308 or LA5256) 1 piezo siren as above (optional) 1 MPX2010DP Freescale Semiconductor pressure sensor (Jaycar ZD-1904 or equivalent) (Sensor1) 1 SPST waterproof momentary switch (Jaycar SP-0732 or equivalent) (S1) 2 SPST micro tactile switches (Jaycar SP-0600 or equivalent) (S2,S3) 1 BCD DIL rotary switch (0-F) (Jaycar SR-1220 or equivalent) (BCD1) 5 2-way PC-mount screw terminals with 5mm or 5.08mm spacing 1 2.5mm DC panel socket 4 3-6.5mm diameter IP68 waterproof cable glands 1 2-way pin header, 2.54mm spacing 1 18-pin DIL IC socket 2 M3 x 20mm screws 6 M3 x 6mm screws 4 M3 nuts 3 PC stakes wave caused by the bucket has almost reached the sensor before pressing S3. You will need to try this at different points around the pool. Quiescent alarm calibration should be done with the filter pump operating and with a typical breeze blowing across the pool. Press S2 (the Quiescent Set switch) during these events to record the water movement levels. The Status LED will light during this time and extinguish after 10 seconds. Note that this quiescent level must be less than the alarm level in order for the return to monitor function and for the set to hold feature to work. Now set the alarm period using VR1, noting that the voltage at TP3 will 1 150mm length of medium duty hookup wire 1 30mm length of 0.8mm tinned copper wire 1 length of 2-pair (4-wire) telephone sheathed cable or 4-core alarm cable (to suit) 2 100mm cable ties 1 150mm length of 3mm ID (5mm OD) vinyl tube 1 145mm length of 5mm ID (6mm OD) metal tubing 1 10kW horizontal trimpot (code 103) (VR1) Semiconductors 1 LMC6064IN quad op amp (IC1) 1 PIC16F88-I/P microcontroller programmed with “Pool Alarm. hex” (IC2) 2 BC337 NPN transistors (Q1,Q2) 3 1N4004 1A diodes (D1-D3) 1 16V 1W zener diode (ZD1) 1 5mm green LED (LED1) 1 5mm red LED (LED2,LED3) Capacitors 2 470mF 16V PC electrolytic 5 100mF 16V PC electrolytic 3 10mF 16V PC electrolytic 2 1mF NP electrolytic 1 100nF MKT polyester 2 470pF ceramic Resistors (0.25W 1%) 2 470kW 2 22kW 1 220kW 3 2.2kW 2 39kW 6 1kW 2 27kW 2 10W show the timeout. A 1V setting gives a 1-minute alarm while 2V gives two minutes and a 5V setting provides a 5-minute alarm – see Table 3. Next, select whether you want the “return to hold” feature with LK1 or LK2 and set BCD1 for the required return to monitor period – see Table 4. If “return to monitor” is used (for settings other than 0), then select the setting that best suits your pool use. If you tend to vacate the pool area after swimming, then the return to monitor period can be set to a short period. If you tend to swim and then sunbake, then a longer period may be necessary to prevent the pool alarm sounding SC when you return for a swim. January 2008  23