SKLW 1900 Series Skye Instruments Ltd., 21 Ddole Enterprise Park, Llandrindod Wells, Powys LD1 6DF UK Tel: +44 (0) 1597 824811 skyemail@skyeinstruments.com www.skyeinstruments.com Iss. 1.2
Skye Instruments Ltd. Skye Instruments is based in the UK and we are very proud to be celebrating being in business since 1983. Our products are designed and built in the UK. We have a very wide product base and our sensors & systems are used for plant & crop research; micro-climate, global climate change studies; environmental monitoring and controlled environment installations. Products include light sensors & systems, weather monitoring sensors, automatic weather stations, plant research systems, soil and water research systems. Feel free to contact us via our e-mail, or any of the methods below: Skye Instruments Ltd. @SkyeInstruments Skye.Instruments SkyeInstrumentsVideo Skye Instruments Ltd. Click on the icons to browse to the sites, or search for the usernames below. Have a Smartphone? Scan this QR code to access our website for more information about your product: Please be aware that the information in this manual was correct at time of issue, and should be 100% relevant to the accompanying product. We take great pride in our ever-evolving range of products, which means that sometimes the product may change slightly due to re-design. If you have any queries, please do not hesitate to contact our technical team by any of the methods above.
CONTENTS Page 1. INTRODUCTION 1 2. OPERATION 2 2.1 Sensing Surface...2 2.2 Cleaning...2 3. CONNECTIONS & POWER SUPPLY 3 3.1 SKLW 1900 Details...3 3.2 SKLW 1902 Details...4 4. OUTPUTS 5 4.1 Voltage Output...5 4.2 Current Output...5 4.3 Sensitivity Control...6 4.4 Connecting to Skye DaaHog...6
1. INTRODUCTION The Skye Surface Wetness sensor SKLW 1900 is designed to be precise, easy to use and simple to connect to recording devices and loggers. It is robust and corrosion free, incorporating a sensitive gold plated grid, designed to detect free water on any similarly exposed surface, e.g. leaves, windows, paint surface etc. The sensing grid is excited by AC voltage and droplets of water on this area, however fine, will change the impedance of the sensing grid. These measuring voltages on the sensor surface are very low, and when the sensor is wet they approach zero, thus the sensor is close to being an inert surface. As the electrical properties of natural water vary from place to place and even day to day, an exact calibration of wetness is not possible. As standard the sensor output is nominally 0-400 mv where 0mV = dry and 400 mv = wet. Other outputs are available at time of ordering, or can be selected on board by the user. Please see later in this manual for details. A sensitivity control is also available, the default factory setting is medium sensitivity, but this can be increased or decreased as preferred by the user. Please see later in this manual for details. The surface wetness sensor is supplied in a weatherproof housing sealed to IP65, which has suitable fixing points for mounting on a flat surface or at any desired angle. Alternatively, it can be used with a mounting arm SKM 226 which enables the sensor to be supported from a mast or boom between 25 and 50 mm diameter. The sensor is stood off from the mast by 330mm with the arm to avoid shadowing effects. 1
2. OPERATION 2.1 Sensing Surface The sensing surface attached to the top of the sensor housing is heavily gold plated and surrounded by epoxy resin. If this is kept fairly clean, it will give good, reproducible results. Some users feel that the surface should not be readily wetted by water and to this end a small amount of silicone polish may be applied. Alternatively, to emulate other surfaces such as stone etc., a layer of clean cloth may be used to give a similar wet delay. If experimenting with different coatings on the grid, remember that a waterproof coating will stop the sensor from working. In most applications the plain gold grid will give the best and most consistent results. 2.2 Cleaning It should be noted that to give consistent results, the sensor surface should be consistent itself. Fallout of ash, pollution and smoke particles etc., will affect the performance of the sensor. Depending on the degree of the pollution the sensor must be cleaned with according frequency to prevent build up of such contaminants. Only water and a soft cloth should be used. In extreme cases a little alcohol may help remove some oily deposits but no other solvents nor any abrasive cleaners should be used. Also avoid damage to the silicone rubber cover on the central grid connection. In general, cleaning every 2-4 weeks is all that should be needed in most cases. 2
3. CONNECTIONS & POWER SUPPLY (SKLW 1900) The SKLW 1900 Surface wetness sensor requires a D.C. power supply between 5 and 15 volts at a current of about 1.8 ma. If the current output option is used this will rise to approximately 13mA when the sensor is wet. 3.1 Connection Details SKLW 1900 The surface wetness sensor is fitted with a screened 4 core cable with the following connections: WIRE COLOUR FUNCTION Red 5-15 VDC Positive power supply Blue Power supply ground Grey * Analogue output ground and cable screen Green -400 / +400 mv dry / wet switching output Yellow ** 0-400 mv analogue output ** * Connected internally to blue wire and power supply ground ** If a nominal -2.5 / +2.5 V switching output is preferred, the yellow wire inside the surface wetness housing can be transferred to Pin 6 on the PCB. Please see below for details. 3
3.2 Connections & POWER SUPPLY (SKLW 1902) The SKLW 1902 Surface wetness sensor requires a D.C. power supply between 5 and 15 volts at a current of about 1.5 ma. If the current output option is used this will rise to approximately 13mA when the sensor is wet. Connection Details SKLW 1902 The surface wetness sensor is fitted with a screened 4 core cable with the following connections: WIRE COLOUR FUNCTION Red 5-15 VDC Positive power supply Blue Power supply ground Grey cable screen Green 0/2V dry / wet switching output Yellow * Signal/ Analogue Ground * Connected internally to blue wire and power supply ground 4
4. OUTPUTS 4.1 Voltage Output The 0-400 mv analogue output is fairly self explanatory. When the sensing grid is dry, the output is close to but slightly above zero (a baseline of several mv will be clearly seen depending on the sensitivity setting), and rises steadily depending on the amount of water on the sensor to a maximum of about 400mV. The switching output is between approximately -400mV and +400mV (or -2.5Vand +2.5V depending which output is chosen). The output remains at the negative value until the surface is half wet i.e., the analogue output has reached 200mV, then it switch to the positive output and remains thus until the surface dries and the analogue output again falls below 200mV. Please remember that these are units of millivolts only, there can be no precise calibration of wetness as suitable units do not exist. 4.2. Current Output The current switching output is activated by moving the shorting link on the PCB inside the sensor housing, shown as Current Output On/Off in Figure 1 from position OFF to position ON. When this is done, the current in the supply cable (Red / Blue) will be approximately 4mA when the sensor is dry, and will rise abruptly to approximately 13mA when the sensor is wet. The transition from low to high current is made in the same way and at the same points as the switching voltage outputs. When this option is selected the sensor will behave as an Industry standard 4-20 ma transducer that is loop powered (except that the supply range is 5-15 VDC). This current switching output is particularly useful if long lengths of cable are required. Low cost 2-core cable can be used since only the supply (red and blue wires) need be connected. The current output can be measured at the datalogger by inserting a 10 ohm resistor (or similar value) in series with the negative supply to the sensor, and measuring the voltage developed across it. With such a 10 ohm resistor, the output will change from approximately 40mV when dry to approximately 130mV when wet. No other connections need be made to the sensor when the current output is used, but care must be taken to ensure that the supply to the sensor is a minimum of 5 volts, i.e., allowance must be made for the voltage drop across the resistor plus the voltage drop in the cable. Please remember that these are units of millivolts or milliamps only, there can be no precise calibration of wetness as suitable units do not exist. It is advised that if the current output is not required then the jumper is left in the OFF position (factory default position). The voltage outputs will work normally at the same time as the current output but the current consumption will be needlessly high if only the voltage outputs are used. 5
4.3 Sensitivity Control There is a sensitivity control on the board which may be set according to local conditions etc. It is shown in Figure 1 as Sensitivity Set. To change it simply pull off the shorting link (pointed pliers may be helpful) and replace it in an alternative position. Keep the orientation as shown in Figure 1. In very polluted rain or fertigation the minimum sensitivity may be best, and in clean areas the maximum sensitivity may be needed. This control simply sets the level of water induced impedance change of the sensing grid that is needed to affect the outputs. Polluted rain will greatly reduce the impedance in small quantities, and pure water much less so. The medium position should be correct for most applications. 4.4 Connecting to Skye DataHog Logger The Skye DataHog2 has two types of voltage inputs, single ended voltage or differential voltage. The surface wetness sensor can be connected to either type. Wiring details are below: 4.4.1 SKLW 1900/I The suffix /I denotes the 5 pin Binder connector, wired to give a voltage (mv) output suitable for a single ended voltage socket labeled 0-2V. The Full Scale Value in the DataHog2 should be set to 5.0000 to give a range of 0 to 1 dry to wet. Please remember that these are arbitrary units, there can be no precise calibration of wetness as suitable units do not exist. DATAHOG PLUG FUNCTION WIRE COLOUR Pin 1 5V power supply to sensor Red Pin 2 0-400 mv analogue sensor output Yellow Pin 3 No connection ---- Pin 4 No connection ---- Pin 5 Ground Blue & cable screen 4.4.2 SKLW 1900/D/I The suffix /D/I denotes the 5 pin Binder connector, wired to give a voltage (mv) output suitable for a differential voltage socket labelled Diff Volts. DATAHOG PLUG FUNCTION WIRE COLOUR Pin 1 5V power supply to sensor Red Pin 2 No connection ---- Pin 3 * Link to Pin 5 ---- Pin 4 0-400 mv analogue sensor output Yellow Pin 5 * Ground Blue & cable screen * Pins 3 and 5 are linked in the DataHog plug. 6