Gypsum Block Soil Sensors with DataHog Interface

Transcription

1 Gypsum Block Soil Sensors with DataHog Interface SKYE INSTRUMENTS LIMITED 21, Ddole Enterprise Park Llandrindod Wells Powys, LD1 6DF UK TEL: FAX: General Technical Sales Web: Updated August 2004

2 CONTENTS 1. INTRODUCTION INSTALLATION USE WITH OTHER DATALOGGERS INTERPRETATION OF READINGS...6 FIGURE 1 - SOIL SUCTION (BARS) V SENSOR RESISTANCE (OHMS)...7 FIGURE 2 - SOIL SUCTION(BARS) V SOIL MOISTURE PERCENTAGE...8 TABLE 1 - GYPSUM BLOCK SENSOR OUTPUTS IN OHMS...9 TABLE 2 - CONVERSION OF SOIL SUCTION UNITS

3 1. INTRODUCTION The gypsum block soil moisture sensor is an electrical resistance type sensor with a signal output in ohms. It measures soil moisture or soil suction in units of centibars where zero centibars represents a saturated soil and 200 centibars represents a very dry soil. In order to be connected to the Skye DataHog or MiniMet datalogger an interface is required to convert the resistance output of the sensor to millivolts. This manual describes the measurements obtained when using the gypsum block sensors with Skye dataloggers via such as interface. 3

4 2. INSTALLATION The gypsum block sensor has been fitted with a logger interface which is inside the small black box, in line with the sensor cable. This box is completely waterproof and can be safely buried in the soil during installation if necessary. This sensor is recommended for applications with low to moderately low saline soils of ph 4.5 or higher. Place the gypsum block in a container of water for ten minutes to saturate the block. Use an auger slightly larger than the sensor (which has approximately a 25 mm diameter) to drill a hole in the soil to the required depth. Make a small amount of soil / water slurry using some earth from the hole, pour this into the hole just enough to cover the gypsum block. Using a pole or broom handle push the saturated gypsum block down to the bottom of the hole, taking care not to damage the signal wire or the interface box. A slot in the base of the pole will help protect the wire during this process. Refill the hole with the original soil, tamping in small amounts at a time. Connect the round 5 pin plug at the cable end to the appropriate socket on the DataHog or MiniMet datalogger (these sockets are usually labelled gypsum block). Allow 24 hours for the gypsum block to stabilise before using the measurements. PLEASE REMEMBER THAT SOCKETS ON THE DATAHOG OR MINIMET LOGGER ARE NOT WATERPROOF UNLESS THERE IS EITHER A SENSOR PLUG OR WATERPROOF DUSTCAP FITTED. WIRING DETAILS FOR DATAHOG CONNECTOR Pin 1 Red Positive power supply (5V) Pin 2 Blue Sensor output Pin 3 Pin 4 not connected not connected Pin 5 Grey (cable screen) Power supply & output ground 4

5 3. USE WITH OTHER DATALOGGERS This gypsum block interface has been designed for use with the Skye DataHog or MiniMet datalogger, using the volt regulated sensor excitation supply. If it is to be used on other dataloggers, please ensure a 5V power supply else the calibration data supplied in this manual will be incorrect. Please also note that the interface is not protected against any power supply reversal. WIRING DETAILS FOR WIRE ENDED SENSORS Red Blue Grey (cable screen) Positive power supply (5V) Sensor output Power supply & output ground 5

6 4. INTERPRETATION OF READINGS The direct output of the gypsum block sensor is resistance in Ohms. The output of the sensor via the logger interface is in millivolts. The gypsum block's relationship between soil moisture in centibars and resistance (in ohms or converted to millivolts by the logger interface) is not a linear one. This relationship is least linear towards the 'wet' soils end, in the region of zero to 20 centibars, whereas at the 'dry' end of the curve (20-200) the relationship becomes much more linear. Figure 1 shows the gypsum block response in resistance v soil moisture in bars. Figure 2 shows how soil moisture in bars relates to soil moisture percentage. The Skye DataHog or MiniMet datalogger reads the direct output from the sensor's interface, giving accurate data over the entire soil moisture range of centibar. These readings are in millivolts however, and require conversion to centibar either manually or in a spreadsheet such as Excel. The more linear end of the gypsum block's relationship curve between 20 and 200 centibars is represented by the equation below. This equation has been used to calculate the soil moisture values in Table 1. For wet soils outside this range the readings calculated using this equation should be regarded as trend indications only. Table 1 gives values of soil moisture in centibars against the gypsum block s resistance output and the DataHog interface s millivolt output. STRAIGHT LINE APPROXIMATION BETWEEN cbar S = R where S = soil moisture in bars (1 bar = 100 cbar) R = gypsum block resistance in ohms 6

7 FIGURE 1 - Soil Suction (bars) v Sensor Resistance (ohms) 7

8 FIGURE 2 - Soil Suction(bars) v Soil Moisture Percentage 8

9 TABLE 1 - Gypsum Block Sensor Outputs in ohms For a DataHog interface fitted with a 15 kohm resistor. The DataHog gives a volt excitation power supply. Soil Moisture Gypsum Block DataHog Interface (centibar) Resistance (ohms) Output (volts)

10 TABLE 2 - Conversion of Soil Suction units FROM TO MULTIPLY BY: bars kpa 100 bars hpa 1000 bars psi 14.5 psi kpa 6.89 psi bars kpa psi kpa bars