OWL2pe and BASIC Stamp to light sensors
The output of the LI-COR radiation sensors is a tiny current, on the order of microamps, that varies in porportion to the light level impinging on the sensor. Each LI-COR sensor comes with a calibration tag. The calibration constant printed on the tag gives you the change in light intensity that a one microamp change in output current. For example, a Pyranometer might have a constant of 11.4 Watts/m2 per microamp, with a full sun current, at 1000 watts/m2 of 87.72 microamps. Or a Quantum PAR sensor might have a constant of 202.43 µM.m-2.s-1 per µamp. (micromoles per meter-squared seconds per microamp.), and a corresponding current output in full sun (2000 µM.m-2.s-1) of 9.88 µamps.
These tiny currents need to be amplified by our UTA module, up to a level of volts compatable with the analog converter inputs on the OWL2c logger. Please see the separate document for a description of the UTA module. Each module will be configured to have a certain LI-COR sensor as its input, and to have a certain full scale output voltage withing the range of the OWL2c. This is called a transconductance amplifier, one that amplifies a current level input from the photodiode up to a voltage level output. The units of transconductance gain are Volts per microamp. The UTA transconductance amplifier keeps zero volts across the sensor itself, and keeps the signal on the inside of the coaxial cable.
To program the OWL2c, you must know the transconductance gain setting of the amplifier and the calibration constant from the LI-COR tag. The calibration constant is entered as a */ multiplier in the program.
' example: solar radiation pyranometer ' via UTA to channel an4 ' calibration constant = 11.4 W/m^2 ' UTA on LI200 input, 0-5 volt output, transconductance gain=0.04 V/uA ADch=3 ' analog input 4 gosub ADread ' millivolts from UTA result=result*/73 ' convert to watts/m^2 gosub EElog2 ' log as word gosub show ' display as 0-->1460 +/- 1 watts/m^2
Why 73 after the */ operator?
Knowing the voltage output of the UTA amplifier, with its transconductance gain of 0.04 V/µA, and the calibration constant for the Pyranometer of 11.4 Watts/m2 per microamp, the formula to get light intensity, PYR, given voltage is:
PYR = volts * 11.4 / 0.04 = volts * 285
= millivolts * 0.285
The output of the ADread routine is millivolts. Calculate 256 * .285 = 73. This is the figure to use on the right side of the */ operator. The BS2 internally calculates result = result * 73 / 256. The division by 256 is implicit in the */ operator. Say the reading is result = 1000 millivolts to start. Then after the calculation, result = 1000 * 73 / 256 = 285 Watts/m2. This is the correct answer. By using the */ operator in this way, you are in effect approximating the 0.285 multiplier by the fraction (73/256).
For greater accuracy you can use the ** operator, which uses 65536 as the denominator in the approximation. 0.285 = 18678/65536. So,
result=result**18678 ' convert to watts/m^2
The constant may be entered directly as shown, or it may be declared at the top of the program, or it may be read from eeprom, with an option for user modification.
The same logic applies for the Quantum PAR sensor.
' example: quantum PAR sensor ' via UTA to channel an5 ' calibration constant = 202.43 µM.m-2.s-1 ' UTA on LI190 input, 0-5 volt output, transconductance gain=0.4 V/uA ADch=4 ' analog input 5 gosub ADread ' millivolts from UTA result=result**33161 ' convert to µM.m-2.s-1 gosub EElog2 ' log as word gosub show ' display as 0->2590 +/- 1 µM.m-2.s-1
Why the 259/2 on the right side of the **operator? Knowing the voltage output of the UTA amplifier, with its transconductance gain of 0.4 V/µA, and the calibration constant for the Pyranometer of 11.4 Watts/m2 per microamp, the formula to get light intensity, PYR, given voltage is:
as a calculator would do it: PYR = volts * 11.4 / 0.04 = volts * 506.075 = millivolts * 0.506
The output of the ADread routine is millivolts, and this formula converts that into PAR. Calculate 65536 * .506 = 33161. The approximation for the BS2 is, 0.506 = 33161/65536.
Light intensity can vary wildly over the course of an hour, or over the course of minutes as clouds occult the sun. Direct sunlight is much more intense than indirect sunlight. It may be desirable to average the light intensity over the course of time. Please see the separate section on averaging.
The UTA amplifier output cannot go all the way to zero. It typically has a minimum reading of 8 or 10 millivolts even in the dark. Sometimes it is necessary for display go to zero. This can be accomplished best by having a minimum value cutoff as part of the ADread subroutine.
Indoor light can hardly register at all on a linear scale that has full sunlight as full scale. Our eyes are remarkably adaptable. Studies of indoor light, with photometers, sometimes need to use much higher transconductance gains than are used out of doors.
Similar sensors are manufactured by Skye Instruments of Wales UK, and by Kip and Zonen, and by Davis Instruments. Kip & Zonen pyranometer produces a small voltage that needs to be amplified by about x100. The emesys model USEA can be used for this purpose. The Davis Instruments sensor and most models of the Skye instruments sensors have a built-in transconductance amplifier, and the output of the sensor can be interfaced directly to the OWL2c.
Another section deals with other light measurement instruments--the radiometers and star pyranometers.