A RAD PRIMER (see Manual at http://www.rmrco.com/prod/rad)
2010-01-30

The Precision Solar Pyranometer (PSP) measures the incoming solar radiation energy. Radiative energy is measured in units of Watts per square meter (W/m^2) which means that the energy passing a 1 m square is one watt. On a sunny day the average radiation would peak at maybe 1200 W/m^2 (think about it, that's a lot of energy, more than a kilowatt, falling on a 1mx1m solar panel. The average over a day and night in Arizona might be 300 W/m^2 continuous power. In full sunlight the output from the PSP would be a voltage of maybe 5/1000 volt, 5 millivolts.

The Precision Infrared Radiometer (PIR) measures the same thing in the infrared range of light. This is radiation from heat--think Predator--and is a much weaker signal, maybe 0.5 mv max. Also with the PIR you need to measure the temperature of the case and of the glass dome. These are an important part of the calculation for total IR radiation. 

So four things must be measured, the PSP voltage, the PIR voltage, and the PIR case and dome temperatures.

A big problem with using the radiometers, PSP and PIR, is that they put out a simple voltage, like a battery, and not a digital output like a computer. This is called an analog output, a simple voltage, instead of a digital output. But the radiometer output is of the order of 1/1000 volt, a millivolt, while a battery as you know is 1.5 V, 1500 mv. So the output of the radiometers is one millionth of a battery and we have to measure that very precisely. Also we want to measure changes of 0.1% in the signals, so actually we are trying to measure to a resolution of one microvolt.

When the radiometers are placed in an electronically noisy environment signals are picked up in the wires, like the static in AM radio, and this electronic noise can ruin the measurement of the voltages from the radiometers. Ships are very noisy places with radars, VHF radio, lightning, and many other things. Therefore one solution is to place a circuit, the RAD, right up next to the radiometers, as close as possible, to read the voltage and convert it to a digital signal called a "serial stream."

A serial output is a stream of ones and zeros in a code where a zero is 5 V and a one is -5 V typically. Ten of these one-zero combinations define a "byte" and a byte can be any character such as 'A', or '1'. The important thing is that a digital string of ones and zeros is +/- 5 V and is much, much less effected by electronic noise. 

So, the "Radiometer Analog to Digital interface" (RAD) is placed next to the radiometers and receives the analog signals from the radiometers and converts it to a digital string that can be read by a computer some distance away. The RAD performs several jobs:

1. High gain analog-to-digital converter for radiometer signals. We have a preamplifier that multiplies the input millivolt signals from the radiometer to a bigger voltage for processing. The gain for the PSP is x125 and the gain for the PIR is x825. This means a 1 mv analog voltage from the PIR is converted to an 825 mv voltage (0.825 V).

2. Uses a precision analog-to-digital converter (ADC) circuit to convert voltage to digital data.

3. Has a tiny computer, the AVR chip, to read the ADC and compute the actual radiation in watts per square meter (W/m^2).

4. Send out a string of information as a serial string. The serial string from the RAD is received by whatever data collection computer, the host computer, the customer wants.