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command. If the air temperature output is connected to AN1 and the RH output is connected to AN2, then
the air temperature voltage will be the first parameter returned and the RH voltage will be the second
parameter returned.
Powered for measurements only
In this case, we need to setup a measurement command for both the air temperature and the relative humidity.
For air temperature, we want to use switched 5V with a 3 second warm-up time. We also want to report
degrees C. With this sensor, zero volts corresponds to -40 C and one volt is +60 C so we have a 100 degree
change per volt which gives a slope of 100. Since we need to report -40 for 0 volts, the offset should be -40.
To configure the M1 command to perform this reading on channel AN2, issue the configuration command:
aXV1+2+G+100-40+X+X+5+3+U+0!
This says that M1 will measure channel 2 with respect to ground. Apply a slope of 100 and offset of -40. It
says to use the default for averaging time and Vref warm-up time. And it says to perform the measurement
unbuffered with an automatic gain. To read the air temperature, the data recorder would issue the
aM1!
command and then about 4 seconds later it would issue the
aD0!
command to read back the air temperature in degrees C.
For Relative Humidity, we will again want to use switched 5V with a 3 second warm-up time. We want to
report %RH. With this sensor, zero volts corresponds to 0% RH and one volt corresponds to 100% RH so we
have 100% change over one volt which gives a slope of 100. We want to report 0 for 0 volts so our offset is
0. To configure the M2 command to perform this reading on channel AN3, issue the configuration command:
aXV2+3+G+100+0+X+X+5+3+U+0!
This says that M2 will measure channel 3 with respect to ground. Apply a slope of 100 and offset of 0. It
says to use the default for averaging time and Vref warm-up time. And it says to perform the measurement
unbuffered with an automatic gain. To read percent relative humidity, the data recorder would issue the
aM2!
command and then about 4 seconds later it would issue the
aD0!
command to read back the percent relative humidity.
Air Temperature Sensor
The 5600-0020 Air Temperature Sensor includes a thermilinear network that produces a voltage division
based upon the temperature. Since the output is dependent upon the input voltage, we will configure this
sensor for a ratio measurement. The equation describing the ratio of the output to the input would produce a
temperature of -72.79 degrees C at a ratio of zero and a temperature of 106.05 at a ratio of one so we have a
178.84 degree change for a change of 1 in ratio which gives a slope of 178.84. To report -72.79 for a ratio of
zero, we set the offset to -72.79. For our connections, we connect the red lead to Vref, the black wire to
Ground, and the output to AN1. We use Vref (2.5V) for the supply since it is a ratiometric sensor. AN1 was
chosen for the input since it is on the same terminal block as Vref and Gnd. This allows the sensor to be
completely wired on one removable terminal block. To configure the M3 command to perform this reading,
issue the command:
aXR3+1+V+178.84-72.79+X+0.1+0+X+B+0!
This says that M3 will measure the ratio of channel 1 to vref using a slope of 178.84 and an offset of -72.89.
It says to use the default time for averaging, but it says to use a tenth of a second for the Vref warm-up time.
It says to leave the switched voltage off during the measurement, that we don‟t care about its warm-up time,
to buffer the input, and to use auto-gain. The thermilinear network is powered off of Vref and appears to the
unit as a relatively high impedance voltage divider. The resistance of the thermilinear network and the
cable‟s capacitance forms a RC time constant. In other words, the output will not appear back at the module
instantaneously, but will be delayed by time that is the product of the resistance of the sensor and the cable‟s
capacitance. The longer the cable, the longer the delay. If the sensor needs more time to charge the cable‟s
