PASCO OS-8542 BLACK BODY LIGHT SOURCE - Manual (Page 8)

Black Body Instruction Manual 012-07105B
4
B. Determining the Temperature of the Black Body Light Source
The temperature of a hot bulb can be calculated from a known resistance at room temperature. The
resistance of the bulb is given by
(13) R = R
0
[1 –
α
0
(T – T
0
)]
where
α
0
is the thermal coefficient at room temperature. The bulb filament is made of tungsten,
which has a coefficient of
α
0
= 4.5 x 10 -3 / 0 K at room temperature. Solving equation (13) for the hot
temperature gives
(14) T = T
0
+
R
R 0
– 1
α
0
The bulb has an approximate resistance of 0.84
Ω at room temperature. For a more exact value,
measure it yourself. You cannot measure the resistance of the bulb while it is still in the holder.
Solder wire leads to one bulb to ensure a good contact, and use this bulb to determine the resistance.
To determine the resistance of the hot bulb, measure the voltage drop across the bulb and the current
passing through it; use Ohm’s law, where V=IR.
The current is determined from the power amplifier output from Channel C. Connect a voltage
sensor to channel B, and place it directly across from the light source. The final equation for the
temperature (in Kelvin) of the bulb becomes
Determining the Temperature of the Bulb Using DataStudio
In DataStudio, the temperature for the bulb in the black body light source can be viewed
from the definition box in the Calculator. To view the equation for temperature, click on
the calculator icon next to temperature (under the “Data” window). The equation is as
follows:
Temp (K) = 300 + ((voltage/current)/0.84 - 1)/.0045
where voltage is the voltage across the bulb
current is the current passing through the bulb
0.84 is the resistance of the bulb at room temperature
.0045 is 4.5 x 10
-3
(15) T=300K +
V I V I
0.84
– 1
4.5 x 10 –3
K
4.5 x 10 –3
K