Campbell CR6 Series, Operator'S Manual

Page: 460 / 690

Section 8. Operation
460
Vibrating Wire Temperature Measurement
Temperature data from the two-wire thermistor that is often embedded in
vibrating wire sensors is used to correct measurement errors caused by thermal
expansion and contraction of the sensor body. Resistance thermistor changes with
sensor temperature. Leads connect to the U terminal pair numerically following
the U terminal pair to which the vibrating wire coil leads are connected.
Measurement of the thermistor is enabled in the VibratingWire() instruction with
an argument other than zero in the ThermFnotch parameter . The measurement is
made as follows: excite U1 , and while exciting, measure differentially U1 U2 , and
then measure differentially across an internal 5.99K ohm resistor. This is the same
sequence as that used in
four-wire half-bridge
measurements, but the wiring has
no provision for long-lead compensation.
Thermistor temperature can be calculated automatically by inputing arguments
other than 0 in the A , B , C parameters in the VibratingWire() instruction. The
values to input are the Steinhart-Hart coefficients supplied by the sensor
manufacturer.
A few vibrating wire sensors use a resistive element other than a thermistor to
measure temperature. Inputing 0 as the argument for A , B , and C results in the
output of resistance expressed as ohms.
Converting Thermistor Resistance to Temperature
The Steinhart-Hart equation is expressed as follows:
T = 1 / (A + B • LN(R) + C • (LN(R))^3) – 273.15
Where T is the temperature of the thermistor, A , B , and C are coefficients, and R is
the resistance of the thermistor.
Note Coefficients for Steinhart-Hart are specific to the thermistor in the
sensor. Obtain coefficients from the sensor manufacturer.
For example, if the coefficients for Steinhart-Hart are provided from the
manufacturer as follows,
A
= 1.4051E –03
B = 2.369E –04
C = 1.019E –07
the equation for converting the resistance measurement to degrees Celsius is:
T = 1 / (1.4051E –03 + 2.369E– 04 • LN(R) + 1.019E – 7 • (LN(R))^3) –
273.15
Where R is the measured resistance. If the measured resistance is 2221 Ω, the
equation is,
T = 1/(1.4051E –03 + 2.369E– 04 • LN(2221) + 1.019E – 7 • (LN(2221))^3) –
273.15