-
AbstractA simple and practical digital thermometer with an
- 上一篇：甲基叔丁基醚MTBE英文文献和翻译
- 下一篇：LabVIEW外文文献和翻译
accuracy better than 0.1 T over a near-rooin-temperature
(from -10 "C to 50 "C) measurement range has been
developed. The instrument is compact and battery
operated and provides for both digital and analog outputs.
A four-lead platinum thermal sensor, driven by a constant
current loop, allows for accurate temperature readings
with high immunity to the contact resistances and to their
variations. A low-noise electronics allows for temperature
measurements with a 1-nK resolution. By experimentally
characterizing the non-linearity of the adopted Pt-100
sensor, a suitable readout correction table has been
calculated in order tal compensate for the sensor
non-linearity. This compensating procedure allows for a
wider (from -50 "C to +200 "C) and higher accuracy
(-0.05 "C) measurement range. The accuracy, up to now,
IS essentially limited by the accuracy of the temperature
standard used for calibration.4827
1. Initroduction
Temperature measurements characterized by the best
accuracy levels use platinum sensors to transduce the
thermal signal into an electric one [l]. The low thermal
coefficient (a = l/R-dR/dT z 0.385 Yd"C for standard
commercial probes 121) and the relatively low ohmic
value of these sensors push toward the need for high-
resolution measurements of the variation in the sensor
resistance. Such measurements require a careful
consideration of the additional resistances arising from
the lead wires and contact weldings arid from the
associated thermal mi;. For this reason, the most
accurate resistance measurements need four-wires
probes [3]. With a four-wire resistor the voltage
difference between the high-impedance voltmetric
terminals can be read, essentially without spurious
resistance effects, when a constant current is flowing
trough the other mperometric wires and terminals. A
particular and quite recent application of this concept is
exploited in the Anderson's loop circuit [4]. This current- 4-lead thermistor Analog ovtput
Km
Measurement
electronic circuR
Figure 1. Block diagram of the digital thermometer.
sensor, the main electronic sections, and both outputs, is
depicted in Fig. 1, while the details of the measurement
electronic circuit are indicated in Fig. 2. The excitation
current is obtained by means of a precise 5-V voltage
reference (Linear Technology, LT1236), an ultra low
noise operational amplifier (Analog Device, OP177), and
a reference resistor R2=100S.1 R (3 ppm/"C). The
doubly differential subtraction section is made of two
instrumentation amplifiers (Burr Brown, INAl18) and of
another OP177 operational amplifier. As a temperature
sensor, a four-wire Pt-100 commercial and cheap
thermistor (RS, 237-1657) has been used. This sensor, in
compliance with BS1904:1984 class B quality, is
characterized by a nominal-resistance value of
Ro = 10033.1 R at a reference temperature T,=O "C and