Microsoft LPF-00004 Manual del operador descargar pdf (Pagina 63)

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380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

available thermocouples have diameters as small as 25 µm and thermal time constants as short as

2 ms (High Temperature Instruments Corp., Omega Engineering).

Table 1. Common Thermocouples

ANSI Type Min Value Max Value Sensitivity Material

(°C) (°C) at 20°C (µV/°C)

E -200 900 60.48 chromel/constantan

J -200 750 51.45 iron/constantan

K -200 1250 40.28 chromel/alumel

R 0 1450 5.80 platinum/Pt-13%rhodium

S 0 1450 5.88 platinum/Pt-10%rhodium

T -200 350 40.28 copper/constantan

Thermocouples require a reference temperature. This was formerly provided by an ice bath but is

now commonly provided by a compensation circuit. Complete signal-conditioning modules such as

the Analog Devices AD594 and 2B50 signal conditioners exist, which contain the differential

amplifiers and the temperature compensation circuitry. Alternatively, voltage-output temperature

transducers such as the LM35A can be used to derive a compensation voltage to apply to the negative

input of the CyberAmp. To interface thermocouples to the CyberAmp, the user must provide an

interface circuit by using a temperature transducer for compensation or by using the Analog Devices

or equivalent conditioning modules.

2) Resistance Temperature Detectors

Resistance thermometers can be made of metals or ceramic-like mixtures. By convention, resistance

thermometers made of metals are called Resistance Temperature Detectors (RTD), while the ceramic-

like resistance thermometers are called thermistors.

The resistance of most metals increases with increasing temperature. The sensitivity is small, less

than 0.4% per degree centigrade. The most commonly used metal is platinum because of its wide

linear resistance vs. temperature relationship.

Platinum RTDs offer better stability and linearity than thermocouples, but are limited to temperatures

below 850°C. Most platinum RTDs have a resistance of 100 Ω at 0°C and a positive temperature

coefficient of 0.385%/°C at 0°C.

The simplest way to configure a platinum RTD is to excite it with a small, accurate DC current.

Currents of 1 mA or less are used in order to minimize self heating. A differential amplifier is used to

measure the voltage across the RTD. If the excitation current is 1 mA, the sensitivity is 385 µV/°C.

To interface RTDs to the CyberAmp, the user must provide a simple interface circuit.

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Microsoft LPF-00004 Manual del operador Pagina 63