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380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

thermistor types. These can be custom interfaced to the CyberAmp 380 by using the AI 490

Connector and AI 491 Cable kits.

2) Temperature transducers that produce an output

current proportional to absolute temperature.

The most notable temperature-dependent current sources are the Analog Devices AD590, with a

temperature range of -55 to +150°C, and the lower-cost AD592, with a temperature range of -25 to

+105°C. These are not available in the wide range of probe configurations provided by thermistor

manufacturers. They are commonly supplied in 0.23" (5.8 mm) diameter metal cans or in

0.25" x 0.093" (6.4 mm x 2.4 mm) ceramic flat packs. For use in the laboratory they must be wired

and insulated. The only temperature probe available from Analog Devices is the AC2626, comprising

an AD590 enclosed in a 4" (102 mm) or 6" (153 mm) long stainless steel sheath, 3/16" (4.8 mm)

outside diameter.

When connected to a DC voltage source (e.g. the +5.000 V excitation voltage of the CyberAmp) these

transducers force the current that flows in the circuit to equal 1 µA per degree Kelvin. The external

circuitry required to use the AD590 is very simple and it is easy to configure it for minimum and

average temperature measurements by placing transducers in series or in parallel, respectively. The

absolute accuracy and interchangeability of these probes is inferior to the Series 700 probes described

above. In their best and most expensive grade, the interchangeability is ±0.5°C at 25°C. The main

advantages of the AD590 are the simplicity of external support circuitry and their suitability for

remote sensing applications. Because the output is insensitive to voltage drops the devices can be

used with twisted-pair cabling hundreds of feet in length.

3) IC temperature transducers that produce an output voltage proportional to absolute temperature.

Two of the most suitable temperature-dependent voltage sources are the LM35A and the LM135A

sensors from National Semiconductor Corp. These have absolute accuracies (interchangeability) of

±0.5°C and ±1.0°C at 25°C respectively. The LM35A has a voltage output of 0 mV at 0°C and a

sensitivity of 10 mV/°C. The LM135A behaves as a zener diode with a voltage output of 0 mV at

0°K and a sensitivity of 10 mV/°C. These devices are generally cheaper than the AD590 transducers.

They are supplied in approximately 0.2" (5 mm) metal cans or plastic packages, but are not available

in ready-to-use probes.

Temperature transducers for extended temperature ranges

Two transducer types are commonly used for temperature measurements beyond the physiological

range.

1) Thermocouples

Thermocouples are economical and rugged transducers and have the advantages of small size, very

fast response time, and a wide temperature range. Thermocouples consist of two dissimilar metals in

contact. The offset potential between the metals is proportional to the temperature. (See Table 1.)

The low sensitivity and broad operating range generally make thermocouples most suitable for

industrial applications. For physiological applications, thermistors, with their greater sensitivity, are

more popular than thermocouples. On the other hand, thermocouples can be fabricated in remarkably

small sizes, permitting some unusual biological applications. For example, thermocouples with

micron dimensions can be inserted into single living cells. (Cain & Welch, 1974). Commercially

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