Question

The range of resistivity of thermistors which are used in medical applications, is:

• A. \( 1 \text{ and } 20 \ \Omega \)
• B. \( 0.1 \text{ and } 100 \ \Omega \)
• C. \( 50 \text{ and } 1000 \ \Omega \)
• D. \( 0.5 \text{ and } 1000 \ \Omega \)

Hint:

Thermistors are chosen for clinical applications over RTDs and thermocouples because of their immense sensitivity. A small change in body temperature ($0.1^\circ\text{C}$) leads to a large, easily readable resistance shift. To prevent measurement errors, the exciting current must be kept extremely low (typically $< 100\,\mu\text{A}$) to avoid internal self-heating errors ($I^2R$ power dissipation raising the sensor's temperature above the true tissue temperature).

Answer 1

The Correct Option is B

Concept: Thermistors (Thermally Sensitive Resistors) are semiconductor-based temperature sensors that exhibit a large, predictable change in electrical resistance proportional to variations in temperature. Most biomedical thermistors belong to the Negative Temperature Coefficient (NTC) class, meaning their resistance drops non-linearly as temperature climbs according to the Steinhart-Hart or exponential relation: \[ R(T) = R_0 \cdot e^{\beta \left( \frac{1}{T} - \frac{1}{T_0} \right)} \] Medical-grade thermistors are constructed out of sintered mixtures of transition metal oxides (such as manganese, nickel, cobalt, iron, and copper).

Step 1: Contextualizing Medical-Grade Thermistor Specifications.
Thermistors optimized for clinical diagnostic applications (such as continuous core body temperature tracking, neonatal incubator monitoring, and thermal dilution catheters) are designed to provide high sensitivity across a narrow biological span ($30^\circ\text{C}$ to $45^\circ\text{C}$). The base material properties of these transition metal oxide semiconductors dictate their electrical resistivity ($\rho$). For common medical thermistor discs, beads, and chips, the typical material bulk resistivity ($\rho$) at room temperature spans a specific range from fractions of an ohm-meter up to several hundred ohm-meters, or when evaluated under standard geometric profiles, corresponds to a raw material resistivity value sequence in the range of 0.1 to 100 \(\Omega\cdot\text{m}\) (often referred to simply as the characteristic operational base resistivity scale of \( 0.1 \text{ to } 100 \ \Omega \)).

Step 2: Eliminating alternative options.

• Low limits near $1\ \Omega$, $50\ \Omega$, or $0.5\ \Omega$ combined with high ceilings like $1000\ \Omega$ represent alternative materials or industrial thermistors built for wide industrial envelopes, rather than specialized medical semiconductor compositions.
• The low base range starting down at $0.1\ \Omega$ allows for the creation of ultra-small bead sensors with manageable nominal resistance values (typically $2\text{ k}\Omega$ to $10\text{ k}\Omega$ at body temperature) that do not introduce excessive self-heating errors when small measurement currents pass through them. Therefore, the range is 0.1 to 100 \(\Omega\), which points to Option (B).