The Clinical Standard of Accuracy: A Definitive Report on the Welch Allyn SureTemp Plus 690 Electronic Thermometer

Update on July 30, 2025, 5:47 a.m.

For any caregiver, whether a parent monitoring a sick child or a clinician making a critical diagnosis, few things are more fundamental than an accurate temperature reading. The market is saturated with consumer-grade thermometers, yet their performance often falls short when precision is paramount. This gap between expectation and reality can have significant consequences. Consider the experience of users who, relying on store-bought devices, have witnessed readings that were off by as much as 3 to 4 degrees Fahrenheit during a health crisis, a discrepancy that can mean the difference between calm reassurance and an emergency room visit [Ellie V. review]. The clinical and emotional stakes are immense: misdiagnosing a fever in a newborn, overlooking a sign of post-operative infection, or making an unnecessary and stressful trip to a medical facility.

This common frustration highlights a critical need for diagnostic certainty. In this context, the Welch Allyn SureTemp Plus 690 Electronic Thermometer (Model 01690-200) emerges not merely as another product, but as a clinical-grade diagnostic instrument. It is a device trusted and utilized by healthcare professionals in hospitals and clinics worldwide, a fact that discerning home users have noted as a primary reason for their investment. This report provides a definitive analysis of the SureTemp Plus 690, deconstructing the technology, design, and user experience that position it as a benchmark for accuracy and reliability. It will explore the scientific principles that justify its premium status and provide the necessary context for users to determine if this professional instrument is the right investment for their needs.

Section 1: A Legacy of Precision: From Galileo’s Thermoscope to the Welch Allyn Standard

The journey to the modern clinical thermometer is a centuries-long quest for precision, driven by the understanding that body temperature is a vital sign of health and disease. This history is not a simple linear progression but a constant effort to resolve a fundamental trilemma: the balance between Accuracy, Speed, and Safety. The design of the Welch Allyn SureTemp Plus 690 is a direct and sophisticated response to this enduring challenge.

The Dawn of Thermometry

The concept of fever as a sign of illness dates back to Hippocrates around 400 BC, who relied on the subjective touch of a hand. For nearly two millennia, temperature remained an unquantified quality. The first major leap occurred around 1593 when Galileo Galilei invented the thermoscope, a device that showed temperature variations but lacked a standardized scale. It was Galileo’s contemporary, the Italian physician Santorio Santorio, who first applied a scaled instrument to medicine around 1612, creating the first true clinical thermometer to measure human body temperature. These early devices, while revolutionary in concept, were cumbersome, slow, and highly impractical, prioritizing the novel idea of accuracy over usability.

The 18th century saw the development of more accurate sealed liquid-in-glass thermometers by Gabriel Daniel Fahrenheit, who also established one of the first reliable temperature scales. However, these instruments were often several feet long and took a considerable amount of time to render a reading. A pivotal moment arrived in 1867 when English physician Sir Thomas Clifford Allbutt developed the compact, six-inch clinical thermometer. This device dramatically improved speed, reducing measurement time from over 20 minutes to just five, making routine temperature monitoring feasible. This innovation solved the speed and usability problem but introduced a new one: safety. The reliance on mercury, a highly toxic substance, posed a significant risk, a factor that would eventually lead to its obsolescence in clinical practice.

The Modern Era: Electronics and the Pursuit of Speed, Safety, and Accuracy

The 20th century ushered in the electronic era, which definitively solved the safety issue by eliminating mercury. However, this technological shift reintroduced the tension between accuracy and speed. Early electronic thermometers were often viewed with skepticism by clinicians accustomed to the proven reliability of mercury-in-glass devices. The core technology that enabled the next leap forward was the thermistor.

The Welch Allyn brand and its SureTemp Plus line of thermometers stand as modern inheritors of this long legacy. The SureTemp Plus 690 is an instrument engineered to resolve the historical trilemma. Its use of predictive algorithms prioritizes speed, while its direct measurement mode offers uncompromising accuracy. Its reliance on single-use disposable probe covers and a design focused on disinfection directly addresses patient safety and infection control. In this context, the device is not just a collection of features; it is a meticulously designed solution to a medical challenge that is centuries old.

Section 2: Deconstructing Clinical-Grade Performance: The Technology of Trust

The clinical reputation and premium cost of the Welch Allyn SureTemp Plus 690 are not products of marketing but are founded upon verifiable scientific principles and sophisticated engineering. To understand its performance, one must look “under the hood” at the three pillars of its precision: the highly sensitive sensor, the intelligent dual-mode processing, and the rigorous mathematical model that guarantees its accuracy. It is the integration of these components that elevates the device from a simple thermometer to a true diagnostic instrument. This instrument should not be viewed as a single tool, but rather as two distinct diagnostic systems housed in one device: a Rapid Screening Tool (Predictive Mode) and a Definitive Measurement Instrument (Monitor Mode). The failure to grasp this fundamental duality is the primary source of user confusion and perceived inaccuracy.

Subsection 2.1: The Heart of the Matter: How NTC Thermistors Achieve Superior Sensitivity

At the core of the SureTemp Plus 690 is a thermistor, a term derived from “thermal resistor.” It is a type of semiconductor made from metallic oxides—typically including manganese, nickel, and cobalt—that is engineered to be exceptionally sensitive to temperature changes. Unlike a standard resistor, whose electrical resistance is relatively stable, a thermistor’s resistance changes predictably and dramatically with even minute fluctuations in temperature.

The SureTemp Plus 690 specifically uses a Negative Temperature Coefficient (NTC) thermistor. In an NTC device, as the temperature increases, its electrical resistance decreases. This inverse relationship is highly non-linear, meaning a small change in body temperature produces a large, easily measurable change in resistance. This high sensitivity is precisely what makes NTC thermistors ideal for clinical applications, as they can detect the subtle temperature variations in the human body with exceptional speed and precision.

Subsection 2.2: The Brains of the Operation: Predictive vs. Monitor Mode

The SureTemp Plus 690 processes the data from its NTC thermistor in two distinct ways, each with a different goal. Understanding this distinction is essential for correct use and interpretation of its readings.

Predictive Mode Explained

The primary appeal of the SureTemp Plus 690 is its speed. Predictive mode is designed to deliver a temperature reading in seconds (4-6 for oral, 10-15 for axillary/rectal) rather than the several minutes required for the probe to reach full thermal equilibrium with the body.18 This is achieved through a sophisticated software algorithm. When the probe is placed, the device’s microprocessor doesn’t wait for the temperature to stabilize; instead, it analyzes the

rate of temperature change at the probe tip. Based on a vast internal database of clinical readings, the algorithm predicts what the final, stable temperature will be and displays that value.

A crucial and often overlooked feature that enables this prediction is the electronic tip warmer. A common source of error in thermometry is the “tissue cooling effect,” where a cold probe placed against warm tissue temporarily lowers the temperature of the measurement site. The SureTemp’s probe tip is actively heated to minimize this temperature differential, allowing the predictive algorithm to start from a more stable baseline and make a more accurate forecast.

Monitor Mode Explained

Monitor Mode provides the most accurate, definitive temperature reading the device can produce, serving as the reference standard for the instrument.21 When this mode is active, the predictive algorithm is bypassed. The thermometer simply functions as a direct-measurement device, continuously displaying the real-time temperature of the probe tip until it reaches

thermal equilibrium—a steady state where the probe and the patient’s tissue are at the same temperature. This process takes approximately three minutes for oral and rectal sites and five minutes for axillary sites.

In clinical settings, this mode is often indicated by a “snail” icon on the display, a feature that has caused confusion among some users. This icon is a signal that the device has switched from prediction to direct, continuous measurement, often because it could not obtain a stable enough reading for the predictive algorithm to work (e.g., due to patient movement).

The proper clinical workflow, therefore, involves using the fast Predictive Mode for initial screening. If a reading is borderline, unexpected, or if the patient is acutely ill where every fraction of a degree is significant, the clinician should then use Monitor Mode to obtain a definitive, reference-grade measurement.

Subsection 2.3: The Steinhart-Hart Equation: The Mathematical Guarantee of Precision

The final piece of the accuracy puzzle is how the thermometer converts the raw resistance measurement from the NTC thermistor into a temperature value. This is not a simple linear conversion. To achieve clinical-grade precision, the SureTemp Plus 690 employs the Steinhart-Hart equation, an industry-gold-standard mathematical model for describing the resistance-temperature relationship of a thermistor.

The equation is:

T1​=A+B(lnR)+C(lnR)3

Where T is the temperature in Kelvin, R is the thermistor’s resistance in Ohms, and A, B, and C are unique coefficients calibrated specifically for the thermistor material.24

For the user, the significance of this complex formula is that it guarantees an extremely precise and reliable conversion of resistance to temperature across the device’s entire operational range. Cheaper thermometers may use less accurate, simplified approximation formulas, which can lead to errors at different temperature points. The implementation of the calibrated Steinhart-Hart equation is a key technical differentiator that underpins the device’s claim to superior accuracy and justifies its place as a clinical standard.

Section 3: A Tour of the SureTemp Plus 690: Design, Features, and Safety

Beyond the internal technology, the SureTemp Plus 690 is engineered for the rigors of clinical use, with a design that prioritizes ergonomics, intuitive operation, and, most critically, patient safety through robust infection control measures.

Subsection 3.1: Ergonomics and Interface

The device is designed for comfortable and secure handling. It measures 8.46 inches x 3.18 inches x 2.43 inches and weighs approximately 12 ounces (357 g), with an ergonomic shape and rubberized grips that provide a firm hold during use.

The user interface is centered around a large, clear LCD display that is easy to read at a glance. It features a simple C/F button to toggle between Fahrenheit and Celsius scales and utilizes flashing icons to clearly identify the selected measurement mode (e.g., a head icon for oral, a torso for axillary). Button functionality is straightforward, with a prominent recall button that allows the user to display the last temperature taken—a convenient feature for tracking and charting.

A key feature for both high-volume clinics and home users is the exceptional battery life. The thermometer operates on three standard 1.5Vdc AA batteries and is capable of performing approximately 6,000 temperature readings on a single set, ensuring long-term reliability and reducing operational costs.

Subsection 3.2: Engineered for Safety: Infection Control by Design

Infection control is a non-negotiable aspect of medical device design. According to the CDC’s Spaulding Classification system, thermometers that come into contact with mucous membranes are considered semi-critical devices, requiring stringent disinfection protocols to prevent cross-contamination. The SureTemp Plus 690 incorporates several features specifically designed to meet and exceed these standards.

The most critical safety feature is the system of interchangeable, removable probe wells. The device uses color-coded wells—blue for oral and axillary measurements, red for rectal—that can be easily detached from the main unit for thorough cleaning and disinfection. This physical separation is paramount for preventing the transfer of pathogens between measurement sites.

The primary barrier against patient-to-patient transmission is the mandatory use of single-use, disposable probe covers for every measurement. The device includes a convenient onboard storage compartment that holds a box of 25 covers, ensuring they are always accessible and encouraging proper workflow. To further minimize contact with contaminated materials, the probe features a push-button ejection mechanism that allows the user to discard the used cover directly into a waste receptacle without touching it.

The official cleaning protocol, aligned with institutional and CDC guidelines, is straightforward: the device body, probe, and probe well should be wiped with a 70-90% ethyl or isopropyl alcohol solution and allowed to air dry. The unit should never be submerged in liquid.

Table 1: Welch Allyn SureTemp Plus 690 - Key Technical Specifications

| Specification | Value |
| — | — | — | — | — |
| Model | SureTemp Plus 690 (01690-200) |
| — | — | — | — | — |
| Measurement Range | 80.0°F to 110.0°F (26.7°C to 43.3°C) |
| — | — | — | — | — |
| Calibration Accuracy (Monitor Mode) | $ \pm 0.2^{\circ}F $ ($ \pm 0.1^{\circ}C $) |
| — | — | — | — | — |
| Predictive Mode Speed | Oral: ~4-6 seconds | Pediatric Axillary (≤17 yrs): ~10-13 seconds 10 | Adult Axillary (≥18 yrs): ~12-15 seconds 10 | Rectal: ~10-13 seconds 10 |
| — | — | — | — | — |
| Monitor Mode Speed (Reference) | ~3 minutes (Oral/Rectal), ~5 minutes (Axillary) |
| — | — | — | — | — |
| Power Source | 3 x 1.5Vdc AA Batteries |
| — | — | — | — | — |
| Battery Life | Approx. 6,000 Readings |
| — | — | — | — | — |
| Dimensions | 8.46” x 3.18” x 2.43” (215mm x 81mm x 62mm) |
| — | — | — | — | — |
| Weight | 12 oz (357 g) |
| — | — | — | — | — |
| Key Features | Interchangeable Probe Wells, Last Temp Recall, °F/°C Toggle, Probe Cover Storage (25) |
| — | — | — | — | — |
| Warranty | 2-year limited on instrument, 1-year on probe |
| — | — | — | — | — |

Section 4: The User Perspective: Voices from the Clinic and Home

While technical specifications define a device’s potential, real-world user experiences reveal its practical performance. Feedback from both home users and clinical professionals provides a balanced and authentic view of the SureTemp Plus 690, highlighting its strengths while also acknowledging its imperfections. This feedback reveals a distinct pattern: users purchase the device based on the trust they place in the Welch Allyn brand and its clinical prevalence. However, to maintain that trust, they must learn to verify its readings and understand its operational nuances, completing a loop from blind faith to informed confidence.

Subsection 4.1: The Case for Investment: Accuracy as a Non-Negotiable

The most consistent praise for the SureTemp Plus 690 centers on its accuracy and reliability. Users who have been let down by consumer-grade thermometers describe the device as “expensive BUT worth it” and “accurate every time” [Ellie V. review]. There is a profound sense of peace of mind that comes from using the same equipment found in professional medical settings; as one user noted, when they bring their temperature logs to their doctor, “there’s no doubt on their part of the temperatures, because this product is one they have their offices as well” [Ellie V. review]. This alignment with clinical standards is a core part of its value proposition.

Beyond accuracy, users commend its overall functionality, fast readings, and ease of use, particularly for those caring for children or managing chronic conditions [AI-generated review summary, Jeff C review]. The sturdy, durable build quality is also frequently praised, reinforcing the idea that this is a long-term investment rather than a disposable gadget [Ellie V. review, Fred K. Fioravanti review].

Subsection 4.2: Addressing the Concerns: Acknowledging Imperfections

No device is without its drawbacks, and the SureTemp Plus 690 is no exception. The most significant barrier for potential buyers is the high price tag, with customer reviews noting that “opinions vary regarding its value for money” [AI-generated review summary].

A more troubling concern is the notable number of user reports of receiving units that were defective or non-functional upon arrival. One user detailed a frustrating experience where a brand-new unit would power on for only five seconds before shutting down. This issue is compounded by the risk of purchasing from third-party sellers. Another user received a defective unit and was denied a refund by the seller, forcing them to purchase a different thermometer from a local store while sick and pregnant. These experiences underscore the importance of purchasing from an authorized dealer with a reliable return policy.

Finally, there are some minor feature omissions. The most cited drawback of the 690 model is the lack of a backlit display, a feature present on the more expensive 692 model and one that users with newborns find particularly useful for nighttime readings [Jeff C review]. Other small complaints, such as the user manual being provided on a CD rather than as a printed pamphlet, have also been noted.

Subsection 4.3: From the Trenches: Unfiltered Insights from Clinical Practice

The most candid feedback comes from clinicians who use the device daily in high-pressure environments. Unfiltered discussions from an emergency medicine forum reveal a more nuanced reality. While generally trusted, clinicians express frustration with the device’s electronic quirks, such as “randomly turning off” or failing to obtain a reading during a busy shift or emergency situation. This highlights a reliability paradox: the device is physically durable but can be electronically finicky under stress.

These clinicians also provide invaluable insights into technique. They note that achieving a fast, accurate predictive reading requires “firm and constant pressure” and a compliant patient. With delirious or uncooperative patients, wrestling with them to get a proper oral reading is often a waste of time, leading clinicians to opt for an axillary measurement instead. This forum also demystified the “snail” icon, with experienced nurses explaining that it indicates the device has switched to the slower Monitor Mode, usually because patient movement prevented the predictive algorithm from locking onto a reading. This “tribal knowledge” is critical, as undertrained staff might misinterpret the real-time reading in this mode and chart an inaccurate, low temperature, posing a significant patient safety risk.

Section 5: Making the Right Choice: SureTemp Plus 690 vs. 692

Welch Allyn offers two models in this line: the SureTemp Plus 690 and the SureTemp Plus 692. While they share the same core measurement technology, they are designed for distinctly different environments. The 690 is optimized for home health, small clinics, or the discerning home user, while the 692 is built specifically for the demands of a high-volume, institutional hospital setting. The additional features of the 692 directly address challenges related to workflow efficiency, low-light visibility, and asset security that are paramount in a large clinical facility. Choosing the correct model is a matter of aligning the device’s features with the user’s specific needs.

Table 2: SureTemp Plus 690 vs. 692 - Feature Comparison

Feature SureTemp Plus 690 SureTemp Plus 692 Benefit / Use Case
Core Technology Identical (NTC Thermistor, Predictive/Monitor Modes) Identical Both models provide the same foundational accuracy and speed.
Backlit LCD Display No Yes Essential for visibility in low-light environments like patient rooms at night.
Pulse Timer No Yes (60-second timer) Streamlines the process of taking multiple vital signs (temperature and pulse/respiration) for clinicians.
Wall Mount Optional accessory Included, with security features Prevents theft and ensures the device is always in a designated, accessible location in a busy hospital.
Location ID No Yes (Configurable on-screen) Allows for asset tracking in a large institution (e.g., labeling a device “ER-Room 3”).
Warranty 2-Year Limited 3-Year Limited Provides a longer warranty for the more expensive, higher-use institutional model.
Target User Home Health, Small Clinic, Discerning Home User Hospital, High-Volume Clinic, Institutional Settings Guides the purchasing decision based on the intended operational environment.

Section 6: Practical Application and Best Practices for Optimal Results

Leveraging the full potential of the SureTemp Plus 690 requires more than just turning it on. Achieving the clinical-grade accuracy for which it was designed depends on proper technique, adherence to best practices, and routine maintenance. This section synthesizes guidance from the official manual with the practical lessons learned by clinical users to serve as a quick-start guide for optimal performance.

Subsection 6.1: Achieving Accurate Readings: A Step-by-Step Guide

For the most reliable results, the following steps and considerations are essential:

  • Pre-Measurement Checklist: For oral temperature measurements, ensure the patient has not eaten, smoked, or consumed hot or cold liquids for at least 20 minutes prior to the reading, as these activities can temporarily alter the temperature of the oral mucosa.
  • Apply a Probe Cover: A new, single-use Welch Allyn disposable probe cover must be securely attached to the probe before every measurement. This is the first and most important line of defense against cross-contamination and is essential for accurate readings.
  • Proper Technique by Site:
  • Oral: Select the oral mode (head icon). Place the probe tip deep into the patient’s sublingual pocket, located to the side of the frenulum under the tongue. The patient’s mouth should remain closed. It is critical that the user holds the probe in place, maintaining firm tissue contact, and instructs the patient not to bite down, which can damage the probe and cause injury.
  • Axillary (Adult & Pediatric): Select the appropriate axillary mode. Place the probe tip vertically, as high as possible into the apex of the axilla (armpit), ensuring it is not caught in any skin folds. Hold the patient’s arm firmly but gently against their side to secure the probe during the measurement cycle.
  • Rectal: Ensure the red probe and red probe well are installed. After applying a probe cover and lubricant, gently insert the probe tip approximately 1.5 cm (5/8 inch) into the rectum—less for infants and small children. Hold the probe securely in place until the measurement is complete, then remove it and eject the probe cover.

Subsection 6.2: Maintaining Your Investment

Proper care and maintenance will ensure the longevity and continued accuracy of the device.

  • Cleaning and Disinfection: After each use, especially in a multi-user environment, the probe, probe well, and thermometer body should be wiped with a 70-90% ethyl or isopropyl alcohol solution and allowed to air dry completely. The device should never be autoclaved or submerged in liquid.
  • Battery Replacement: The LCD will display a single flashing battery segment to indicate low batteries; operation is not yet affected, but batteries should be replaced soon. An open, non-flashing battery symbol indicates the batteries are dead, and the device will not operate. The three AA batteries are located inside the probe cover storage compartment.
  • Storage: When not in use, the thermometer should be stored in its holder. For long-term storage, it is recommended to remove the batteries to prevent potential damage from leakage.

Table 3: Temperature Measurement Guide (Site, Time, Best Practices)

Measurement Site Probe/Well Color Predictive Time Monitor Time (Reference) Key Best Practices
Oral Blue 4-6 sec 3 min Place deep in sublingual pocket. Patient’s mouth must be closed. Do not bite. Wait 20 mins after eating/drinking/smoking.
Axillary (Pediatric, ≤17) Blue 10-13 sec 5 min Place high in axilla, avoiding folds. Hold arm firmly against the side.
Axillary (Adult, ≥18) Blue 12-15 sec 5 min Place high in axilla, avoiding folds. Hold arm firmly against the side.
Rectal Red 10-13 sec 3 min Use lubricant. Insert only 1.5 cm (5/8”). Hold probe securely in place.

Conclusion: Beyond Temperature—An Investment in Confidence and Care

The Welch Allyn SureTemp Plus 690 is demonstrably more than a simple thermometer; it is a complex and capable diagnostic instrument. Its higher cost is not an arbitrary premium but a direct reflection of the sophisticated technology, robust engineering, and thoughtful design required to meet the exacting standards of clinical practice. This report has deconstructed the device, revealing that its value proposition rests upon three integrated pillars:

  1. Clinical-Grade Accuracy: Achieved through the superior sensitivity of its NTC thermistor technology and validated by the mathematical rigor of the Steinhart-Hart equation, ensuring that its readings are both precise and repeatable.
  2. Uncompromising Safety: Delivered through a design philosophy centered on infection control, from its interchangeable, color-coded probe wells to its mandatory use of single-use covers and touch-free ejection system.
  3. Versatile Performance: Offered by its unique dual-mode system, which provides the speed of a rapid screening tool (Predictive Mode) and the certainty of a definitive measurement instrument (Monitor Mode) within a single device.

Ultimately, the decision to invest in the SureTemp Plus 690 is a decision to prioritize certainty. For the parent of a newborn, the individual managing a chronic illness, or the small clinical practice, the cost of inaccuracy—in anxiety, in misdiagnosis, in unnecessary medical intervention—is far greater than the price of the device. By understanding the technology within, respecting the proper techniques for its use, and acknowledging its operational nuances, the owner of a SureTemp Plus 690 is not just taking a temperature. They are leveraging a powerful, professional-grade tool for making confident, informed healthcare decisions. It is an investment in confidence, and an investment in care.