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Welch-Allyn SureTemp 690 Digital Thermometer


The fastest and most accurate way to take the temperature is to use a digital thermometer. Not all thermometers are the same, though. Some stand out as more reliable than others. Welch-Allyn SureTemp 690 digital thermometer is considered among the best of its kind, but in this post, we're going to discuss it in greater detail.

History of thermometers

The truth is we do not often think about the long history of devices we use on a daily basis. The thermometer is one of them. For that reason, it's useful to address the history of thermometers before focusing on Welch-Allyn SureTemp 690 digital thermometer.

The practice of checking temperature has been used for diagnostic purposes since the earliest days of medicine. As early as 400 BC, Hippocrates taught it's possible to determine the presence of fever with the human hand. It will take several centuries for the first thermometer to see the light of day, though. More precisely, the earliest thermometers were developed in the 16th and 17th centuries AD. At that point, thermometers were made in a way to trap air in glass tubes while the open ending of the tube was submerged in a water reservoir. These devices were called thermoscopes.

Galileo Galilei opted for wine instead of water and was the first person to develop an alcohol thermometer in 1610. Later, it was discovered the changing atmospheric pressure affects the level in the tube when carrying this device up the mountain. Before the concept of temperature has even been recognized, these devices have demonstrated alterations in sensible heat. Therefore, even though Galileo is credited to this day as the inventor of the first thermometer, the device he invented was actually a thermoscope.

Galileo also discovered that glass spheres filled with different densities of aqueous alcohol rise and fall with temperature changes.

In 1638, Robert Fludd (prominent English physician) described a thermoscope with a scale, which can be considered a thermometer. A few years before that, in 1612, an Italian physiologist called Santorio Santorio calibrated the tube and attempted to take the temperature with his thermoscope device. The thermoscope had a bulb at the end of the sealed tube. The bulb was blown of the optimal size and inserted into the mouth. The open end of the device was submerged in water. The oral temperature would expand air and thereby expel fluid from the tube. Later, the air was cooled, bulb removed, and fluid level would increase in a calibrated tube.

The first thermometer to depend on liquid expansion/contraction independently of barometric pressure was developed in 1654 by Ferdinand II de' Medici, grand duke of Tuscany. Later on, several kinds of the same concept were developed. Each variety was also unique because the standard scale was nonexistent until 1665, when Christiaan Huygens, a Dutch mathematician and physicist, recommend the standards should be the points of boiling water and melting ice. This was the beginning of the standard scale, which went through a few tweaks over the years. In 1701, sir Isaac Newton proposed there should be a scale of 12°C between the body temperature and melting ice.

The first commercially available thermometer was the Albutt clinical thermometer, named after a well-known British physician, Sir Thomas Clifford Albutt. A local company in Leeds, England, first manufactured this thermometer in 1867. Albutt made his design available to everyone, which is why many other physicists developed their own thermometers on the same principle[i].

All these inventions paved the way for the development of the digital thermometer, which is widely used today.

How does digital thermometer work?

An electronic or digital thermometer works by putting voltage across the metal probe. Then, the probe measures how much current flows through it. The thermometer has a microchip inside. The main job of this microchip is to measure the resistance and convert it into temperature measurement.

Electronic thermometers take temperature readings in the mouth, armpit, or rectum. The greatest advantage of digital thermometers is that they provide accurate temperature readings in one minute or less. The gold standard for temperature measurements in children and babies is rectal thermometers since they cannot hold the device safely in the mouth.

There are different kinds of thermometers and their uses. In a healthcare setting the practicality is crucial, which is why versatile devices are much-needed. One of those devices is Welch-Allyn SureTemp 690 digital thermometer which allows healthcare professionals to take a patient's temperature orally, axillary, or rectally.

What is Welch-Allyn SureTemp 690 digital thermometer used for?

Founded in 1915, Welch-Allyn is one of the most reputable manufacturers of patient monitoring systems and medical devices. The SureTemp 690 electrical thermometer allows healthcare professionals to measure oral, axillary, and rectal temperature in adult and pediatric patients alike. It is a thermistor-based thermometer.

A thermistor is a resistance thermometer, and it works based on resistance changes. The barrier value changes significantly in the presence of temperature changes. There are two types of thermistor thermometers: NTC and PTC. The latter is a type of thermometer wherein resistance value increases with upgrading temperature. On the other hand, NTC is a type of thermometer wherein the value of resistance decreases in response to the increasing temperature. At this point, NTC thermometers have more uses. The NTC technology is also used for purposes such as digital thermometers like SureTemp 690. Thanks to advances in NTC technology, digital thermometers are widely used and prove to be more reliable than mercury-based counterparts since they are safer and easier to read. Mercury thermometers can rupture, which can be quite dangerous as evaporated mercury can harm the heart, kidneys, lungs, and brain.

Elevated body temperature is a symptom of many health problems, which is why accurate measurements are important for diagnostic purposes. Fever happens when the hypothalamus shifts the set point of the normal temperature upward. As you're already aware, normal body temperature varies during the day. It is generally lower in the morning and higher in the afternoon and night. Causes of elevated body temperature include:

•     Bacterial infections - in case of bacterial infections, fever, or elevated body temperature results from the body's attempts to neutralize the infection-causing bacteria. In a state of elevated body temperature, it is more difficult for these bacteria to survive. Additionally, fever activates the immune system defenses. High fever is usually present in bacterial infections such as salmonella, shigella, and Shiga toxin-producing E. coli. On the other hand, it is often low-grade or absent in cases of cholera, enteropathogenic E. coli, and other diseases.[ii]

•     Virus - just like with bacterial infections, viral infections are linked to elevated body temperature because the body is trying to kill the pathogen or invader. Elevated temperature can destroy proteins in viruses and thereby prevent them from multiplying. Viral fever often makes patients experience fatigue, irritability, and aches. Other symptoms may be present, too, but they vary based on the virus that causes the illness.

•     Malignant tumors - tumors can produce pyrogens and cause infections or impair the normal function of the hypothalamus. Additionally, cancer treatments can directly cause fever and affect white blood cells, thus weakening the immune system and making a patient more vulnerable to infection and inflammation. Fever caused by cancer may come in cycles and occur at the same time each day, but there are also days when a patient doesn't have an elevated temperature.

•     Acute brain injury from trauma or vascular event - accidents and vascular events can damage the part of the brain that controls body temperature. In most cases, fever caused by head trauma is acute. Sometimes inflammation or faulty immune response may be behind fever as the brain mistakenly responds as though a patient has infection even though they are not. Drug reactions, deep vein thrombosis can also cause fever in persons with acute brain injury.

•     Endocrine fever, e.g., due to hyperthyroidism - fever can be a presenting manifestation of endocrine-related problems such as subacute thyroiditis and hyperthyroidism. In some cases, although uncommon, fever can result from primary or secondary adrenocortical insufficiency.

•     Immunizations after some vaccines, including those against COVID-19 - since fever is a normal part of the immune response, it may present itself as a symptom after vaccination. In this case, the immune system responds to the vaccine and builds immunity against the virus or bacteria targeted by that specific vaccine.

•     Medications such as antibiotics and drugs for seizures and hypertension - drug administration may impair the usual balance that maintains body temperature and thereby cause fever. Some medications may increase metabolism, interfere with heat dissipation peripherally, induce an immune response in the cells, or damaged tissues, all of which can lead to elevated temperature. Familial predisposition to hypersensitivity to medications is also observed in patients, especially those with renal dysfunction or taking thiazide diuretics.[iii]

•     Inflammatory conditions including rheumatoid arthritis - fever is considered a system-wide sign of inflammation that elevates body temperature and induces response of the immune system. In diseases such as rheumatoid arthritis, a patient's immune system is active but is attacking the body's tissues rather than viruses or bacteria. The autoimmune response could be behind low-grade fever in inflammatory conditions.

•     Sepsis (accounts for 74% of fever cases in hospitalized patients)[iv] - fever is a common symptom of sepsis, but it may be absent in immunosuppressed patients and the elderly. In sepsis, the hypothalamus resets, meaning both production and loss of heat are balanced in favor of elevated body temperature.

In many cases, the cause of fever is unclear, and other tests are necessary for differential diagnosis.

Thanks to Welch-Allyn SureTemp 690 digital thermometer, healthcare professionals can take temperature measurements quickly and easily, using a single device for different uses on all patients regardless of age.

What are the main features of the SureTemp 690 digital thermometer?

Electronic thermometers often come with a wide range of features that allow healthcare professionals to provide a better level of care. The primary features of the Welch-Allyn SureTemp 690 digital thermometer include:

•     Ease of use - the thermometer has a sleek, clean, and neat design with a few buttons and options for added ease of use. The whole process of selecting the mode and other parameters is simple and straightforward. Plus, the device features an easy-to-read LCD display with flashing icons. The icons communicate clearly in any language. Besides an intuitive user interface, Welch-Allyn SureTemp 690 digital thermometer has an ergonomic shape, designed specifically for busy healthcare environments.

•     Fast processing - this electronic thermometer captures oral temperature in four to six seconds, the rectal temperature in 10 to 13 seconds, adult axillary in 12 to 15 seconds, and pediatric axillary temperature in 10 to 13 seconds.

•     High accuracy - not only is this thermometer fast in measuring temperature, but it also does it accurately thanks to the technology that sets the standard for accuracy. More specifically, accuracy is 0.2 degrees Fahrenheit to 0.1 degrees Celsius.

•     Versatility - the thermometer works for patients of all age groups, in adults and children alike.

•     Safety - device improves patient safety thanks to the interchangeable probe that decreases the cross-contamination risk. This is important because, according to a CDC report, one in 31 hospital patients has at least one healthcare-associated infection on any given day.[v]. The removable or interchangeable probes for oral, axillary, and rectal temperature also serve to improve staff satisfaction, besides patient safety. It's also important to mention removable probes are color-coded for additional safety and ease of use of this digital thermometer. For example, a blue probe well is used for axillary and oral temperature measurement, while a red probe well serves to take the temperature rectally.

The temperature range of SureTemp 690 electronic thermometer is 80 to 110°F (26.7 to 43.3°C). The device needs three AA batteries to operate.

The ergonomic design of this digital thermometer allows for rubberized grip and stronghold in the busy and hectic healthcare environment. Under a large, easy-to-read LCD display, three buttons are located. One button allows users to switch between Celsius and Fahrenheit; the other button is for selecting the mode (oral, adult axillary, pediatric axillary), while the prominent lower button on the device is reserved for recall to display the last temperature measured. The display itself is neat, has larger digits, and features a battery life indicator, temperature value in Celsius or Fahrenheit, mode, and whether the temperature is measured on an adult or child.

Connected to the thermometer is a removable probe well and interchangeable probe with a push-button feature to eject the probe cover. Probe shafts are waterproof and made of stainless steel. The probe well is removable to allow for easy replacement and cleaning.

While the device is handheld, there is also an option to get a wall mount.


Probe and covers for Welch-Allyn SureTemp 690 digital thermometer

As mentioned above, this electronic thermometer can take a patient's temperature in three different ways: oral, rectal, and axillary. To reduce the risk of cross-contamination that would occur by using the same probe, the device comes with removable or interchangeable probe options.

Welch-Allyn SureTemp temperature probe covers packaging comes with 250 cover pieces. The covers are compatible across all thermistor-based thermometers made by this brand. They are disposable and serve to protect the patient as their rectal temperature is measured with Welch-Allyn SureTemp rectal probe. The probe comes with a well kit included and a 4-foot cord. Additionally, the probe is non-sterile and reusable. For that reason, covers serve as a layer of protection that prevents cross-contamination. At the same time, healthcare professionals tend to save up on costs they'd spend on disposable probes, which would require greater costs to accommodate more patients. Not only would this have a financial impact, but it is also not suitable for the environment.

The environment part is particularly important if we bear in mind hospitals in North America are responsible for 20% to 33% of total waste[vi]. And it's all mainly due to the high usage of reusable items and devices. This problem is becoming worse because many supplies have been produced and marked as disposable or single-use items due to cost, convenience, and patient safety. However, they may turn out to be more expensive in the long run. For that reason, a reusable probe proves to be a better fit for the healthcare setting.

The detachable probes are available in short- and long-cord versions. They include a mechanical probe cover ejection mechanism. The user simply triggers this mechanism with a finger press. The probes were specifically designed for easy exchange and replacement in a busy and hectic clinical setting.


Temperature measurement is one of the most common diagnostic tests in a clinical setting, and it has been the case since ancient times. Even in the ages before the thermometer was invented, physicians would assess a patient to determine the presence of fever, a common symptom in many health conditions. Today, it's easier to measure a patient's temperature since thermometers have come a long way and evolved to the point it takes a few seconds to know the values. In this post, we focused on Welch-Allyn SureTemp 690 digital thermometer, which allows healthcare professionals to take a patient's temperature orally, rectally, and under the armpit. The device is fast, accurate, ergonomically designed, and easy to use in a hectic and stressful clinical setting.


[i] Grodzinsky, E., & Sund Levander, M. (2019). History of the Thermometer. Understanding Fever and Body Temperature: A Cross-disciplinary Approach to Clinical Practice, 23-35.

[ii] El-Radhi A. S. (2019). Fever in Common Infectious Diseases. Clinical Manual of Fever in Children, 85-140.

[iv] Walter, E. J., Hanna-Jumma, S., Carraretto, M., & Forni, L. (2016). The pathophysiological basis and consequences of fever. Critical care (London, England)20(1), 200.

[v] Healthcare-associated infections. CDC.

[vi] Kagoma Y, Stall N, Rubinstein E, Naudie D. (2012). People, planet, and profits: the case for greening operating rooms. CMAJ, 184(17), 1905-1911.