Why Is My Thermometer Giving Different Readings? A Guide to Accurate Non-Contact Use

You point the non-contact thermometer at your child’s forehead and click. 98.4°F. Seems normal.

Just to be sure, you try again. 99.3°F.

You pause, confused, and try a third time. 97.9°F.

This experience is incredibly frustrating, and it’s the number one reason people lose faith in these powerful devices. You’re left wondering, “Is this thing broken?” or “How can I get a number I can actually trust?”

Welcome to the club. As a mentor who has guided many new parents and caregivers, I can tell you: The thermometer is almost certainly not broken. It’s just misunderstood.

The problem is that we think of it as a simple “point-and-shoot” gadget, but it’s actually a sensitive scientific instrument. It doesn’t “send” anything. It’s a passive infrared sensor that reads the invisible heat energy radiating from a surface.

To get a stable, accurate reading, you must control the variables. This guide will turn you from a frustrated user into a confident expert, using the popular Berrcom JXB-178 as our working example.

The Mental Shift: It’s a Camera, Not a Magic Wand

Think of your thermometer like a high-tech thermal camera. If you try to take a photo of someone through a screen door, while standing in a breezy hallway, with a smudge on the lens… you’re going to get a bad picture.

Your thermometer faces the same challenges. Its “picture” can be blurred by:
1. The Wrong Settings (Using “Surface” mode for a “Body” reading)
2. A Poor Target (A sweaty forehead or hair)
3. A Bad Environment (A cold room or a drafty A/C vent)

Once you learn to control these three things, your readings will become rock-solid.

Part 1: The 3 Most Common (and Easiest) Fixes

More than 90% of reading errors come from these simple procedural mistakes. Let’s fix them right now.

1. You’re In the Wrong Mode (The “Body” vs. “Surface” Trap)

This is the single biggest error. Most quality non-contact thermometers, including the Berrcom JXB-178, have at least three modes. * Body Mode: This is the only mode you should use for a person. It measures the forehead’s surface heat and then uses a special algorithm to calculate the equivalent internal body temperature. * Surface Temp Mode: This measures the true, unadjusted surface temperature of an object. It’s perfect for checking a baby’s bottle, bathwater, or food. * Room Mode: This measures the ambient temperature of the room.

The Mistake: If you accidentally measure a forehead in “Surface Temp” mode, the reading will look shockingly low (e.g., 95.0°F), because human skin is cooler than our internal core temperature.
The Fix: Before every use, press the “MODE” button until you clearly see “Body” on the screen.

2. The Target is “Contaminated” (Sweat and Hair)

Your thermometer is reading the heat from the skin. Anything that blocks the skin or artificially cools it will ruin the reading. * Sweat/Moisture: Even a light sheen of perspiration will cause “evaporative cooling,” just like a breeze cools you down. This will make the reading artificially low. * Hair & Hats: Hair is an insulator. Measuring over hair will read the temperature of the hair, not the skin beneath. * Cosmetics: Thick makeup or lotions can also act as an insulating barrier.

The Fix: Gently wipe the forehead dry with a cloth. Brush any hair aside completely. Wait about 10 minutes if the person just came in from outside or was physically active before taking a reading.

3. The Distance is Wrong (The “Optical Sweet Spot”)

The manual for the Berrcom JXB-178 specifies a distance of 1.2 to 2 inches (3-5 cm). This is not a casual suggestion—it’s a strict optical requirement.

Think of the sensor’s lens like a magnifying glass trying to focus sunlight. There is one perfect “focal point” where the beam is sharpest. For your thermometer, that “sweet spot” is 3-5 cm.

• Too Far: If you hold it 6 inches away, the sensor isn’t just reading the forehead. It’s also reading the cooler ambient air and background objects in a wide, out-of-focus cone. This averages the reading down, making it inaccurate.
• Too Close: Holding it too close (or touching) can distort the reading and risks cross-contamination.

The Fix: Practice holding the device at the correct 3-5 cm distance. Many people use their other hand to frame the distance (e.g., holding two or three fingers up to the forehead) until they get a feel for it.

Part 2: The “Hidden” Variable: The Environment

Okay, so you’ve mastered the technique. But your readings are still inconsistent. Now, we look at the environment.

The “Acclimation” Rule

This is the expert-level tip that most people skip. Your thermometer’s sensor is sensitive to its own temperature. * The Problem: If you store the thermometer in a cold drawer (68°F) and then bring it into a warm bathroom (75°F) to take a reading, the device’s internal reference point is wrong. It will interpret the “warm” forehead as “hotter” than it really is, giving you a high, false reading. * The Fix: The Berrcom JXB-178 manual suggests waiting 10-15 minutes for the device to “warm up and adjust to the room temperature” when first installing batteries. This rule also applies if the device has been in a place with a significantly different temperature (like a cold car or a drafty bag).

Let the thermometer sit in the same room as the patient for at least 10 minutes before you use it. This allows its internal sensor to reach thermal equilibrium, ensuring its baseline is correct.

The “Air Draft” Rule

The sensor is reading a delicate column of infrared heat. Any air movement between the device and the forehead can disrupt this column and skew the data.

The Fix: Do not take a measurement: * Directly under an A/C or heating vent. * In front of a strong fan or open window. * Immediately after a person has had a cold or warm compress on their forehead.

Move the patient to a “still air” part of the room for the most stable and reliable results.

Part 3: Advanced Troubleshooting (The “F4” Calibration)

You’ve done everything right. You’ve acclimated the device, set it to “Body” mode, the forehead is dry, and your distance is perfect. But you still feel the readings are off compared to an old-fashioned oral thermometer.

You may not need to. Many high-quality models, including the Berrcom JXB-178, have a hidden “offset” calibration menu. This allows you to make micro-adjustments to align the device with a trusted reference (like your doctor’s thermometer or a high-quality oral one).

Disclaimer: Only attempt this after you have mastered all the steps above and are certain your reference thermometer is accurate. Do not do this right out of the box.

How to Access the F4 Menu (on Berrcom JXB-178):
1. Turn the device on by pulling the trigger.
2. Press and hold the “MODE” button for 2 seconds. The screen will display “F1”.
3. Press the “MEM” button twice (skipping F2 and F3). The screen will display “F4”.
4. Press the “MODE” button to start adjusting the offset. You can add or subtract in small increments (from -3°C to +3°C, or -5.4°F to +5.4°F).
5. Let’s say your Berrcom consistently reads 0.5°F lower than your trusted oral thermometer. You would use the “MODE” button to adjust the offset to “+0.5”.
6. Once set, press the “MEM” button to save and exit.

This F4 function is the ultimate tool for achieving peace of mind, allowing you to “tune” your device to your specific environment and reference.

Your Path to Confidence

A non-contact thermometer is an amazing tool. Its speed and hygiene are unmatched, especially for a sleeping child. That “jumping numbers” frustration you felt was valid, but it wasn’t a sign of a broken device—it was a sign of a sensitive instrument needing the correct procedure.

By mastering the “Big 3” (Mode, Surface, Distance), respecting the environment (Acclimation, Drafts), and knowing how to use the advanced calibration, you can now take a reading in 1 second and—more importantly—trust the number you see.