Inaccurate or Inconsistent Temperature Readings of Soldering Irons

Why is the preset temperature different from actual measured temperature?

There may be situations where the temperature of your soldering tip (as measured by the Hakko FG-100B Thermometer) is different from the temperature selected on the soldering station.

This temperature difference could be due to one or multiple issues. Take a look through the options below to identify the cause(s) of the temperature variance.

Oxidation of any part of the soldering iron or soldering tip may cause lower heat conductivity. This affects the temperature, causing it to be lower than what was selected on the soldering station.

Oxidation of Hakko 907 Soldering Iron

Replace the oxidized parts and clean the soldering tip before measuring the temperature. If the issue still remains, it may be caused by one of the other issues below. 

Different tip shapes have different length and weight, and as such, their thermal capacity is different, even if they are from the same series. Swapping out the tips during soldering but maintaining the same preset temperature affects the thermal capacity, causing the temperature to deviate from the preset.

For example:

If the original tip is 900M-T-B with a set temperature of 350ºC, changing to:

  • 900M-T-I will cause the displayed temperature to be approximately 10ºC lower
  • 900M-T-K will cause the displayed temperature to be approximately 30ºC higher

As such, to maintain the same tip temperature, it is necessary to perform fine adjustment (calibration or offset value adjustments).

If the sensor is deteriorated due to corrosion by solder, the temperature may be shown lower in some cases. A sensor can last approx. 50 times in general; however, if lead-free solder is used, the sensor must be replaced more frequently.

Replace the sensor before measuring the temperature. If the issue remains, move on to check the other options. 

When the batteries are low in power, the temperature reading is affected and may show errors.

For Hakko FG-102

When the batteries are low on power, the battery icon circled in red in the photo flashes. When the batteries are further consumed, the icon becomes steadily lit. If you continue to use the unit while the battery icon is steadily lit, it may display an incorrect temperature and may turn off during operation. In some cases, the screen may display "Low Battery" when the power is turned on, and then immediately turn off.

Hakko FG-102 Thermometer Display

For Hakko FG-100B or Hakko FG-100 (discontinued)

In the upper picture, the screen displays the room temperature of 22ºC. When batteries are running out of power “B” mark will appear on the screen as circled in red in the picture AS shown in the lower picture when the “B” mark is on the screen it may display the temperature nearly 100ºC higher than the actual room temperature. And the screen display will gradually fade out and eventually will disappear as the batteries run out.

Hakko FG-100B Thermometer Display

For Hakko 191 (Discontinued)

The upper image shows the display when 23ºC of the room temperature was measured. When batteries are low on power, the "B" mark will be lit at the position indicated by the arrow in the photo. If batteries are further consumed, an extremely-low temperature or minus temperature will be displayed as shown in the lower photo. The indications will gradually become lighter and finally no indications will be displayed. Meanwhile, a temperature higher than the actual temperature by approx. 100ºC will be displayed in some cases.

Hakko 191 Thermometer Display

Replace the battery in the thermometer before measuring again. If it still displays a variance in temperature, it could be due to one of the other reasons on this page. 

If the wrong method is performed for measurements, there tends to be errors in the temperature readings.

HAKKO recommends that the same tip undergoes measurement several times and the measured maximum temperature is adopted as a correct tip temperature. Since the temperature may be displayed lower for I-type and 1C-type tips whose tip end is thin in particular, take sufficient time for measurement and read the temperature when the displayed data are stable.

Kindly refer to the instruction manual for measuring instruments manufactured by other companies. 

Measuring of Temperature

  1. Firstly, remove solder that has been stuck on the tip prior to measurement. Use a damp cleaning sponge or the Hakko 599B Cleaning Wire to remove solder residue.
  2. Apply new solder onto the tip.
  3. Place the tip onto the measuring point and apply solder on it.
    measuring soldering tip temperature
  4. If too much solder is applied, heat is absorbed by the solder, causing inaccurate temperature readings. 
  5. The way of placing the tip onto the measuring point should be changed according to the tip shape. The image shown above is for the Shape B (Round Shaped) tip.
  6. Do not touch the slide pin with the tip nor should you solder directly on it. The slide pin is weak to high temperatures. 
    Slide Pin
  7. Record the displayed value. Repeat the steps a couple of times and use the maximum recorded temperature. 

How to Measure Temperature Correctly

The tip should be placed horizontally on the measuring point for the correct temperature measurement

Soldering Tip Temperature Measurement

For the B type, I type and D type, in the case of a tip with standard solder plating, place the center of the solder plating to the measuring point as shown left. How to place the plated face to the measuring point for the BC type and C type.

For the BC type and C type, in the case of a face with solder plating, place the plated face to the measuring point as shown below.

Tip Measurement for tinned surface


It is impossible to measure while applying hot air of Hot Air Rework System (HAKKO FR-801, etc.) directly. Use the Temperature Probe (No. C1541) specifically for Hot Air Rework Systems.

Additionally, due to human error or the differences amongst individuals, it is likely that different readings are generated if the soldering irons are measured by different people. Hakko recommends measurements to be done by the same person under the same conditions. 

If you have performed all these checks and the problem still persists, feel free to reach out to our sales staff who would be happy to assist you!

Back to blog