Common Misunderstandings About Temperature Calibration

1. "Calibrating the Readout is Enough"

• Misunderstanding: Believing that calibrating the digital indicator or transmitter (the readout) is sufficient for the entire system.

• Reality: A temperature measurement system is a chain: sensor → connection → transmitter → readout. The sensor (e.g., RTD, thermocouple) is often the component that drifts the most. A proper calibration must apply a known temperature to the sensor itself and compare the entire system's output to the standard. Just calibrating the readout with a simulator only checks one link in the chain.

2. "All Calibration Sources are Created Equal"

• Misunderstanding: Assuming a dry-block calibrator is just as accurate as a liquid bath for any application.

• Reality: The choice of calibration source is critical.

• Dry-Blocks: Are convenient and portable but can have significant axial and radial uniformity errors. The depth of insertion and the fit of the thermometer well can greatly impact accuracy.

• Liquid Baths: Provide superior uniformity and stability, especially for high-accuracy work or calibrating irregularly shaped sensors. They are the right choice when the highest accuracy is required.

3. "An Ice Bath is Always 0.00°C"

• Misunderstanding: Thinking that simply mixing ice and water in a cup will automatically create a perfect 0°C reference point.

• Reality: A proper ice bath requires a specific technique to achieve 0.00°C ±0.02°C. Using large ice chunks (not crushed), distilled water, and ensuring a slushy consistency with no air pockets is essential. A poorly made ice bath can easily be off by 0.5°C or more.

4. "Stability Means the Displayed Value is Ready to Record"

• Misunderstanding: Seeing that the number on the calibrator's display has stopped changing and immediately recording the result.

• Reality: The calibrator's display shows the temperature of its internal sensor. The device under test (DUT), especially if it has a larger thermal mass, may still be stabilizing. You must wait for the DUT's reading to stabilize, not just the calibrator's display.

5. "Calibration is the Same as Adjustment"

• Misunderstanding: Using the terms "calibration" and "adjustment" interchangeably.

• Reality: They are two distinct steps.

• Calibration: The process of comparing the measurement of the DUT to a reference standard and documenting the error.

• Adjustment (or Trimming): The process of physically or digitally correcting the DUT's output to minimize its error.
  You can calibrate without adjusting, but adjusting without first calibrating is guesswork.

Common Misunderstandings About Humidity Calibration

1. "Humidity Calibration is Just Like Temperature Calibration"

• Misunderstanding: Approaching humidity calibration with the same mindset and equipment as temperature calibration.

• Reality: Humidity calibration is vastly more complex. While temperature is a scalar quantity, humidity has many units (RH%, Dew Point, PPM) and is highly sensitive to temperature. The equipment required (e.g., chilled-mirror hygrometers, two-pressure/two-temperature generators) is more specialized and expensive.

2. "A Humidity Generator Only Needs to be Calibrated Once"

• Misunderstanding: Assuming a high-end humidity generator maintains its accuracy indefinitely.

• Reality: The built-in reference sensors in a humidity generator (often capacitive RH sensors) themselves drift and require regular recalibration against a primary standard (like a NIST-traceable chilled-mirror hygrometer) to maintain their stated uncertainty.

3. "Response Time Isn't Important"

• Misunderstanding: Not accounting for how long a humidity sensor takes to reach equilibrium at a new set point.

• Reality: Humidity sensors, especially capacitive polymer sensors, can have very slow response times (minutes to hours). Recording data before the sensor and the environment have fully stabilized is a primary source of error. The calibration chamber must be given ample time to stabilize at each new set point.

4. "Any Environment is Fine for Benchtop Calibration"

• Misunderstanding: Performing humidity calibration on an open lab bench without considering the ambient conditions.

• Reality: The ambient lab humidity can significantly affect the calibration, especially if the generator's ports are open or there are leaks in the test chamber. The calibration setup should be isolated from drafts and rapid changes in the lab environment.

5. "All Humidity Sensors are Calibrated the Same Way"

• Misunderstanding: Assuming a single method works for all sensor types.

• Reality: The best practice depends on the sensor:

• Relative Humidity (RH%) Sensors: Require a calibrated humidity generator to create specific RH levels at a controlled temperature.

• Dew Point Sensors: Are best calibrated using a primary standard like a NIST-traceable chilled-mirror hygrometer, which directly measures the fundamental dew point temperature.

The Universal Misunderstanding

"If it Passes Calibration, it's Accurate Until the Next Cycle"

• Reality: Calibration is a snapshot in time. It verifies performance on that day. It does not guarantee the instrument will not fail five minutes later. The purpose of periodic calibration is to catch drift over time. Instruments used in harsh conditions or that show a history of drift should have shorter calibration intervals.

In summary: The core misunderstanding across both fields is underestimating the complexity of creating a known, stable, and uniform physical condition (temperature or humidity) and properly comparing a device to it. Avoiding these pitfalls requires understanding the physics of measurement, the limitations of equipment, and a rigorous, documented process.