We've outlined A LOT of information on calibration below. Use the Table of Contents to navigate to different topics on calibration, recalibration, and much more.
Table of Contents
A calibration is a comparison between two measurements. Dickson’s calibration lab compares a device (unit under test) with a more accurate device (the standard). The standard tells us exactly where 70F, 0C, 137F, etc. are. After the comparison is made we adjust the device under test so it reads the same as the standard. At the end of all of that, we do one last check at one or multiple points to fully guarantee the device is within its specified accuracy.
Calibration is essential to all devices that measure a variable. However, we often get the questions, “Why isn’t it accurate already?” Isn’t it made to be accurate? The answers are: it is, and yes.
However, while our devices are accurate without calibration, we can’t be positive they are accurate to a specific measurable degree (and thus can’t prove their accuracy) unless we perform a calibration. We wish it was as simple as “Yes, this device is accurate,” or “No, this device is not accurate,” However, measuring temperature, humidity, and pressure are delicate sciences.
As stated above, a calibration is a comparison between two measurements, and then the subsequent adjustment of one of the measurements if the two measurements are not reading the same value. When you need to calibrate your device, Dickson places it and a standard in a special temperature chamber that allows us to change the environment your device and the standard reside in. We then set the temperature, humidity or pressure values of the chamber using the standard’s measurements. If your device reads the same as the standard, then it is good to go and accurate! If it does not, then we have to adjust its measurements to better match the standard. Dickson will run through this process multiple times, or until the device is as accurate as it is supposed to be (within specification or within “spec”). Once we are sure that your device is accurately measuring temperature, humidity, or pressure, we send it back to you with documentation.
Over the years, especially with the creation of environmental monitoring regulations for a never-ending list of industries, calibration has become synonymous with documentation. It’s no longer enough for calibration labs to tell you that the device you are purchasing has been calibrated, it must be a documented process that you can point to whenever your devices are called into question.
The documentation you receive from Dickson comes labeled as a Certificate of Calibration. This document will tell you at what points Dickson checked your device (at the end of the calibration process) for its accuracy. Depending on the type of calibration you ordered from Dickson, the certificate will display 1 or 3 checked points (shown in values of Fahrenheit or Celsius). These documents will also tell you standard your device was compared to and how accurately your device measures temperature, humidity, or pressure.
We discussed the process above, but we figured we would expand a bit further . . .
The calibration process changes depending on the variables you are measuring and the calibration labs performing the calibration. At Dickson, we have SOP’s in place for every unit that goes through a temperature, humidity, temperature and humidity, and pressure calibration, to ensure units are calibrated to as accurate a degree as we have specified. Our process works like this:
A. We place the sensor and a standard in a stable environment.
We say “sensor” above to distinguish some of our devices from others. The “sensor” is what is actually taking the temperature, humidity, or pressure. Sometimes that sensor is within the device, making the device synonymous with its sensor. However, other times, like in the case of our Replaceable Sensors, this is not the case, as the sensor is able to be removed from the device and calibrated on its own.
“Stable environments” are essential to calibrations. We can’t assume any temperature, humidity, or pressure value without the use of a standard and confined, stable environment. These environments are chambers, where temperatures can be changed quickly and to the exact value we desire.
B. We compare the sensors readings to that of the standard across a range of temperatures.
After the sensor and standard are both within the stable environment, we run comparison tests for the entire range of temperatures that a sensor can read, recording by how much the sensor deviated from the standard at each temperature value.
C. If there are any differences between the sensor and the standard, we adjust the sensor to align with the standard.
If your sensor isn’t reading temperatures, humidity, or pressure values accurately, we want to fix it. After the comparison is made across the spectrum of temperature, humidity, or pressure values, we take a look at how far off your sensor was off from our standard. If the device is out of its stated accuracy, we adjust it.
How do we adjust the sensor? We don’t physically adjust it any way-bending, tearing, taping doesn’t happen-we promise. Instead, we adjust the readings it is sending to the data logger or chart recorder. We want to keep your sensor intact, so we simply adjust the values (via an advanced software system) that it believes it is reading, to account for the miscue in its accuracy.
At Dickson, we use two types of adjustments when making your sensor read more accurately. Before we get to them however, it’s important to understand that your device is calibrated on a curve. How we manipulate that curve determines how we get your sensor to be within its accuracy specifications.
Zero Adjustment: A zero adjustment is also called an offset. All of Dickson’s units are calibrated on a curve. The zero-adjustment moves that curve up or down to equal what the standard is reading. If your standard reads 26F, but the device displays 24F, your device is off by 2F. You need to adjust the device two degrees in order for it to read accurately. A zero adjustment will shift the entire scale by 2F even if the extreme ends of the scale were not off by 2F.
Span Adjustment: A span adjustment is when you adjust the slope of the calibration curve. In the example above, we’d adjust the device by 2F at 24F, but the extremes wouldn’t need any adjustment and therefore would be relatively unaffected.
D. We run though the above process multiple times, each time adjusting the device as it is compared at multiple temperatures (more on this below, too).
We don’t just run through the process once-we do it over and over again until the unit is within our specifications. Because we perform both a zero-point and span adjustment, sometimes multiple calibrations and adjustments are needed to ensure device accuracy.
E. We perform a final check of one or more points (depending on your order) and create the necessary calibration certificate.
This final check and subsequent documentation is exactly what many of our customers are looking for. When we perform the final comparison and calibration, we generate a document based on how many calibration points you would like to be validated as accurate by us. While we ensure that all units calibrated are within a stated specific accuracy, some industries need proof of that. So, we offer documentation generated from this final check. That documentation will state that the unit has a specific temperature, humidity, or pressure accuracy at a certain temperature, humidity, or pressure. We label that document a “Certificate of Calibration.” For hospitals, pharma manufacturers and distributors, and those in the food industry, auditors may show up and ask to see that document, so having it is probably a good idea.
When you are ordering a calibration from Dickson, you have the choice to choose between a 1-point, 3-point, or customer points calibrations. The Certificate of Calibration will reflect which option you choose. For many industries, a 1-point calibration is plenty. As a fair warning, we will usually perform the final check at room temperature, if you are monitoring at a specific temperature that is not between 60-80F, you may want to have a custom calibration point done. We offer a 3-point calibration for those industries who want or need documentation of accurate readings across a larger range. We will typically do a high, middle, and low calibration check across the temperature spectrum that your sensor or probe monitors. For a simple digital ambient sensor, this would be from -20 to 120F. Our custom calibrations are for those organizations who want to see documentation that a validation check was done at a specific temperature. If you know what temperature you will be monitoring at a custom point calibration may be the best documentation for you.
Recalibration is just a calibration for a second time. Your temperature, temperature and humidity, or pressure sensor becomes less and less accurate over time, and eventually, you need to “Recalibrate” it to get it back within its specified temperature accuracy.
Due to natural wear and tear, temperature and humidity sensors lose their pinpoint accuracy. Environmental factors such as dust, dirt, and humidity can negatively affect a sensor’s accuracy. This is called drift. These devices probably won’t read 95F in a refrigerator that feels cool, but they may be off by a few degrees. When you calibrate your device you are accounting for the natural drift of the sensor.
Those few degrees matter to a lot of organizations and a lot of industries, so we at Dickson recalibrate our manufactured devices in our A2LA accredited laboratory.
After 90 years in the business, we recommend re-calibrating every year: it’s what we’ve found works best.
Now, some factors may change that. These include:
Environment: The more extreme the conditions (really hot or really cold) the more likely the device will drift over time. Dust, dirt, moisture, air particles can all contribute to a device needing to me recalibrated more often as well.
SOPs: Your standard operating procedure (SOP) may require you to recalibrate more often than our recommendation.
Regulations: Different industries are subject to different regulations and may require calibrations more or less often than our recommendation. It is best to check with your regulatory agency.
Auditors: Some auditors have their own preferences on how often a device should be calibrated. Most often they adhere to the regulations for their industry/regulating body, but this isn’t always the case. Be sure to ask them what they’ll be expecting.
Physical damage - Dropping, kicking, bumping, or otherwise damaging your device may mean it will need to be recalirated more often.
You can also use a before and after calibration to hone in on the perfect calibration cycles.