Calibration of Thermo-Hygrometer (Temperature and Humidity Meter) -Single Point Method

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Dwyer Thermo-Hygrometer Pen
Extech 445703 Big Digit Hygro-Thermometer
Extech Humidity and Temperature Recorder

 What is Thermo-Hygrometer?

Most are familiar with these instruments because if you have been to a hospital or even a mall these days, you can see it mounted in a wall. A thermo-hygrometer is an instrument used to measure and monitor temperature and humidity at a time, it is also known as Temperature Humidity Meter. It is a combination of a thermometer and a humidity meter or hygrometer.

Since both parameters affect each other, it is good that we can monitor them simultaneously. Some Thermo-hygrometers has a recorder included with a chart to properly display and monitor the readings. We are already familiar with a temperature which is the degree of hotness or coldness of a body or system, while humidity is the amount of moisture or water vapor in the air.

Why use a Thermo-hygrometer?

In our typical environment outside, when it is very hot and very humid, what do you feel? If you observed, you will perspire faster and that perspiration in your skin will not evaporate easily so you feel very sticky and uncomfortable. On the other hand, when it is very cold and low humid, our skin and lips are very dry which also affects our throat and breathing where it makes us to cough.

But how do we know that temperature and humidity are low or high? It is through the use of a thermo-hygrometer.  By monitoring temperature and humidity, a desired environmental conditions can be controlled and achieved. Thermo-hygrometer will monitor the environment and inform us what needs to be done so as not to affect our equipment and our process. 

The Importance of Thermo-Hygrometer

There were lots of reasons why during calibration, environmental conditions must be acceptable before starting a calibration work.

In dimensional calibration, the temperature is a big factor in maintaining an accurate measuring process. A gauge block, for example, will be affected by thermal expansion and this will result in an additional source of error.

In electrical systems, the presence of high humidity can result in moisture in which some electrical components particularly made of metals will get rusty or oxidize which can result in damage or malfunction. 

On the other hand, an area environment with low humidity will cause high resistance in the air where a problem with electrostatic discharge will be observed.

In some food industry, humidity is kept low to avoid molds to grow in their products and to maintain dryness and preserve its good quality. Others kept temperature and humidity higher to grow bacteria to ferment their products.

And other applications that require humidity and temperature controlled environment.

Where to use a Thermo-hygrometer?

Thermo-hygrometers are used especially in an area or room where temperature and humidity are critical in a process just like in laboratories, manufacturing areas, pharmaceuticals, food industries and more. Critical in a way that it affects your process where a continues monitoring should be properly implemented. Some products or procedure also requires certain temperature and humidity in order to be processed properly.

Please visit my other post to read more regarding this topic.Temperature-and-humidity-why-do-we-need-to-monitor

Why Calibrate a Thermo-hygrometer?

As for other instruments that are used for measuring, thermo-hygrometer should be calibrated to ensure accurate readings. Thermo-hygrometer has a sensor that can degrade and drift anytime so a regular calibration must be applied.

How to calibrate a Thermo-hygrometer?


Thermo-hygrometers have different types of sensors, but regardless of its sensor types, the procedure in calibrating them is mostly the same. As long as the sensors are accessible and intact, we can calibrate it with this simple method.

It is purely comparison between a standard or more accurate Thermo-hygrometer and a regular thermo-hygrometer which we call a unit under calibration (UUC) where adjustment is not required unless the instruments have the capability to be adjusted.

You can use this method at any ambient room temperature in an enclosed area where the environment is stable. This method is suitable for those thermo-hygrometers that are permanently installed in a location where removing them is not possible.

Also, this method is applicable when you are not requiring a very high accuracy of results or you have a wide tolerance for your process monitoring.

See for more details:

Let us go to our procedure below.


To define the verification procedure on thermo-hygrometers with a probe using a Fluke 971 as the reference standard. We will be using a single point calibration since the environment that we will measure is the existing environmental condition in a stable room.

Calibration Method:

This is accomplished by comparing the temperatures displayed from Fluke 971, which is the reference standard,  on the display of the Unit Under Calibration (UUC) which is the Extech RH520A Humidity and Temperature Chart Recorder.

Click this link to see my review for  Extech RH520A Humidity and Temperature Chart Recorder


  • Warm-up time (UUC): At least 1 hour for proper stabilization
    Sample Measurement Data Sheet (MDS)
    Sample Measurement Data Sheet (MDS)
  • Temperature:   23 +/- 5 deg C
  • Humidity:         50 +/- 30%
  • Measurement Data Sheet (MDS)

Reference standard to use:

  • Temperature Humidity Meter Fluke  971

Other materials:

  • Holder
  • Cleaning materials

Calibration procedure:

  1. Check the thermo-hygrometer for any visual defects that can affect its accuracy. Discontinue calibration if any defect is noted.
  2. Clean the thermo-hygrometer with a soft cloth, carefully check the sensor for any signs of dust or contamination. Check if it has good batteries, replace low powered batteries.
  3. Prepare the measurement data sheet (MDS) and record all necessary details or information ( Brand, Model, serial #, etc).
  4. Calibration will be based on the room or ambient condition, so make sure that the room has a stable temperature. A confined space where air movements are very minimal is better.
  5. Place the Fluke 971 and the Unit Under Calibration(UUC) together, the sensors of both instruments must be close together and at the same level, use a holder if necessary. Allow the units to stabilize for at least 30 minutes. Temperature and humidity will be obtained in the same way.
  6. Once stabilized, get the readings on their display.
  7. Perform 3 times and record readings on the MDS.
  8. Check readings if within the accuracy defined by the manufacturer, for the example above ( Extech RH520A),  accuracy = +/-3% for humidity and +/-1 DegC for temperature.
  9. If the readings are already within limits update the corresponding record, do labeling and sealing and issue to the owner, otherwise, do necessary repair or adjustment.
  10. End

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8 Responses

  1. Kevin
    | Reply

    Hi Edwin,
    your calibration procedure is very helpful. Just want to know several things for the calibration process. Is there any international standard that states the number of set point required for calibration proses and number of repetition for each set point?

    Thank you.

    • edsponce
      | Reply

      Hi Kevin,

      Thank you for the positive comment. I have read a British Standard which details a calibration procedure that requires three set points of interest with three repetitions. You may check this standard, BS 1339-3:2004. It has an updated version but I did not check it yet. I appreciate if you can update me just in case you read the updated one.

      Hope this can help.

      Best regards,

  2. Gregor Mendell Rabang
    | Reply

    Hi, what is the frequency of Calibration of this Hygrometers. Is calibration for every 2 years valid?

    • edsponce
      | Reply

      H Gregor,
      Thank you for reading my post. Usually, there are no set of rules for the frequency of calibration of an instrument. But the majority of the users have defaulted to a frequency of 1 year. This is based on experience and the stability of the environment where the hygrometer is placed and being used.

      2 years frequency is valid considering that you have a history or a record of its past performance where it has a stable calibration record that you can rely on.

      If you have a new hygrometer, and your budget can allow it, my recommendation is to start 6 months then increase your calibration frequency if readings are stable for every calibration until you reach 2 years and can stay there as your calibration interval. At this point, you are more confident that 2 years is good, you even have a justification if ever an auditor will question it.

      I hope this helps, please do not hesitate to comment further.

      Thanks and regards,

  3. rhea
    | Reply

    hello good day! what is/are your reference/s for the procedure?

    • edsponce
      | Reply

      Hi Ms. Rhea,
      You may check this procedure under the British Standard-BS 1339-3:2004 section 9.4. It is not the latest version but you can read it there.

      Hope this helps,

  4. hk combs
    | Reply

    I am in a situation unusual to my normal field which is Mechanical HVAC and building controls exct. We are managing a building just over a year old. It is unique as it is a gallery/teaching center for the arts. The Gallery rotates very high end displays along with up and coming artists for all ages and around the world.
    I have been asked to get base line humidity % and temps to confirm or dispute the accuracy of the devices already in place. I was pleased at first to see the fluke 971 getting an 55% average but in cross reference one being a humid-stat for a humidification system and the second being a near high end trusted stand alone I am not so sure. The other two devices are closer to each other then the readings from the fluke 971. The two to one ratio is a very questionable along with an average 12% deviation average. The humidity dead band is consistent on all the devices but the start points are not. The short Question is how can I solidly confirm which one is out of calibration?

    • edsponce
      | Reply

      Hi HK,
      Thank you for visiting my site.
      You have performed a good verification of the humidity readings. But in order to have a valid verification, you need to have a good reference, in which the reference should be a calibrated instrument. In this way, any results you compare with the reference standard will tell you if it is acceptable or not.

      If you are using the Fluke 971 as the reference, be sure that it is still within the calibration date (interval) with the calibration certificate at hand. Since it is calibrated, you are confident that it is reading accurately.

      The next thing after you compare readings, check the calibration certificate, there is a chance that Fluke 971 has a correction factor that you can use. Apply the correction factor if applicable (check my related post here about correction factor). Also, be sure that when you performed the verification, the fluke 971 sensor is close to the sensors in question.

      The 2:1 accuracy ratio is not a big deal as long as the verification result is acceptable to you or within the indicated tolerance.

      Hope this helps

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