What is a thermo-hygrometer?
Most are familiar with this 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 use 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 a humidity is the amount of moisture or water vapor in the air.
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.
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 a thermal expansion and this will result to an additional sources of error.
In electrical systems, the presence of high humidity can result to moisture in which some electrical components particularly made of metals will get rusty or oxidize which can result to damage or malfunction. On the other hand, an area environment with low humidity will cause a 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 a 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 a 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 is 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.
Thermo- hygrometer can also be calibrated by using a saturated salt solution ( a mixture of salt and a little bit of water) if the temperature is not that critical in your process or if the use of your thermo-hygrometer is just a hygrometer only (humidity only).
You can use this method at any ambient room temperature in an enclosed area ( can be a self-sealed plastic) and the resulted or expected humidity is 75%+/-0.3%. This is much applicable when you are at home or office where no further documentation is required and time is not a big factor because this will take more time than the first method.
See for more details: https://en.wikipedia.org/wiki/Hygrometer
Let us go to our procedure below.
To define 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 room or a laboratory.
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
- Temperature: 23 +/- 5 deg C
- Humidity: 50 +/- 30%
- Measurement Data Sheet (MDS)
Reference standard to use:
- Temperature Humidity Meter Fluke 971
- Cleaning materials
- Check the thermo-hygrometer for any visual defects that can affect its accuracy. Discontinue calibration if any defect is noted.
- Clean the thermo-hygrometer with a soft cloth, carefully check the bi- metallic sensor for any signs of dust or contamination. Check if it has good batteries, replace low powered batteries.
- Prepare the measurement data sheet (MDS) and record all necessary details or information ( Brand, Model, serial #, etc).
- 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.
- 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.
- Once stabilized, get the readings on their display.
- Perform 3 times and record readings on the MDS.
- 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.
- 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.
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