SOP for Calibration of Thermohygrometer

What is Thermo-Hygrometer?

A thermo-hygrometer is an instrument used to measure and monitor temperature and humidity at a time, it is also known as a 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 have a recorder included with a chart to properly display and monitor the readings. We are already familiar with 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 Thermohygrometer?

• Thermo hygrometers are the most common type of humidity sensor and you’ll find them in handheld units and transmitters. They commonly display temperature and humidity simultaneously and have a response rate of about four seconds. Their sensors read resistance or capacitance. 
• Typical applications include:

1. Breweries and fermentation
2. Environmental research
3. Food ripening and storage
4. HVAC
5. Pharmaceutical incubators
6. Pharmaceutical tablet coating
7. Plant growth chambers

• With features that can include large, back-lit displays, calculation of dew point, min/max, data hold, data logging, infrared temperature measurements, RS232 output, printing capabilities, and more. 

Advantages of Using a Thermohygrometer

1. Low cost
2. Quick sensor response time
3. Repeatability at ambient RH values
4. Portability

Disadvantages of Using a Thermohygrometer

1. Susceptibility to losing calibration
2. The level of accuracy is medium or moderate
3. Probes are not interchangeable

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 continuous monitoring should be properly implemented. Some products or procedure also requires a certain temperature and humidity in order to be processed properly.

Why Calibrate a Thermo-hygrometer?

• As for other instruments that are used for measuring, a 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.
• Some thermo-hygrometers or hygrometers can drift easily and cannot be adjusted. Once your Thermo hygrometer is calibrated, a calibration certificate will display the errors, and therefore, you can use it to apply corrections during measurements.
• It is also advisable to monitor the performance by comparing it regularly to another calibrated thermo-hygrometer. This will ensure that drift can be detected immediately while it is within the calibration interval.
• Regular calibration will also provide a good record for us to decide if we increase or decrease the calibration frequency we use.
• A recommended calibration interval for thermo-hygrometers that are situated in a stable environment is 6 to 12 months.

How to calibrate a Thermo-hygrometer?

• The standard way for a Thermo hygrometer and/or hygrometer calibration is to use a special chamber or a humidity generator, this is usually performed in a lab. The humidity generator can be varied to a different range of humidity and temperature.
• For single-range calibration, calibration should be done at the location where the Thermo hygrometer is being used.
• For multi-range calibration, calibration should be done in a humidity generator. the location where the Thermo hygrometer is being used.
• Thermo-hygrometers have different types of sensors, but regardless of their sensor types, the procedure for calibrating them is mostly the same. As long as the sensors are accessible and intact, we can calibrate them with this simple method.
• It is purely a 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.

SOP for Calibration of Thermo Hygrometer

Objective

To lay down a procedure for the Calibration of the Thermo Hygrometer.

Scope

This procedure outlines the proper procedure and frequency for evaluating and certifying Thermo Hygrometer at PharmaInfo Ltd.

Responsibility

• Engineering personnel and QC analyst responsible for calibration.
• It is the responsibility of the Department supervisor to ensure that equipment is appropriately cleaned, maintained in good working order, and available to engineering personnel as requested.
• Department Head responsible for overall compliance with this procedure.

ALSO READ: SOP for Calibration of Electrolab Dissolution Apparatus

Equipment Use

• Thermo Hygrometer units located throughout the facility are used for the temperature and humidity records. These units are certified at least annually and this certification is memorialized by a written label affixed to that unit.

Procedure

• The Engineering personnel and QC analyst are responsible for the annual calibration of the Thermo Hygrometer.
• Calibration must be performed using a certified traceable standard or a new or recently certified unit that can be traceable to a standard as a reference.

• Each Thermo Hygrometer should be uniquely identified as to the room and/or area in which it is used.
• Collects all units from the facility and brings them to an area for evaluation and calibration. 
• The units being evaluated and the reference standard unit is allowed a few minutes to equilibrate to the ambient room temperature and humidity.

ALSO READ: SOP for Temperature Mapping of Incubator and Refrigerator

Single Point Calibration Procedure:

• 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, and carefully check the sensor for any signs of dust or contamination. Check if it has good batteries, and replace low-powered batteries.
• Calibration is 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 Standard 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 in annexure-1.
• Check readings if, within the accuracy defined by the manufacturer, 

Accuracy = 

+/- 5% for Relative Humidity 

+/- 1 Deg C for Temperature.

• If the readings are already within limits update the corresponding record, do labeling and fill the information in annexure-1.

(Note: Accuracy limits from user manual of thermohygrometer).

Multi-Point Calibration Procedure:

• Before starting the calibration, ensure that the Humidity generator System is placed on a clean and dust-free platform.
• Ensure that the instrument has been connected to the power supply. Ensure that all communication cords between the units are connected properly.
• Switch on the Humidity Generator System.
• Set the temperature at 10°C and humidity at 20% and wait for the condition to stabilize.
• Place the Standard and the Unit Under Calibration (UUC) together, the sensors of both instruments must be close together and at the same level. 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 and record the readings in annexure-2.

• Set the temperature at 20°C and humidity at 40% and wait for the condition to stabilize for Humidity Generator System.
• Place the Standard and the Unit Under Calibration (UUC) together, the sensors of both instruments must be close together and at the same level. 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 and record the readings in annexure-2.
• Set the temperature at 30°C and humidity at 60% and wait for the condition to stabilize for Humidity Generator System.
• Place the Standard and the Unit Under Calibration (UUC) together, the sensors of both instruments must be close together and at the same level. 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 and record the readings in annexure-2.

• Check readings if, within the accuracy defined by the manufacturer.

Accuracy = 

+/- 5% for Relative Humidity 

+/- 1 Deg C for Temperature.

• If the readings are already within limits update the corresponding record, do labeling and fill the information in annexure-2.

(Note: Accuracy limits from user manual of thermohygrometer).

• Units with temperature readings within +/- 1°C and humidity readings within +/- 5% of the reference standard are considered to be within calibration limits and are certified and memorialized by a written label affixed to the unit.
• Units with temperature readings > +/- 1°C or humidity readings > +/- 5% of the reference standard are considered out of acceptable limits and are replaced.
• The Thermo-Hygrometer Certification Record includes the following information:

1. Instrument Description
2. Location of unit
3. Date of Calibration
4. Next Due Date
5. Standard Details
6. Calibration Result

• The certification label should include:

1. Calibration Date
2. Due Date
3. Initials of the person certifying the unit.

• The department Manager maintains the Thermo-Hygrometer Certification Record for 1 year in accordance with SOP for Document distribution, Control, Storage and Disposal procedure.

Maintenance

• Clean units periodically by wiping them down with a mild disinfectant and a soft cloth.
• Any additional maintenance/service should be performed by authorized personnel.

Reference

Manufacturer’s manual.

Annexure

Annexure-1 : Single Point Calibration Record of Thermo Hygrometer

Annexure-2 : Multi Point Calibration Record of Thermo Hygrometer

Abbreviation

SOP : Standard Operating Procedure

°C : Degree centigrade

REVISION HISTORY

Nil

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