# Where to place a temperature & humidity sensor in a cold room,warehouse,van or refrigerator?

This article is particularly about cold rooms and walk-in freezers for storage of medicines and vaccines. We are explaining on deciding the number of sensors and location of sensors of a temperature monitoring and alert system. The operator or owner of the cold room has to continuously monitor and record the temperature. Sometimes it may be essential to record humidity also. The question is where to place the sensors in the cold room continuous monitoring system. These are the sensors which generate alarms in case the temperature or humidity goes above the permitted limits.

## How to decide the number and locations of sensors of temperature alert system?

First of all, we have to carry out a temperature mapping study to decide the hot and cold points. Based on these, we can decide the number and location of sensors. We can see that in many cold rooms, the operator places sensors of a monitoring system without a mapping study. The sensor will be located at his most convenient location. Sometimes this may be close to the door, which is irrational. If an auditor asks a question as to how the location was decided, there won’t be any answer.

Hot cold points cold room

We will explain how to decide the same.

After conducting a mapping study, we can identify the hot and cold points of the cold room. As an example, we will consider a small sized cold room. We assume that we use 10 data loggers to conduct the mapping study. After analyzing the complete data of a loaded test, we can find out the hottest and the coldest points. We should not consider data of the empty test, door opening test and power failure test for this purpose. The air circulation and stabilization of the cold room will be tested in a loaded test.

The hottest point and coldest point will be normally in different places. We have to place one temperature sensor at the hottest point and one sensor at the coldest point. Please see the following examples for further clarification. In both the examples we are assuming cold storage of 2 to 8°C for storage of medicines. It is assumed that we used 10 data loggers for the mapping study. DL1 is data logger1, DL2 is data logger 2 and so on.

### Example 1 for cold room temperature monitoring & alert system

In this example we will consider the following maximum and minimum values of the data loggers:

1. DL1 : 7.2°C, 3.1°C
2. DL2 : 7.1°C, 3.3°C
3. DL3: 7.0°C, 3.1°C
4. DL4:7.1°C, 3.4°C
5. DL5:6.9°C, 3.3°C
6. DL6:7.1°C, 2.6°C
7. DL7:7.2°C, 3.3°C
8. DL8:7.7°C, 2.7°C
9. DL9: 7.1°C, 3.1°C
10. DL10:7.3°C, 3.5°C

In this case, you can see that location of DL 6 is the cold point. Similarly, the location of DL8 is the hottest point. These are the weakest points in the cold room. Now you have to place one data logger at the location of DL6 and one data logger at the location of DL8.

### Example 2 for cold room temperature monitoring

In this example we will consider the following maximum and minimum values of the data loggers:

1. DL1: 7.2°C, 3.1°C
2. DL2: 7.1°C, 3.3°C
3. DL3: 7.0°C, 2.6°C
4. DL4:7.1°C, 3.4°C
5. DL5:6.9°C, 3.3°C
6. DL6:7.1°C, 2.6°C
7. DL7:7.2°C, 3.3°C
8. DL8:7.7°C, 2.7°C
9. DL9: 7.1°C, 3.1°C
10. DL10:7.3°C, 3.5°C

In this case, you can see that locations of DL 6 & DL3 are the cold points. There are two cold points. However, there is only one hot point which is DL8. These are the weakest points in the cold room. In this case, you have to place one data logger each at the locations of DL3, DL6 and DL8.

#### Data logger location for humidity monitoring of cold room (humidity sensor)

For humidity, there are no hot and cold points. Generally, the humidity quickly equalizes across the entire cold room, once we close the room. Hence, you can consider the following general rules:

1. Cold room temperature distribution

We should not place the Temperature & humidity data logger near to the door. The humidity near the door increases as soon as the door is opened.

2. The humidity data logger should not be directly in front of the airflow of the chiller unit.
3. If you have one of the temperature data loggers deep inside the cold room, it is better to place the humidity sensor of the data logger near to this sensor. You will be able to correlate humidity variation with respect to temperature variations.
4. For a small to medium-sized room, only one humidity sensor is sufficient irrespective of the number of temperature sensors.
5. If you have a dehumidifier inside the room, you should place the humidity sensor far away from the outlet of the dehumidifier. The humidity at the inlet of the dehumidifier will be always high and at the outlet will be very low. Placing a humidity data logger near to the dehumidifier will generate readings which are not in line with the distributed humidity,

However, if humidity is a very critical parameter for your stored goods, you may have to consider more humidity data loggers for the continuous monitoring system.

#### How to decide the location of temperature sensors in Reefers or Vans?

In principle, the methodology is the same as above. You should take data of temperature mapping study during a loaded test excluding data of power failure test, door opening test etc. You have to decide at least one each of hot point and cold point and place sensors accordingly.

The main limitation is that you can place a sensor only on the wall. So even if you find a hot point somewhere in the middle of the van, you cannot place a sensor there. This is because inside a van you always have a different pattern of loading. Hence you can install the sensor for continuous monitoring somewhere on the nearby wall or ceiling.

The logic for selection of the sensors point is to closely monitor the variations. You should receive an alert as soon as the temperature goes above or below the limits. You have to select the location and number of sensors accordingly. In case the cooing unit is not working, the hottest point will be the first one to cross the upper limit. Hence you will receive an alert as soon as the hottest point crosses the limit. Same is the case for the cold spot.

### FAQ on a temperature monitoring system for medicines and vaccines

1. What are the regulatory requirements for the selection of a temperature recording system in pharma & food segments? If you are in the USA, you should have systems complying with 21CFR Part 11 standards. Many other countries follow this or other standards. There is no common standard across the world. World Health Organization (WHO) also has a standard specification for Distribution of Pharma products.
2. What are the minimum ideal technical requirements for a temperature monitoring system for pharma and food segments? (1) It should be able to generate a phone, SMS or local siren alerts (2) The system should ideally work even in case of a power failure (3) The system should ideally work even in case of a WiFi or network failure (4) It should be possible to calibrate sensors individually (5) There should not be any loss of existing data in case of a power or network failure (6) You should be able to program the sampling interval based on the critical nature of the goods. (7) you should preferably able to use a single system for the warehouse, cold room, refrigerator, trucks etc. This makes your life easy. (8) It is best to use sensors with inbuilt battery without the need for external power supply. This is extremely essential for warehouses. Pulling the power supply to the sensors to various sensors will be challenging. Also during operations, damages may occur to these power cables.
3. Is it possible to use a single temperature recording system for warehouse, cold room, trucks etc? Yes, there are such systems available. We highly recommend using such systems. Thus you will be able to use Big Data Analytics to improve the efficiency of operations.
4. What kind of power supply is ideal for temperature monitoring and alert systems? It is ideal to use systems with internal battery back up. In this case, you will receive a phone call or SMS alerts even in case of power supply.
5. What kind of an alert system is ideal for a temperature alert system? A phone call alert system using a SIM card with battery back up is most ideal. In such a case you will receive phone calls without power supply and network connection. This may be the most dependable alert if you have costly assets. The system should be able to make phone call alerts to multiple operators.

We provide all kind of temperature mapping study, temperature qualification study, temperature monitoring systems and temperature alert systems across the world.

###### 5 comments on “Where to place a temperature & humidity sensor in a cold room,warehouse,van or refrigerator?”
1. Sajan Paul says:

Requirement of sensors for Temperature and Humidity

• Vacker says:

One of our sales engineers will contact you

2. KR Krishnakumar says:

Hi Can you please send me Vacker India contact details.

3. Abhijit says:

During routine temperaturing monitoring in hot point.How we place the temperature sensor?
1. Temperature sensor will touch with wall or it should be in air. Please send reference guideline also.

• Vacker says:

Hot points are identified in a temperature mapping study. Due to practical limitations, it might be sometimes difficult to place a sensor at the exact hot points. Examples of limitations are areas with the movement of the forklift, middle of a walkway etc. In such cases, the sensor is ideally placed in the nearest possible location.

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