# 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 used for storage of medicines, vaccines etc. The operator or owner of the cold room has to continuously monitor and record the temperature and sometimes humidity also. The question is where to place the sensors in the cold room continuous monitoring system. These are the sensors which are used to generate alarms in case the temperature or humidity goes above the permitted limits for the specified duration.

## How to decide number and locations of sensors of data loggers?

First of all, we have to carry out a temperature mapping study to decide the number and location of sensors. We can see that in many cold rooms, the operator places a sensor before carrying out the mapping study. The sensor will be located at the most convenient location including near to a door. However if an auditor asks a question as to how the location was decided, there can’t be any answer.

Hot cold points cold room

We will explain how to decide the same.

After conducting a mapping study, the hot and cold points of the cold room have to be decided. As an example we will consider a cold room in which 10 data loggers are used 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 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 at 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 all the examples we are assuming a 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

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

DL1 : 7.2°C, 3.1°C

DL2 : 7.1°C, 3.3°C

DL3: 7.0°C, 3.1°C

DL4:7.1°C, 3.4°C

DL5:6.9°C, 3.3°C

DL6:7.1°C, 2.6°C

DL7:7.2°C, 3.3°C

DL8:7.7°C, 2.7°C

DL9: 7.1°C, 3.1°C

DL10:7.3°C, 3.5°C

In this case, you can see that location of DL 6 is the cold point and 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:

DL1 : 7.2°C, 3.1°C

DL2 : 7.1°C, 3.3°C

Cold room temperature distribution

DL3: 7.0°C, 2.6°C

DL4:7.1°C, 3.4°C

DL5:6.9°C, 3.3°C

DL6:7.1°C, 2.6°C

DL7:7.2°C, 3.3°C

DL8:7.7°C, 2.7°C

DL9: 7.1°C, 3.1°C

DL10:7.3°C, 3.5°C

In this case, you can see that locations of DL 6 & DL3 are the cold points and location of DL8 is the hottest point. 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. 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 air flow of the chiller unit.
3. If you have one of the temperature data logger 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 data logger sensors in Reefers or Vans?

In principle, the methodology is 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.

###### 4 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. 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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