# Improving the temperature stability of a temperature controlled water bath

• jonlg_uk
In summary: Another benefit to using a metal container of oil is that it will dampen down the temperature fluctuations that occur in the water. This will help to reduce the thermal noise or 'jitter' that can occur. Overall, I think this method would be a good way to improve temperature stability. However, I do have some questions that I would like to ask someone with more experience in this area. For example, what is the thermal diffusivity of different oils? How much energy does it take to achieve thermal equilibrium? What is the target
jonlg_uk
Hello, I have a temperature controlled water bath that is capable of controlling the temperature of the water to 0.2 degrees C. However this isn't good enough for my application and I require a greater level of stability. I want to place a metal box filled with oil inside the temperature controlled water bath in order to dampen down the temperature fluctuations that occur in the water.

Will this work? My intuition tells me that in order for this to work the specific heat capacity of the oil inside the metal box needs to be greater than the specific heat capacity of the water to have any effect.

Another factor I have consider is the viscosity of the oil. Because the oil is more thick than the water I may get areas inside that are hotter than others and because the oil does not circulate as readily as water I may get hot areas. For example the oil near the inside walls of the metal box will be hotter than the oil at the surface of oil as I do not intend to fully submerse the metal box in the water because I am worried that water will leak through the list of the metal box. I need a uniform temperature throughout the oil.

How large a volume do you require to have the temperature controlled to better than 0.2 K? How well do you have to control the temperature? What is the target temperature (i.e, 0 C? 40 C? 700C?)?

When we needed to hold temperature variations to less than 0.05K, we used Peltier devices, but the active volume was only about 1 cm x 1 cm x 10 cm.

The dimensions of the temperature bath is 274 x 330 x 559 (H x W x L). The volume of the metal box filled with inside the bath is 150 x 300 x 240.

The best performance I can get from the temperature controlled bath is 0.2 degrees C/hour when it is set to 35 degrees C.

I want to use the oil to dampen down this and see if I can reduce it to maybe 0.1 degrees per hour when it is set to 35 degrees C.

I am sure you can calculate the thermal fluctuations of the oil as a result of the water temperature varying. I just don't know about thermodynamics, thermal mass, diffusivity and heat capacity can solve this...N.B I am using silicone oil with a viscosity of 50 Centistokes.

Can anyone offer some help for my problem?

I thank you in advance :)

In the interest of offering something back to the website I will paste my notes I made on this subject:

I have investigated the effect of using a bain-marie method for improving temperature stability. The idea is to place a metal container of oil inside the temperature controlled water bath. I have been trying to figure out why this will work last night.

If we examine the equation for thermal diffusivity:

where:
k : thermal conductivity (SI units: W/(m·K))
p : density (kg/m³)
cp : specific heat capacity (J/(kg·K))

The definitions for the terms are:

Specific heat capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount

Thermal conductivity is the property of a material's ability to conduct heat. Heat transfer across materials of high thermal conductivity will occur at a faster rate than across materials of low thermal conductivity.

The denominator of the thermal diffusivity expression above, p*cp , can be identified as the volumetric heat capacity with the SI unit of J/(m³·K). Also know as volume-specific heat capacity that describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change. It is very similar to the specific heat capacity. However specific heat capacity is based on the mass of the material, while volumetric heat capacity is based on a given volume. Multiplying the specific heat capacity(cp) by the material density (p) will give us volumetric heat capacity.

Substances with high thermal diffusivity rapidly adjust their temperature to that of their surroundings because they conduct heat quickly in comparison to their volumetric heat capacity or 'thermal bulk' and they generally do not require much energy from their surroundings to reach thermal equilibrium.

The thermal diffusivity of the materials that we are concerned with have been calculated in the following table:

By observing the calculated values for diffusivity in the table it is possible to conclude what will happen with a Bain-marie setup using silicon oil and water with a stainless steel container/pot.

As we can see water has a volumetric heat capacity three times greater than silicone oil. This means it has a greater the ability for a given volume to store internal energy while undergoing a given temperature change. Basically it means that water will take longer to reach equilibrium and has greater thermal inertia.
This is a good, because the main volume of the liquid will be water that will be less subject to temperature fluctuation.

Water also has a thermal conductivity 6 times greater than silicone oil. This means that any heat transferred from ambient or the heater coil will transfer at a faster rate across water than it would silicone oil.
This is again good, because it means that water will have more temperature uniformity and there will be less chances of having any localized hot spots.

Stainless steel has a thermal conductivity 26.666 times greater than water and 160 times greater than silicone oil. This means that the stainless steel container/pot will quickly reach temperature uniformity throughout, so it will heat the silicon oil inside of it equally.

Silicone oils volumetric heat capacity is 3 times less than water In heat transfer, meaning that it will take a shorter time to reach equilibrium. However its thermal silicone oil is 6 times less than water. Meaning silicone oils thermal diffusivity is half as much of water. This basically means that it will be slower and therefore take longer to adjust its temperature to its surroundings. This provides the is the dampening effect we are looking for.

So if we are seeing 0.1 degrees C increase in water temperature over a 5 min period, ignoring the stainless steel container/pot we can say that the temperature increase over that 5 min period in the oil will be 0.05 degrees C (0.1 *0.5).

*******UPDATE***********

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These calculations assume there is a equal volume of oil and water

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## 1. How can the temperature stability of a temperature controlled water bath be improved?

The temperature stability of a temperature controlled water bath can be improved by regularly calibrating the temperature sensor and adjusting the temperature settings accordingly. It is also important to regularly clean and maintain the water bath to prevent any external factors from affecting its temperature stability.

## 2. What is the ideal temperature range for a temperature controlled water bath?

The ideal temperature range for a temperature controlled water bath depends on the specific application or experiment being conducted. However, in general, most water baths have a temperature range of 5-100 degrees Celsius.

## 3. Can the material of the water bath affect its temperature stability?

Yes, the material of the water bath can greatly affect its temperature stability. Certain materials, such as stainless steel, have better insulation properties and can maintain a more stable temperature compared to other materials like plastic. It is recommended to use a water bath made of a material with good insulation properties for optimal temperature stability.

## 4. How often should a temperature controlled water bath be recalibrated?

The frequency of recalibration depends on the usage and the manufacturer's recommendations. However, it is generally recommended to recalibrate a temperature controlled water bath at least once a year or whenever there are significant changes in temperature stability.

## 5. What are some common causes of temperature instability in a water bath?

Some common causes of temperature instability in a water bath include incorrect temperature sensor placement, external factors such as drafts or fluctuations in room temperature, and a malfunctioning heating or cooling system. Regular maintenance and checking for these factors can help improve the temperature stability of a water bath.

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