Laboratory

Water Bath- Working, Operation, Handling, and Maintenance

The water bath is a type of vessel, with heated water in it, that helps to heat the samples. The samples can be the serum for the blood tests, inflammable chemicals, etc. They are mainly of three types. They are non-circulating water bath, circulating water bath, and shaking water bath. Here, we will be talking about the general operation and the maintenance of a water bath. Thus, these things will be applicable to all kinds of water baths.

Water Bath Principle

A water bath helps to maintain the temperature of water to a set value. This is possible with a help of a temperature sensor and a temperature controller.

Here I am going to explain to you some basic designs and workings of the non-circulating water bath. But in the market, you may find a water bath with many more features than the one shown in the figures below.

1. Analog Water Bath

Design

Analog Water Bath
Construction of Analog Water Bath

The water bath connects to an external three-pin power socket. From the power socket, the live wire goes to the thermostat through the fuse and switch.  From the thermostat, the wire then goes to the water immersion heater (heating rod). The mains indicator is used to indicate the condition of power from the external supply. Similarly, the heating indicator is used to indicate the condition of the heating rod.

The entire system of fuse, switch, indicator, and thermostat lies inside the water bath surrounded by glass wool. 

Glass wool covers the three sides of the water bath. It acts as an insulator and prevents the leakage of heat into the surrounding.

The fourth side of the bath is open. Inside the bath lies a water immersion heater and the bulb of the thermostat.

Working

When the machine is given a power supply, the mains indicator will glow. Then the current will go to the thermostat. From the thermostat, the current will flow through the water immersion heater.

When the temperature of the water is less than the set value, the thermostat will allow the current to flow through the heating rod. Thus, the temperature of the water will begin to rise up. Once the temperature of the water reaches the set value, the thermostat will cut off. Thus, the current will not flow through the heating rod. Then, the temperature of the water begins to go down. The heating indicator will also stop glowing.

2. Digital Water Bath

Design

Digital Water Bath
Construction of Digital Water Bath

The design of a digital water bath is similar to that of an analog water bath. The only difference is that instead of a thermostat there is a solid-state relay and the PID temperature controller. Similarly, a temperature sensor is also present.

Working

The working mechanism is almost similar to that of the analog water bath. Here, the PID controller and the solid-state relay (SSR) acts as a controller for the water immersion heater. When the temperature of the water is lower than that of the set value, the controller will supply a certain DC voltage to the relay. Then the relay will get activated causing the flow of AC current to the heating rod. Thus the heater begins to heat up the water.

When the actual temperature of the water goes closer to the set value, the PID controller will activate and deactivate the relay continuously. Thus the heating rod will go ON/OFF continuously. So, by controlling the duration of the ON and OFF stage, the water bath helps to maintain an almost accurate result with not more than 1°C of variation.

The resistance temperature detector (RTD) such as the PT100 sensor, senses the temperature of the water. Then, it converts the temperature to the resistance value and supply to the controller. The controller will compare this incoming signal with the set value.

Analog Vs Digital Water Bath

An analog water bath is the simplest type of water bath. It is cheap and easy to operate. But, there are few problems with it. The first problem is that the accuracy of the analog water bath is less than that of the digital water bath. Besides that, there is no display board. So, you will not be able to see the actual temperature of the water.

A digital water bath is an advanced version of an analog water bath. It is quite expensive. However, it is easy to operate. It has got a better accuracy. Similarly, the display board present in it allows the user to see the actual temperature of the water present in the bath.

Water Bath Temperature Range

Theoretically speaking, the temperature range of the water bath is from room temperature to 100°C.

Since the water bath does not generally have a cooling system, it can not maintain the temperature below room temperature. Similarly, on full heating, the water should boil. At that moment, the temperature of the water should be at 100°C. But due to the leakage of vapor from the cover/ lid, the water may not reach that level. I personally have found 95.5°C of water temperature at the maximum setting. 

How to use water bath in laboratory?

(Water bath procedure)

  • First of all, pour distilled or deionized water into the water bath. The level of water should be above the level of the heating rod.
  • Then, turn on the machine.  The digital water bath will show the display of the present value (PV) and the set value (SV). If needed,  set the set value (SV).

If the water bath is analog, just turn the knob of the thermostat to the required set value.

  • Allow the temperature of the water to reach the set value.
  • When, the water temperature reaches the set value,  put the sample inside the water bath.
  • Finally, cover the top of the bath with a lid and set the timer.

Handling of Water Bath

  • Put the power source away from the machine.
  • Put the machine on a stable and solid surface.
  • Do not add tap water because tap water consists of ions and minerals. The ions and minerals can accelerate the rate of corrosion of the bath.
  • During operation, the level of water should be above the level of the water immersion heater. Else, the heater will overheat and will burn.
  • Empty your water bath at the end of the day. Then clean it dry to prevent it from early rusting. Rusting can be the cause of contamination of the samples.
  • You can use rust-removing chemicals to get rid of rust. Similarly, you can use Collins to get rid of dust and dirt particles.
  • If you think that the water bath is not working properly, then take a mercury thermometer and measure the temperature of the water. Then compare that value with the preset value of the display.

Maintenance of Water Bath

The water bath can undergo a number of problems. So, in those cases, you may need to open the machines. Since the machine consists of glass wool you need to wear nitrile gloves, a face mask, and glass for safety purposes.

Here I will be discussing some of the problems and the method to deal with them.

1. No power supply?

If the machine does not get any power supply, you will not see a display in the PID temperature controller. Similarly, the indicator of the mains switch and the heater indicator does not light up.

Solution

There can be a problem with the power supply in the laboratory. The problem can also be due to the loose connection of the power cable to the supply. So, try to fix the cable in a suitable plug. The cable can also be broken. Besides that, there can be a defect of a switch or the fuse.

Similarly, the indicator of the mains switch and the heater switch can also be damaged. In that case, you may or may not replace it with a new one

2. Heating rod does not work?

If the heating rod does not work, then the temperature of the water will not rise up.

Solution

If the heating rod is burned/ damaged, then the rod will not operate. You can observe the condition of the rod. If damaged, the shape of the rod will be irregular. It will also turn black. In that case, replace a rod with the suitable one. Generally, 1KW, 2KW, or 3KW rods are available in the market. You can put any rod that you like. But if the rod is of low wattage, the process of heating the water becomes slow.  Similarly, if the rod is of high wattage, the process of heating the water becomes quick. But, there can be chances of burning the internal wiring system. Due to the overload, the fuse (MCB) of the room can trip.

The solid-state relay (SSR) supplies current to the rod. In other words, it acts as a switch for the switching of the rod. You can also check the condition of the SSR, which I have stated in the topic of SSR.  Make sure to check the LED present in the relay. If the LED glows, but the heating rod does not operate, then there can be a fault in the relay. There can also be a problem in the wiring system from the relay to the rod.

If the LED does not glow, then there can be a problem with the PID controller. In that case, the PID controller may not supply DC voltage to the input of the SSR. So, the relay will not operate. You can check the input supply to the PID controller and the output voltage given by the controller to the relay. Else, there can be a problem in the internal programming. We will talk about the PID controller later.

3. Water does not heat properly?

Sometimes, the water will heat, but the temperature will not reach the set value.

Solution

If the heating rod is damaged, it may work partially. Similarly, if the supply voltage from the plug is lower than the specification, then the rod will not get sufficient voltage to operate at full capacity. In that case, use a voltage regulator to control the input voltage.

The temperature sensor (Pt 100 sensor) may not work problem. In that case, you need to check it. There are a number of ways to check the condition of the temperature sensor, which I have stated in the topic of the Pt100 sensor.

The problem can also be in the PID controller.

4. Error in the Present Value (PV)?

The present value shown by the PID controller may not be the same as that of the actual temperature measured by the mercury thermometer.

Solution

The problem can be due to the poor connection of the temperature sensor to the PID controller. In such a case, remove the wires of the sensor from the controller. Then reconnect it. If the sensor is defective then replace it with a new one.

The problem can also be with the PID controller.

5. Error with PID controller?

PID temperature controller is the brain for the digital water bath. So, if the PID becomes defective, then the machine can undergo a number of problems. Some of them are as follows.

  • The solid-state relay may not activate.
  • The water may not heat. Even if it heats, the temperature may not reach the set value.
  • The display may show a faulty reading of the present value.
  • Since the display is embedded in the PID controller, a defective controller may not display any value.

Solution

There are few ways to deal with the problem of the controller. For example, you can do an auto-tuning of the controller. Auto-tuning is a kind of internal calibration to calibrate the temperature. If you are trying to do an auto-tuning, then pour the water into the water bath above the level of the heating rod. Then close the cover of the water bath. Set the value to a certain temperature. Then press the button of the auto-tuning. Then the process of auto-tuning will begin. During that time, the display will keep on blinking. This process can take about half an hour or more, based on the size of the water bath. Once the tuning is over, the display will stop blinking.

Another problem can be due to the wrong programming of the controller. Generally, there will not be any error in the programming. But sometimes, the users can mistakenly alter the programming. In that case, you need to set the values properly. For that, you need to know the values of the parameters shown in the display. Since different models of PID controllers can have different programming, I can not tell you the values. However, you can see the model of your controller. Then, search for the manual on the internet. If you find it, then look for the programming and auto-tuning process.

6. Problem with Thermostat?

If the water bath is an analog type, then there will not be any PID controller. Instead of that, there might be a thermostat. For more details about the working and the testing of the thermostat go through my post related to it.

 

 

 

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