Introduction to transformer oil level gauges


Oil level indicators are essential transformer equipment

Transformer oil level indicators - also known as liquid level indicators or OLIs for short - are a standard piece of equipment on most transformers. Over time, oil leakages lead to a drop in oil level and reduced cooling performance of the transformer. Having a clear indication of the oil level inside the tank of a transformer, the conservator and in the on-load tap changer provides important information about the condition of the transformer.


The Qualitrol OLI Product Range

Qualitrol is one of the leading providers of transformer oil level gauges globally - and an Insulect supplier partner. So to explain the different types of OLIs available, we’re using the Qualitrol product range as a reference. It is one of the most extensive range of oil level indicators available, having been developed over the past 70+ years.


Understanding transformer oil level indicators

The 3 main assemblies

In order to identify the different types of transformer oil gauges, it helps to first understand their major components. Each gauge consists of three assemblies:

  • The Case Assembly, which houses the dial (face) where you read the temperature, as well as the switches.

  • The Flange Assembly, which consists of the flange that connects to the tank. The flange assembly also consists of the support tube, which extends from the back of the flange.

  • The Float Rod Assembly, consisting of the float and float arm, which is supported by the flange assembly.


OLI Mounting Type

There are two main mounting types available for OLIs.

  • Direct Mount oil level indicators

  • Remote Mount oil level indicators

Most transformer oil level indicators are Direct Mount devices, meaning the case assembly, flange assembly and float rod assembly are a single integrated unit. These can be side mounted or top mounted.

Side mount OLIs generally have a float assembly that consists of a float on the end of a rotating arm. Whereas top mount OLIs (aka vertical oil level indicators) have a float within their vertical support tube.

Remote mount OLIs by contrast are designed for use where the point of measurement is not easily viewed by personnel, thus requiring separate or remote indication. For example on a conservator tank. In practice this means the Case Assembly (with the visual dial) is separate from the Float Assembly, connected by a capillary tube.


Oil Gauge Size

Oil level indicators generally range from a small two inch dial, all the way up to this six inch dial. This is to suit the requirements of different apparatus, such as the transformer main tank, conservator, on load tap changer, and related assets.

The size required is usually determined by the physical dimensions of the device being measured and the level of visual indication required (from how far away does it need to be viewed, etc).


Magnetic Coupling

Many modern oil level indicators have the case assembly magnetically coupled to the flange assembly. The advantage of this design is leak free operation. It makes it very easy to disassemble the OLI out in the field and replace the case assembly (swap it with another) when required.

Because these are magnetically coupled, you don't run into the risk of needing to put the transformer offline or lower the oil level to change the case or do any maintenance work.

qualitrol magnetic coupled oil level indicator.png

The Float

The float on an oil level indicator usually comes in one of two styles for use in various tank geometry:

  • lever driven float arm (radial)

  • heavy duty gear driven float arm

The float type can be changed based on the application whether there's a compensation bag or not.


The Dial Face

The dial face is what provides the visual indication of the oil level.

You can generally change the dial face to suit operational requirements, including:

  • the markings

  • the colour of the pointer

  • and the background.

As well as having a temperature reading to allow for easy oil filling of the tank. As mentioned earlier, the dial is field replaceable and can be exchanged without lowering oil level or compromising tank seal.


The Case

All Qualitrol cases are all IP65 rated, meaning they can be used in arduous environment, including where high humidity is a factor. They housing is generally die-cast aluminium with a thermoset powder coat finish.

For extreme conditions or corrosive environments, most OEMs provide options such as C5M painted finishes (according to ISO 12944), stainless steel housings, tin plated for water applications, and UV resistant polycarbonate lenses.



Most models include the option to specify switches for remote indication back to a control centre. The number of switches available will depend on the product model.

Commonly switches will be used to monitor the oil level (indicate low level, or multiple levels) and for limit, trip and alarm control functions.


Customising an oil level gauge

Most transformer oil level gauges are highly customisable, to be able to fit a wide range of equipment types, shapes and sizes. In addition to the many variances we have already covered, you can often also change the following:

  • float support tube

  • float rod length

  • modify the flange mounting angles

  • modify the gasket groove

  • modify the orientation of the screws


Get expert transformer advice

Whether you are needing to replace an oil level indicator on an existing transformer or specify a new device, an Insulect specialist can provide you with the expert advice needed to select the right product for your application. Don’t hesitate to get in touch with us.