Gold Star Restorations, LLC. Hollister California

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Capillary-type temperature gauges are (pressure gauges) commonly found in automotive and aircraft applications.
Capillary type gauges utilize a Bourdon tube mechanism, and do not require electricity to function.
Temperature gauges used in aircraft and automobiles are of two general types: electrical, or pressure type. The electrical temperature gauge relies on a sender unit connected via wires to the gauge head. The sender produces varying electrical resistance according to the temperature it encounters. The gauge measures this resistance, and generally displays the difference by moving a needle on the gauge face.

The pressure type temperature gauge uses a Bourdon tube mechanism. This mechanism has been around since 1859 and was used in steam gauges, and continues in use today.
The Bourdon tube mechanism, or pressure gauge, has a gauge head with a needle which moves against a scale. The head of the gauge is connected to a very small diameter tube, which terminates in a bulb, or small reservoir which is immersed in the liquid (oil, steam, or coolant) whose temperature it will measure. The tube which connects the bulb and the gauge head is called a capillary tube, and it is hollow.

Principal of Operation

The Bourdon tube gauge actually measures changes in pressure within the capillary tube and bulb assembly. This assembly contains a gas which expands with heat, and contracts with cold, and thus moves a needle indicator.
The bulb, or gas reservoir, is immersed in the stuff whose temperature needs to be monitored. When the water or oil surrounding the bulb is hot, the pressure inside the gauge's capillary tube expands against the moving needle in the head of the instrument. Cooling the captive gas does the reverse.

What Goes Wrong

The capillary tube and bulb assembly are somewhat delicate items. They typically fail due to either vibration stress over a long period of time in service, or due to twisting forces applied when an old assembly is removed from a corroded engine or cooling system.
Many times if the break in the capillary tube is not complete, there may be no way for the naked eye to see the break in the tube, yet the unit will not function. The break only need be large enough to release the gas which was under pressure inside the bulb and tube assembly in order to fail.

We have successfully repaired hundreds of failed oil and water temperature gauges.
Repairing rather than replacing often saves originality.
We also create gauges with custom length capillaries for special applications.

For more information and detailed how-to-request a repair, download our flyer!