Vacuum Indicator – Definition

Vacuum indicator the engine, turn it on and let it reach the maximum operating temperature. Then, connect your vacuum indicator to an empty vacuum port on the intake manifold (anywhere BEYOND the throttle body). You may need to click a vacuum line, which may not have come in your kit. Once you have connected the indicator, you should see a CONSTANT reading idling with the throttle closed. Consequently, that reading should be between 17 and 22 inches.

Vacuum sensors use to measure vacuum or sub-atmospheric pressures. Vacuum means pressure below atmospheric. Since the actual vacuum was never attaining, the measurement is regarding a near absence of gas pressure. Vacuum can be measured using a conventional pressure sensor; however, they typically do not resolve low gas concentrations due to poor signal-noise ratio. Vacuum sensors rely on the physical properties of gaseous molecules related to the number of such molecules per volume of space.

The vacuum Indicator Readings Tell you

A high and constant vacuum reading is a sign of a healthy engine. If at altitude, these readings may be lower. A low vacuum reading means you have deficient piston rings or oil rings. You may also notice lower vacuum readings when; running overlapping camshafts or changing camshaft timing.

Suppose the engine passes that test; open and close the throttle again. Doing this quickly should reveal a vacuum drop, perhaps to 0-3 or so, and then it should go back to 25 before settling back to 17-22. Consequently, this means that the valves and rings are acceptable. Step on the accelerator and go down to 0, and then a climb to say only 20-22 will possibly indicate worn rings. If it only fits where it was or just one hair above, you may be dealing with worn piston rings. It would be best to temporarily jump higher (3-4 inches or so) and settle down.

 As a Vacuum Indication that Shows Slow Movement Between Several Readings

a vacuum is a space that is partially exhausted by artificial means (such as an air pump). This definition refers to a high or hard vacuum. Illustrates that the relationship of absolute and gauge pressure with 0 PSIA equals a high or hard vacuum.

Sensors that work in the vacuum range use some physical displacement or material property change to make a measurement. Medium to high vacuum sensors uses properties of the environment, such as thermal conductivity and ionization, to measure.

Variable capacitance

pressure instruments use the capacitance change results from the movement of a diaphragm element to measure pressure. The device uses a thin diaphragm as one plate of a capacitor. The applied pressure causes the diaphragm to deflect and the capacitance to change. The deflection of the diaphragm causes a change in capacitance that is detected by a bridge circuit. Capacitive absolute pressure sensors with a vacuum between the plates are ideal for preventing error by keeping the dielectric constant of the material constant.

Medium-High Vacuums

It is more accurate to measure vacuum with thermal and molecular devices at medium and high vacuums.

Thermal conductivity – The thermal conductivity of gas is measured using a Pirani gauge. It is a simple device that contains a heated plate and measures the amount of heat lost by the scale. The amount of heat lost depends on the gas pressure. There are several designs of the Pirani gauge. One method includes using two plates with different temperatures. The amount of power spent on heating is the measure of gas pressure.

Another design uses a single scale to measure the thermal conductivity of gas by heat loss to the surrounding area. The gauge in the image below uses a thermal balance technique by dividing the sensing chamber into two sections and filling one with gas at a reference pressure. The other connectedes to the vacuum that is being measured. Each room is the same size, shape, and construction and contains a single heated plate. The temperature measurement is done with a thermocouple.

Additional Pressure Readings of Vacuum Indicator

Many vacuum sensors can perform additional pressure readings such as absolute, differential, gauge, compound, and sealed pressure. The total pressure is a pressure measurement relative to a perfect vacuum.


Differential pressure of Vacuum Indicator the Difference Between two input Pressures.

Gauge pressure is the pressure measured above the local atmospheric pressure. It is the most common pressure measurement. Sealed gauge pressure is relative to one atmosphere at sea level (14.7 psi), regardless of local atmospheric pressure. A compound vacuum sensor can measure positive and negative (vacuum) pressures. Read How to Select Pressure Sensors for more information on pressure readings.

Performance Specifications

Vacuum range is the most important specification to consider when selecting vacuum sensors. Vacuum range is the span of pressures from the lowest vacuum pressure to the highest vacuum pressure. When reading the chart below, it is essential to remember that a high- vacuum is more downward pressure than a low vacuum. Operating temperature is another critical element to consider when selecting a vacuum sensor. Working temperature is the full-required range of ambient operating temperature. Temperature and pressure are directly related to each other. If the temperature of the working environment increases, the tension in the system will increase. To prevent equipment damage, it is essential to know the extreme temperature ranges of the area.


Therefore, checking your engine with a vacuum indicator is still the fastest test. While vacuum pressure gauge readings don’t necessarily tell you with 100% certainty what the problem is, they will warn you, something is undoubtedly wrong.

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