Comparison Tests on Tire Pressure Gauges

Accurate tire inflation pressure measurement requires accurate tire pressure gauges. The Alberta Farm Machinery Research Centre (AFMRC) tested a range of tire pressure gauges to evaluate their performance. Readability, accuracy, durability, repeatability and resistance to damage from tractor tire ballast solutions were evaluated.

Types of gauges
The tire gauges commonly used on farms are of three major types: pencil or "stick", dial and digital readout. Individual gauge models cover a range of pressures and are usually purchased for a specific range of pressure measurement.

Pencil gauges consist of a metal or plastic casing, a pocket clip and a deep set chuck, usually with a pressure release bump on the opposite side. Pressure scales are marked on an indicator bar that extends when air pressure is applied to the gauge. To use the gauge, the chuck end is placed on a tire valve and the indicator bar slides out of the gauge a distance corresponding to the pressure in the tire.

Dial gauges consist of a protective casing around a dial indicator with a pressure scale on the dial background. An air chuck is attached to the dial either rigidly or with a section of flexible tubing. To use the gauge, the chuck is placed on the tire valve and the indicator needle rotates over the scale corresponding to the pressure.

Digital readout gauges consist of a protective casing around some form of digital readout with the necessary electronics to drive the readout. An air chuck is attached to the casing either rigidly or with a section of flexible tubing. To use the gauge, the chuck is placed on the tire valve, the gauge is turned on, and the digital display indicates the pressure.

Gauges tested
Representative samples from each of the three pressure gauge types were tested, including six different pencil gauges, three different dial gauges and one digital gauge. Three gauges were marked only in english units while the other seven had both english and metric units. The english unit scale was always pound per square inch (psi) while the metric unit scales varied between kilopascal (kPa), bar and kilogram per square centimetre (kg/cm2). The pencil and dial gauges tested were in various pressure ranges and have been grouped as low pressure or higher pressure gauges. Low pressure gauges were defined as those from 0 to 20 psi (0 to 138 kPa) and higher pressure gauges as those from 5 to 60 psi (34 kPa to 413 kPa). The digital gauge tested had a pressure range of 0.5 to 150 psi (3.5 to 1034 kPa).

Gauge Names and Model Numbers:
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Low Pressure	Higher Pressure
Pencil - Bridgeport 40-399 - Milton s917 - Motomaster	Pencil - Bridgeport 40-402 - Milton s928 - Superex
Dial - Milton s901 - Superex	Dial - Power Fist
Digital - Accutire

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Summary of results
Three samples of each gauge were tested to evaluate accuracy, repeatability, readability, durability and resistance to damage from tractor tire ballast solutions. Test details and evaluation criteria are described in the Test Description section. Table 1 ranks the various gauges in order of overall performance and shows their performance in each of the evaluation categories.

Accuracy - Accuracy is a measure of the difference between the indicated gauge reading and the actual pressure reading. Average pressure readings that were within 5 percent of the true pressure were considered acceptable. Three of the gauges, the Milton s928 pencil gauge, the Superex dial gauge and the Power Fist dial gauge showed unacceptable accuracy levels throughout their range and were rated as poor. The remainder of the gauges were acceptable. Two gauges, the Superex pencil gauge and the Milton s901 dial gauge showed average readings within 2 percent of the true readings and were rated excellent.

Repeatability - Repeatability is a measure of the ability of a gauge to show the same value when taking multiple readings of the same pressure. The repeatability of three of the gauges, the Milton s928 pencil gauge, the Superex dial gauge and the Power Fist dial gauge was poor. For these gauges, the differences in readings within the sample set varied as much as 4 psi (28 kPa). The repeatability of the other gauges was acceptable. For these gauges, the repeated measurements taken by individual gauges were within 0.3 psi (2 kPa) on average. The differences in readings within each set of these gauges was within 0.6 psi (4 kPa).

Readability - Readability is a measure of the ease of determining a pressure with a gauge. Considering the readability of the gauges tested, the digital gauge was the easiest to read, followed by the dial gauges and finally the pencil gauges. No estimation or interpretation was required or possible when reading the digital gauge. Being able to see the entire scale on the dial gauges made it easier to estimate between two marked points with the dial gauges than with the pencil gauges.

Table 1. Gauge Performance Summary (ranked from best to worst).

Gauge	Type	Price CDN$	Pressure (psi)	Accuracy	Repeatability	Readability	Durability	Ballast
Superex Motomaster Bridgeport 40-402 Bridgeport 40-399 Accutire Milton s901 Milton s917 Milton s928 Superex Dial Power Fist	Pencil Pencil Pencil Pencil Digital Dial Pencil Pencil Dial Dial	6.00 6.00 9.00 8.00 30.00 20.00 8.00 8.00 8.00 5.00	10 to 50 0 to 20 5 to 45 0 to 20 1 to 150 0 to 15 0 to 20 5 to 50 8 to 60 8 to 60	Excellent Acceptable Acceptable Acceptable Acceptable Excellent Poor Poor Poor Poor	Excellent Acceptable Excellent Acceptable Acceptable Excellent Acceptable Poor Poor Poor	Excellent Excellent Acceptable Excellent Acceptable Acceptable Acceptable Acceptable Excellent Excellent	Excellent Acceptable Acceptable Acceptable Acceptable Poor Acceptable Acceptable Poor Poor	Acceptable Acceptable Acceptable Poor Poor Poor Poor Poor Poor Poor

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Durability - The digital and pencil gauges were unaffected in the durability tests and were ranked as acceptable. All the dial gauges were damaged to varying degrees and were ranked as poor.

Ballast - Exposure to liquid chloride ballast solution affected all the gauges. In general, if the gauges were not cleaned well after being exposed to ballast, their accuracy and reliability suffered. Assuming a vigorous water rinse after each use, three pencil gauges, the Superex, Motomaster and Bridgeport 40-402, were ranked as acceptable with chloride ballast. Even with careful cleaning the other gauges were significantly affected by the ballast and were ranked as poor. The Milton s928 pencil gauge was specifically designed to be used with ballasted tires but suffered a significant and continuing decrease in accuracy with continuing exposure to ballast.

Conclusions
In selecting and using tire pressure gauges for farm tractor tires, the following can be done to improve the accuracy of the resulting measurements:
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1.	Select gauges that are designed to measure in the desired pressure range. The best accuracy is obtained when using gauges to measure pressures that are not near the limits of the designed range of the gauge.
2.	Select gauges with scale graduations that are easy to read in the desired pressure range.
3.	Check that gauges are accurate within the desired pressure measurement range. Gauge accuracy should preferably be checked against a known reference or at least against another gauge.
4.	Recognize that exposure to liquid ballast can affect the accuracy of any tire gauge whether designed to be used with ballast or not. After using a gauge on a ballasted tire, rinse out the gauge thoroughly with water and if appropriate for the gauge, oil the mechanism. Regularly recheck the calibration on gauges that have been exposed to ballast.
5.	Store tire gauges in a clean, protected and moisture free environment.
6.	Avoid dropping, jarring and exceeding the maximum pressure on tire gauges. If a gauge is dropped or over-pressured, check the gauge against a reference to see if it still reads correctly.
7.	Dispose of any gauges found to be damaged or inaccurate.

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The Alberta Farm Machinery Research Centre (AFMRC) is at the forefront of machinery evaluations, applied and scientific research, and development of innovative agricultural technologies.

Producers, manufacturers and any other interested parties are invited to contact AFMRC for more information.