Under Pressure: Pressure Gauge vs. Ambient Temperature

Monday, April 12, 2010 by Woody Rogers
Originally written for Grassroots Motorsports magazine late last year.

As we fine-tune our setups, we often make small tire pressure adjustments to help balance the car and manage tire wear. Small changes can make a difference, and the gap between winning and second place can be smaller still. So how accurate is your tire pressure gauge?

To find out how the various commercially available tire pressuregauges behave in real-world conditions, we tested several types against a calibrated, high-end digital unit. We used a tire and
wheel assembly initially set at 40 psi, and took multiple samples with each gauge. Readings were taken back to back with the reference gauge to minimize the influence of air loss after those multiple readings. The reference gauge and tire were kept in a climate-controlled room for consistency.

The Gauges
Reference This high-end digital gauge has been calibrated for 1/10 psi accuracy.
$50 0-60 psi dial gauge w/bleeder valve This gauge has been in service for several years, replacing one ruined by a single drop onto the pavement.
$18 low-cost digital gauge: We’ve used this one daily for nearly a year and have dropped and banged it a number of times.
$5 pocket digital gauge This one’s brand-new.
$1 pencil-style stick gauge: This gauge has led an unknown but lengthy service life.

 

Room Temperature (72 Degrees F)
Dial gauge 0.9 psi low, 2.26 percent error
Low-cost digital gauge 0.4 psi high, 0.98 percent error
Pocket digital gauge 6.4 psi high, 16.11 percent error
Pencil gauge 5.6 psi low, 14.18 percent error

The low-cost digital gauge proved to be consistent and accurate despite its hard
service life. The $50 dial gauge was also reasonably close, but it was off by more than
2 percent. Think all digital gauges are the same? The pocket digital gauge had a 6.4
psi error versus the reference—that’s 16 percent—along with a 2 psi variance in its
readings. And since most pencil gauges are known to be inaccurate, our example’s 6
psi error wasn’t much of a surprise.

Below Freezing (4.5 Degrees F)
Dial gauge 2.0 psi low, 5.13 percent error
Low-cost digital gauge not functioning, no reading
Pocket digital gauge not functioning, no reading
Pencil gauge 4.8 psi low, 12.37 percent error

Have you ever left your gauge in a glovebox or unheated garage during cold winter months? Just as tire pressures change with temperature, so can the readings on your mechanical gauge.
Why? Its pressure readings rely on a temperature-sensitive spring.

To measure the effects of a chilly environment on our gauges, we stuck them in the freezer for 18 hours. At subfreezing temperatures, the dial gauge read 2 psi—5 percent—lower than
the reference. Interestingly, the pencil gauge was slow to give its final reading, taking more than 2 seconds. Alarmingly, neither of the digital gauges worked. Their LED flashlights continued to
function, so we knew their internal batteries still had some power.

In the Sun (105 Degrees F)
Dial gauge 0.8 psi low, 2.06 percent error
Low-cost digital gauge 0.2 psi high, 0.52 percent error
Pocket digital gauge not functioning, no reading
Pencil gauge 1.5 psi low, 3.9 percent error

After giving the gauges 24 hours to return to room temperature, we placed them in the sun for several hours. We wanted to simulate a gauge left on the pit wall or tool box. The air temperature was 79 degrees F, but the gauges heated up to 105 degrees.

Conclusions

Our tire gauge tests taught us a few things, and helped us come up with some recommendations for gathering the best readings possible:
  1. Treat your tire pressure gauge like the precision instrument it is.
  2. Go digital. Even $20 will buy an accurate gauge; spending more adds features and may allow for recalibration, but probably won’t improve accuracy.
  3. Mechanical gauges are more prone to fluctuations in temperature. They can also be permanently damaged by even minor bangs and bumps.
  4. Always use the same pressure gauge. Different gauges are likely to give different readings. Using the same gauge will at least keep all of your readings relative.
  5. Have your gauge regularly checked for accuracy.
  6. If your gauge reads low, you are over-inflating your tires. If your gauge reads high, you are unknowingly under-inflating your tires.
And keep in mind: We conducted our test with our wheel and tire set to 40 psi. If your pressures are higher or lower, or if you’re using a different brand of gauge, your results are likely to be different. After all, even variance is variable.

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