Other Articles |  April - 2022

Can You Handle the Pressure? (Part 1) How to Construct an Affordable Electronic Pressure Gauge

If you’ve been reading my articles in GEARS Magazine, you are probably aware that I love scopes. I think they provide a deep look into the operation of a component or vehicle system. In addition to the obvious scope uses, such as checking electrical circuits, I use scopes for everything from noise diagnostics, engine mechanical analysis, and just about anything that generates pressure. This article is divided between two issues because of length and detail. In this month’s article, I focus on how you can construct an electronic pressure gauge with various options regarding scopes, sensors, and controls. Next month we’ll compare the quality of the sensor’s measurements with the different scopes.

I know many of you don’t have a scope, or possibly you have one but never use it. So, I’m going to use an inexpensive Pico scope 2204a ($150) and a slightly more expensive Autel Maxiscope ($400) to demonstrate how you can make an electronic pressure gauge without having to invest thousands into a tool you might not use frequently. I’ll also compare these two scopes to the expensive flagship Pico Scope 4425 to see if the significant expense of the high-end scope is worth the cost (figure 1).

I will explain and demonstrate a few different configurations on using a scope to measure various pressures, including transmission pressures. If you have absolutely none of these components, you could build yourself a scope-based pressure gauge for less than $200, and yes, that includes the scope! Now, suppose your budget can go a bit higher. In that case, you could build an even more capable setup that measures just about anything that deals with pressure. Applications include an engine’s in-cylinder pressure to gain insight on engine sealing, valve issues, pressure losses, exhaust and intake restrictions, and valve timing.

What’s needed:

Scope: To use an electronic pressure transducer, you’ll need a scope that includes software that allows you to set up a “custom probe” or at least one that will enable you to perform math. This is because of the typical voltage in which these pressure transducers’ output is usually between 0.5 and 4.5 volts. Most pressure transducers don’t output in a direct millivolt to psi relationship, so for example, a transducer that outputs 325 millivolts won’t necessarily equal 325psi. For that reason, we’ll use the scope’s software to teach the scope the transducer’s voltage-to-pressure relationship. Pico and Autel make this relatively painless by allowing users to create custom probes.

Another fact about these scopes – the more you spend, the better the resolution, speed, and channel options. Without boring you with specs and details, the inexpensive Pico 2204a is an 8-bit scope, which means its resolution isn’t as great as the Autel or Pico 4425, which are 12-bit scopes. It’s like watching a game on a 720p TV versus a new 4K TV. The picture isn’t as great, but if you can learn to work around that limitation, you can still accomplish what you need. Also, the 2204a is a two-channel scope versus the Autel and Pico 4425, which are 4-channel scopes. Two channels allow the user to measure and compare two signals at once, and the four-channel scope allows the user to measure and compare up to four signals.

Power Supply: The pressure transducers used in this article are simple three-wire sensors with a five-volt input, ground, and signal wire. There are numerous ways to supply 5-volts to these sensors, but I will explain the three I have created.

The most straightforward (but least favorite) power supply option is a USB port, either from a computer or a backup battery pack. They output an excellent clean 5-volt supply, and we all have USB cables laying around. Just chop off the end of the USB cable and separate the wires. The outer red and black wires are the power and ground, and you could use one of the middle wires as your signal circuit. I strip away a little bit of the insulation at the other end of my USB cable to connect my signal wire to the scope. This way, my signal wire is mainly protected by the shielding, which will help reduce noise on the circuit. Also, make sure you create a pigtail from the USB cable ground for the scope’s ground lead because the scope needs to connect to the power source’s ground. There you have it. A simple 5-volt power supply (figure 2).

Another option, which I prefer to the USB cable, is to build a 5-volt power box. In figure 3, you can see a 5-volt regulator attached to an inexpensive 9-volt battery box with a built-in switch. Solder in some power and ground leads and provide an extra pigtail for the scope ground. You can install as many pigtails as you need for multiple sensors. Don’t worry; these sensors draw minimal current so that you won’t overtax the power supply.

Lastly, I made a more robust and aesthetically appealing power supply using a project box, switch, and several DIN connections, as shown in figure 4. This allows for an easy and more secure connection. It keeps me from searching for that ground or signal lead that popped off, causing me to lose my readings.

Wiring: I prefer to give myself plenty of space between the scope and the pressure transducer. If you opted to use a USB cable as your power source, you are limited to the length of that cable, but if you decided to build a power supply box, you could use whatever length cable you desire. Here are a couple of suggestions. Network cable is suitable because it’s cheap and bundled in a narrow conduit, but find stranded wire instead of solid wire. Solid wire still works, but all the bending and flexing during use will likely result in broken wires at some point.

Also, it’s best to use shielded wiring. Shielding prevents electromagnetic interference from ignition systems, solenoids, and electric motors from introducing noise into your scope patterns. It’s not a deal-breaker, but in a perfect world, you’ll find a 3-wire cable with shielding. Figure 5 shows different conductors and shielding.

Pressure sensors:

Do a quick Google search for pressure transducers, and you’ll see pages of options. The inexpensive, 25-dollar transducers listed all over eBay and Amazon work just fine for measuring general pressures where signal speed isn’t necessary. Applications include transmission and engine oil pressure, fuel pressure, AC system pressures, and brake system pressures. But if you plan to use your new setup for measuring in-cylinder engine pressures, you’ll need a faster sensor. Honeywell ($85), SSI ($125), and many other manufacturers make excellent sensors that quickly report their measurements. All three sensors used in this article are shown in figure 6. The sensor characteristics you want to look out for include:

Voltage range – ensure the output voltage range is between 0.5 and 4.5 volts to provide an easy setup for your scope to perform measurements. Be aware that some sensors alter amperage instead of voltage, and those would require more complicated hardware to be usable. This article relates to the 0.5 – 4.5v sensors, not the milliamp sensors.

Response time – Check the published response time. This spec defines how frequently the sensor reports its voltage. These sensors are tiny microprocessors of their own, so ideally, they’d report the signal output more frequently than 1ms intervals. On the sensors I tested, the inexpensive sensor was very slow (3ms+ intervals) compared to the Honeywell and SSI (less than 0.5ms intervals). 3ms is plenty fast for general pressures, but when testing fast switching pressures, like an engine’s in-cylinder pressures, you need a sensor that outputs faster than 1ms intervals.

Connections – Keep in mind how the sensor connects to the pressure source. There are various options, but you’ll likely want 1/8” or .” male pipe threads.

Gauge Type – Know what type of gauge you are picking. You’ll see options such as “gauge,” “absolute,” and “sealed.” Since these sensors are used with a scope that can generate custom probes and ranges, it’s not a huge deal what type of gauge you pick, but it does change the voltages that your sensor outputs when simply measuring atmospheric pressure. Sensors labeled as “gauge” sensors will output 0.5 volts when disconnected and open to the free air. It doesn’t matter if you are at sea level in Florida or a mile high in Denver; it starts at 0.5v and goes up with pressure and down with vacuum.

An absolute gauge is calibrated for sea level and will generate a voltage when open to the free air based on the actual atmospheric pressure. For example, the atmospheric pressure is higher at sea level in Florida than in Denver, so the voltage values represent the actual air pressure of your location. Maybe you’ve never considered it because it’s a common thought that there’s zero pressure around you wherever you’re at, but an absolute pressure sensor will show you the actual air pressure squeezing on your body!

To conclude this month’s article, I’ll leave you with a pressure test from a 2019 GMC Sierra with a 10L transmission (figure 7). The sensors were all tapped into the line pressure port. You can see that the pressures look identical and that all sensors, including the inexpensive eBay sensor, performed well. In this capture, I have 20 seconds on the screen. You can see where the engine started (A) and line pressure stabilized (B). The first squiggle (C) was where I shifted from park to reverse. The line pressure changes resulted from brake torque blips (D). Then the other two squiggles (E and F) were from shifting to neutral and drive. Now I can drag the horizontal rulers down to determine pressure values.

In next month’s issue, I’ll show a comparison between all these sensors with the inexpensive Pico 2204a scope, the Autel MaxiScope, and the pricey Pico 4425.

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