While scrolling through generator listings and checking out specs, apart from the more familiar ones, such as weight or noise, you may have come across some less familiar: PSI, GPM, CUs. Any idea what they mean? No?
That’s probably the exact reason why you’ve clicked on this article. If so, you’ve come to the right place to learn about them!
This article is dedicated to those of you who simply want to know what these three values mean, how they are related and how you can use them to judge if the pressure washer you’re looking at is any good.
PSI and GPM
PSI, GPM and so-called cleaning unit’s (CUs) are the three most important parameters which describe how well a pressure washer does its job. While the first two, PSI and GPM, refer to actual physical units, the third, CU, is a specialty that the pressure washer industry came up with, to give customers an index that combines both. Let’s see how that works.
PSI, or pound-force per square inch, is a unit of pressure (opens in a new tab) which describes the force or its equivalent in weight, applied per a surface area. Simply put, this value is proportional to the power of a pressure washer’s pump.
However, there is a catch. A pressure washer’s PSI at its pumps head and at its tip are two different values. Since “force” in PSI is measured per surface area by definition, by simply narrowing down the surface – the diameter of the tip or nozzle, the value of PSI can be easily manipulated – more on that later.
PSI would be a perfectly acceptable metric to compare the power of any given pressure washers, if only pressure washer manufacturers could agree on the same measuring standards. However, that’s not the case and in addition, they do not address the pressure at the washer’s tip and in turn, the user’s cleaning experience. Thus, PSI values must be taken with a grain of salt.
GPM, or gallons per minute, are a unit of volumetric flow rate (opens in a new tab). Simply put, the value describes how much water a pressure washer can pump out in a minute.
Unlike PSI, GPM is a value much less dependent on the conditions of the measurement. In fact, under ideal conditions of the tubing and at constant temperature, GPM is considered to be constant, regardless of the dimensions you chose for its measurement (e.g., the diameter of the tip).
For these reasons, GPM values are much better for comparing pumps of given pressure washers. A pump that can pump out water faster is by definition a better pump. But does a better pump necessarily clean better?
GPMs are arguably the best way to measure the power of a pump. But when push comes to shove, the fact is that a pressure washer cleans with… well… pressure. Does that mean that PSI is the way to go instead? Well, yes and no. There is an argument to be made that a washer which can achieve the same pressure as another, but on twice as big surface (e.g., with twice as much water pumped out), will clean better – firstly, because it is covering a larger surface and secondly, because by simply changing the tip to match the measuring conditions of its opponent, the pressure will be twice as large.
Therefore, both PSI and GPM values must be taken into account when comparing washers. Sounds easy enough, but try doing that with 10 different washers and you’ll quickly find out that one easily loses track of all the numbers. This is where cleaning unit’s come into play.
CUs are simply a multiplication – either of GPM by PSI or vice versa. The product of the two values scales in proportion and therefore, accounts both for the volume and the force of the stream that is measured, if both are measured under the same conditions.
Even though the definition of CUs may seem too trivial to be of any value, they annihilate the differences in between the conditions set by the design of individual devices and the testing methods. The result is a leveled playing field upon which pressure washers can compete fairly, considering that they use the same standards for measuring PSI and GPM values.
Notes on Measurement Standards
Sadly, we do not live in a perfect world. Cleaning Unit’s representability relies on the accurate measurement of both the GPM and PSI. If conditions are changed for the two measurements, even CUs can be manipulated.
Consumer protection laws should be sufficient enough to prevent manufacturers from outright lying. However, in principle, standards by which any of these values are measured are far from unified or transparent. This is especially the case when ordering from overseas.
The measuring methodology, such as where the values are measured (head of the pump, a pressure washer’s tip, etc.), for how long, how many times and under what conditions (e.g., water temperature), can all affect the final value that appears on the product sheet and that, in many cases, will differ from what the use will actually experience.
Many manufacturers run their devices through independent testing companies, which guarantee the accuracy of the measured PSI and GPM with a certificate. Even though this system is not perfect, they add an air of transparency and legitimacy to the values listed on the washer’s product sheet.
Since anybody can, and often will, in practice, make up their own “independent” certification for their product, we recommend sticking to the devices which are certified by the following:
- Any OSHA Nationally recognized testing laboratory (opens in a new tab), such as CSA, UL, etc.
- PW101-2010, from PWMA (opens in a new tab), the Pressure Washer Manufacturer’s Association
- CPC100 from CETA (opens in a new tab), the Cleaning Equipment Trade Association
In conclusion, we have explained that PSI and GPM are units of pressure and volumetric flow rate respectively. To accurately determine the cleaning capability of a device, values of both must be considered. For practical reasons, Cleaning Units (CUs) have been established to aid in comparing any given devices.
Lastly, we have discussed the current state of measurement standards, or rather lack of them and what to look out for when considering reliability of values given by the manufacturer. Hopefully, this information will aid you in a more informed purchase.