NOSHOK is one of many vendors that provide liquid or air filled pressure gauges suitable for tuning up your Olympia espresso machine’s pressurestat setting. (Their products are available through McMaster-Carr.) As explained in an application note on the NOSHOK website, process temperature can negatively impact pressure measurement accuracy [emphasis added]:
For every 18 °F (10 °C) shift in temperature from which the gauge is calibrated, the user can experience up to a ±0.4% additional error. The cause is the change in the elasticity or spring rate of the Bourdon tube element with temperature. While it is difficult to circumvent the influence of ambient temperature, we can address the influence of process temperature. In steam service, the common practice is to install coil syphons or pigtail syphons to dissipate process heat. Another common practice is to install a diaphragm seal with capillary to separate the gauge from the high heat source. There are many options available with fill fluid in the seal and capillary system to withstand temperatures up to 600 °F. In severe cold ambient conditions, many users elect to heat trace their instrumentation via electric or steam trace. Process and ambient temperature is an important consideration when selecting and applying pressure gauges.
Many people have successfully used 140°F-rated pressure gauges with the Richard Penney boiler neck adapter, as shown here and here. However, some of these gauges are not rated for the process temperatures inside of an espresso machine boiler, which obviously exceed 212°F. Further, the gauges themselves are generally installed in close proximity to the boiler, which will tend to compromise measurement accuracy. If you are willing to spend the extra money, it is possible to address these issues through gauge selection and measurement device configuration.
As a general rule, all-stainless steel pressure gauges are rated for higher media temperatures than gauges with brass parts. For example, this 2-1/2-inch 30-PSI WIKA gauge from Valworx, which is shown in the photo above, has a stainless steel case as well as stainless steel wetted parts; it is rated for media temperatures up to 212°F and ambient temperatures of 140°F. Best of all, its mid-scale accuracy is 1%, which means it is ideally suited for capturing accurate boiler pressure measurements—provided you can address the negative impacts of elevated process temperatures.
You can address process temperature impacts in Olympia espresso machine applications by installing a cooling tower or diaphragm seal or similar between the Richard Penney boiler neck adapter and your pressure gauge. In the photo above, I am using a Model A-240-A perforated cooling tower from Dwyer to moderate process temperature effects. This cooling tower is basically a heat sink that pulls heat out of a small-diameter pipe spiraling up inside the perforated tower. During a typical boiler gauge pressure measurement, the base of this tower is too hot to touch with a bare hand; however, the top of the tower is only warm to the touch.
By taking these extra steps, you can capture accurate boiler pressure measurements on your Cremina. Note that in order to ensure measurement accuracy over time, you will also need to protect your pressure gauge against the vacuum that the boiler pulls during cool down. To do this, simply open and close the steam wand periodically as the machine cools down, in much the same way as you relieve false pressure in the boiler prior to taking any official pressure measurements.