Testo 557 Formal Evaluation

The Testo 557 Evaluation

Altitude/Barometric Pressure Compensated Sensors
One of the little touted features of the very first generation 523 was absolute pressure sensors which offered extremely high accuracy at all elevations compared to the relative pressure sensors used by the rest of the industry. (Again, Testo was simply ahead of their time.) The downfall of this, aside from the higher cost, was the fact that the instrument required a dead-band of about +/- 3 psi to hide changes in barometric pressure when used in the PSIG mode, (gauge pressure versus absolute) making them unusable for some refrigeration applications that operated close to 0 or in a slight vacuum. (This confused and still confuses many technicians.)  
A relative gauge (like all typical manifolds) reads 0 psig when no pressure is measured. When you "zero" the instrument you expect the sensors to zero. The truth is however that 0 psig is relative to a single pressure which is relative to 29.92"hg at sea-level. If the elevation or pressure is outside of standard atmospheric conditions the pressure is no longer standard and the pressure is no longer a true 0 with respect to the current conditions. When you press the zero key on any other (competitive) digital gauge today you are simply telling the sensors that there is no pressure on them and to show 0 psig on the display for the current atmospheric conditions. In other words, atmospheric pressure is not considered or accounted for in the measurement. This can result in significant errors where precise measurement is required, and is an inherent inaccuracy for all applications that utilize relative pressure sensors.
One of the most intriguing features of the Testo 557 is the use of an on-board absolute sensor to measure and compensate the relative pressure sensors for elevation and pressure. (This is available as a manual adjustment on the 550.) When the 557 is turned on and nulled (zeroed), barometric pressure is measured and the temperature pressure charts are adjusted for the current conditions relative to the atmospheric conditions and elevation you are working. The relative gauge will still show 0, but the temperature pressure chart is corrected. Since the gauge displays both pressure and temperature when a critical application is encountered, the evaporating or condensing temperature is displayed with a higher degree of accuracy than any other digital manifold gauge on the market. Utilizing this technology the dead-band is now 0.7 psig. The Testo 557 and 550 both use matched relative pressure transducers that are 0.5% accurate over the entire measurement range.
We stripped the 557 down to see how it is built. The Testo 557 is constructed extremely well with an aluminum valve body. Sensors are temperature compensated to assure accuracy.
4 Valve Manifold
The 557 uses a vacuum rated manifold that has been helium tested for leakage. Utilizing 4 ports allows for the connection of the vacuum pump (3/8") and refrigerant tank (1/4") at the same time minimizing the possibility of exposing the system to atmosphere when commissioning a system. The use of vacuum rated hoses is strongly recommended for this type of system as standard hoses are not adequate or rated for evacuation.
Testo 557 in case with 6' VACUUM RATED HOSES (Hoses not included in the kit, but available at TruTech Tools)
Grounded Thermistor Temperature Probes
Static build up is a potential problem with all digital and analog refrigeration gauges. When refrigerant (a very good dielectric) is passed through a hose a static charge can be generated. If not dissipated it can be a source of temperature measurement errors, pressure errors, an uncomfortable shock for the user, and/or damage to the meter or sensors. The Testo 557 and 550* use grounded temperature probes eliminating the need of a grounding strap when charging or recovering refrigerant. Static charges are bled off instead of building in the instrument eliminating errors, annoying static shocks and/or damage to the instrument.
The thermistor technology used in the 550 and 557 has many advantages over a thermocouple, first and foremost is accuracy. While thermocouples are useful in applications where a wide temperature operating range is anticipated (0-1450°F). Thermistors are recommended for applications with a specified temperature range (-60 to 300°F) and where interchangeability without recalibration is required. Negative temperature coefficient (NTC) thermistors are the most sensitive temperature sensors with an extremely fast response time. Instruments and control systems do not have to be recalibrated when a thermistor of the same nominal value is replaced. Additionally, NTC thermistors are able to handle mechanical and thermal shocks better than any other temperature measuring device. Sensor calibration is a process that should be left to calibration labs. While there are methods of field verification of calibration they are not to be considered calibration methods due to the inherent error with them. Although a K-type thermocouple is typically field adjustable it is only to a single point of calibration. While a K-type can be reasonably adjusted to 212° or 32°F, its accuracy at other temperatures will stray. A single point calibration can only be guaranteed at a single point. Accurate temperature calibrations require reference class instrumentation and a three point calibration. Additionally if K-type thermocouples are used it is imperative that the channel that the thermocouple is adjusted to remains the channel it is used in as the calibration setting is internal to the instrument and relative to the probe used for calibration- that is - once calibrated to a channel, there is no swapping of others' K-Type line temp. probes.
Stronger Hook/Gasketed Battery Door/Protected Display
When you examine the 557, you will notice a few changes in construction, especially if you have an early model 550. The hook used on the 557 and the newer 550's has been significantly improved in strength. The new hook base is a much larger diameter which will eliminate hook failures. The battery door is sealed with an o-ring to provide additional  resistance to moisture and dirt. The display (still a backlit LCD) now has a Plexiglas shield to protect against breakage. 
New hook (left) versus old hook style (right). Notice the beefier construction.
The door is sealed with an o-ring to prevent moisture and dirt from entering the instrument. Batteries are protected from vibration with foam.
Vacuum Measurement
The testo 557 measures vacuum with a barometric pressure compensated relative pressure sensor. The sensor is limited as it has a 500 micron resolution, but it does provide an adequate indication of vacuum for most users. When evacuating a system after our testing, we recommend that the pump is operated until the 557 display reaches a level of 000 microns indicating it is somewhere below 500. The 557 does not have the accuracy or the resolution for a standing pressure test (to indicate leakage) in a vacuum, but it does have a temperature compensated pressure test for this purpose. For all refrigerants especially those utilizing POE oils like R410a, a standing test in a vacuum is not recommended as a means of testing for system tightness as any leaks will introduce moisture into the system thereby contaminating the refrigerant oil and possibly damaging the system. After evacuation is complete, the 4 valve manifold allows for the isolation of the vacuum sensor from the vacuum pump. After a short stabilization period, it is easy to determine if the level of vacuum is adequate. We recommend the use of vacuum rated hoses and core removal tools with the 557 to minimize the potential leaks.
Testo 557 shown in vacuum mode (Note: Saturation temperature of water is not exactly 212 F, as it is derived from current barometric pressure, which factors in altitude.)
High Performance
Two temperature inputs allow simultaneous calculation and display of superheat and subcooling. With just a press of a button, the differential temperatures are also displayed. With another press of a button, you can quickly switch between the different measurements. The testo 557 features a large, backlit LCD display. Large numbers help you quickly and easily read the values. Intuitive operation provides you the ability to quickly analyze and diagnose the system without changing connections or performing tedious calculations. In a few seconds the instrument displays all of the values you need for an accurate and precise analysis. 
Extraordinary Value
The testo 557 incorporates a compact design for ease of use, transport and set up. The testo 557 has a durable housing that protects the analyzer from the everyday bumps and drops. It is built for the field. The testo 557 is a full function analyzer. With 39 refrigerant profiles plus H2Ostored on the processor and unique temperature and atmospheric pressure compensated sensors, the testo 557 is one of the most accurate analyzers on the market. Say goodbye to the old fashioned PT charts and temperature probes!
Blazing Speed
With a testo 557 Refrigeration System Analyzer, multiple tests are performed and calculated all at once. Just select the results you want displayed and away you go!  Once connected, you can perform a leak test, system evacuation, and monitor charging. Refrigerant flow can be monitored in the integrated (magnified) sight glass. Superheat and subcooling can be measured and displayed in real time!
  • Atmospheric compensation of the relative pressure sensors. (This feature alone justifies the purchase.)
  • Overall much better construction than the first release of the 550. Testo paid attention to the details
  • No grounding strap required (One less step.)
  • Hook issue has been addressed
  • 3/8" vacuum port allows for faster evacuation and recovery
  • Temperature compensated tightness test is very useful. This can help minimize moisture contamination in 410a systems.
  • Thermistors that do not require field calibration and hold calibration much more accurately over time
  • Super easy to use
  • Not a big fan of vacuum in the manifold. Hoses tend to leak! Use vacuum rated hoses (TruTech Tools has them) if you plan to use the vacuum feature of the instrument. The good news is that the sensor cannot be contaminated!
  • Vacuum resolution. I would have like to see a resolution of at least 100 microns. (What can I say we are purists! We like to see vacuum in 10ths of a micron.)
  • New temperature probes do not eliminate the grounding strap for the early 550s. First generation 550's will always require the grounding strap as there are hardware changes in both the meter and the probe.
*early Testo 550's require the use of a grounding strap.
Copyright TruTech Tools 2011. No part of this evaluation may be copied or redistributed without prior written permission.
25th Jul 2023 TruTech Tools, LTD

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