Detailed Review of the Testo 557 and 570 Technology
Altitude/Barometric Pressure Compensated Sensors
One of the most untouted 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. (Testo was simply ahead of their time.) The downfall aside from the high 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, (gauged 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 and 570 is the use of an on board absolute sensor (A manual adjustment on the 550.) to measure and compensate the relative pressure sensors for elevation and pressure. When the 557/570 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 557/570 and 550 both use matched relative pressure transducers that are .5% accurate over the entire measurement range.
4 Valve Manifold
The Testo 557 & 570 use 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.
Grounded Thermistor Temperature Probes
Static build up is a potential problem with all digital and analog refrigeration gauges. When refrigerant which is 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, and or damage to the meter or sensors. The Testo 557/570 and 550* use grounded temperature probes eliminating the need of a grounding strap when charging or recovering refrigerant. Static charges are bleed off instead of building in the instrument eliminating errors, annoying static shocks and/or damage to the instrument.
A thermistor 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. 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 and 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 do 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 requires 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.
Temperature Compensated Clamp Probes (570)
Reducing measurement uncertainty is a key component of the engineering that went into the Testo 557/570. Because all clamp probes a have a slight influence on the temperature reading due to the portion of the clamp contacting the sensor, a compensation factor has been incorporated into the instrument to better reflect the temperature of the measured medium. (Generally a pipe)
Data Logging Software
Testo was first-to-market with a complete line of refrigeration system analyzers that incorporate data logging. This allows the service technician and or system analyst to evaluate system performance over a period of time from a snapshot (single measurement) to 72 hours at intervals as little as 1 second. In addition, no laptop is required to store the readings as onboard storage allows for standalone monitoring.
The most significant advantages come when data from a Testo 570 is read, analyzed and managed in the Testo PC Software. It is now possible for the technician designer, engineer, service manager, or a lead technician to spot trends, benchmark systems, verify proper/design operation, provide real-time system operation to a manufacturer or other interested party in an tamperproof data format that can be graphed to provide a “digital window” into the refrigeration/air conditioning system. System high and low side pressures, saturation pressures, measured temperatures, along with superheat, subcooling, oil pressure and or compressor current can be viewed on an auto-scaling graph. All measurement or each individual measurement can be viewed at once. Sections of the graph can be zoomed in on by dragging a box over the suspect area for further investigation when warranted.
Stronger Hook/Gasketed Battery Door/Protected Display
If you have an early model 550, when you examine the 557/570, you will notice a few changes in construction. The hook used on the 557/570 and the 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 which is sealed with an o-ring to provide additional moisture resistance. The display which is still an LCD is now overlaid with a Plexiglas shield to protect the display against unintentional breakage.
The testo 557/570 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 it reaches a level of 000 microns indicating it is somewhere below 500. The 557/570 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 to the system 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 tools with the 557/570 to minimize the potential leaks.
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 557/570 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.
The testo 557/570 incorporate a compact design for ease of use, transport and set up. The testo 557/570 has a durable housing that protects the analyzer from the everyday bumps and drops. It is built for the field. The testo 557/570 is a full function analyzer. With 39 refrigerant profiles plus H20 stored on the processor and unique temperature and atmospheric pressure compensated sensors, the testo 557/570 is one of the most accurate analyzers on the market. Say goodbye to the old fashioned PT charts and temperature probes.
With a testo 557/570 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!
*older Testo 550's require the use of a grounding strap.