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Measuring fluid pressures

By autotech-nic on July 27, 2016

By Steve Smith, Automotive Application Specialist at Pico Technology

A lot has been written about Pico Technology’s WPS500X pressure transducer with regards to engine evaluation. I am referring to in-cylinder measurements, exhaust gas pulsations, crankcase and intake manifold pressure.

Although these are invaluable tests, the fact that the WPS500X can also measure fluid pressures is often overlooked, for example: fuel priming pressures (positive and negative), engine oil pressure, cooling system pressure, ATF pressure, and diesel injector back-leakage pressure/pulsations – with the distinct advantage of plotting pressure and pulsations against time.

Given that the WPS500X has a response time of 100 µs, the tiniest fluctuation in fluid pressure can be captured and reviewed for close analysis.

We can relate fluid pulsations to events such as injection, where a momentary pressure drop (attributed to injection) can be displayed and compared with subsequent injection events.

  • Pumping chamber contribution can be analysed, where multiple pump chambers combine to produce a total pressure. Early detection of underperforming pumping chambers can avoid disaster.
  • Pump rise times can be measured in response to demand or increase in engine speeds, to confirm the integrity of the pump and pumping circuit.
  • Cooling system activity detected inside the expansion bottle can be measured as a result of the water pump impellor acceleration (mechanical or electrical pump), confirming circulation and integrity of the pump impellor.
  • Diesel injector back-leakage pressure and pulsations can be measured at a single return point from all injectors.

These are just a few of the fluid measurements that can be performed in any application, assuming the pressures do not exceed the maximum 34.5 bar operating pressure of the WPS500X.

Let’s see it in action 

The WPS500X can be connected to numerous existing pressure testing kits (AST or Sykes-Pickavant) by using Pico Technology’s conversion sets (PP970 or TA250). In this way, you can replace the conventional pressure gauge while utilising the existing adaptors contained in kits already in use.









Figure 1 looks at engine oil pressure with the WPS500X. It highlights pumping chamber contribution (seen at idle speed) while proving rise time and peak oil pressure in relation to gas pedal position and engine speed. These captures provide date and time-stamped proof that engine oil pressure has been measured and, more importantly, confirm the integrity of the oil pump and the pumping circuit.

By using the zoom and scaling features in PicoScope, closer inspection of peak oil pump pulsations at idle speed reveal how they align with peak compression from the engine. A math channel is used to graph the change in frequency of the crankshaft sensor signal in relation to the deceleration of the crankshaft during compression, and the acceleration during combustion, see Figure 2.

Figure 2 Pulsation Analysis








Moving on to cooling system testing – apart from the obvious pressurisation of cooling systems as a direct result of head gasket failure, the underlying cause of water pump impellor failure can be notoriously difficult to evaluate and document. By using the expansion bottle pressure cap adaptors with the WPS500X, the water pump impellor’s activity can be compared against a known good vehicle or previously saved or shared data. You can also document the results for proof of a conclusive repair.

Figure 3 Water pump activity








Take a look at Figure 3, it is interesting to observe the delay in peak pressure arriving in the coolant expansion bottle after the decay in water pump speed. We can conclude that the water pump impellor is ‘active’ and contributing to circulation in the cooling system.

Diesel injector back-leakage evaluation can be challenging when injectors are located within the engine, under the camshaft covers, with only a single outlet pipe for injector spill accessible. Connecting the WPS500X in series with the single back-leakage injector return pipe allows you to analyse pulsations, which can be attributed to injection events, present in the return line. Using it with a current clamp on injector number one, see Figure 4, allows you to identify the pulsations using firing order.

FIgure 4










While the traditional back leakage test remains essential to the evaluation of fluid volume, the WPS500X offers an additional test with minimal intrusion that will provide an insight into
back-leakage activity, see Figure 5. It also gives you the ability to road-test the vehicle while connected, in order to monitor pulsations under full engine load conditions. In addition, we have a hard copy of test results which can be accompanied by additional engine parameters, such as RPM, MAF, MAP and fuel pressure, via the common rail pressure sensor.

Figure 5 Back Leakage Pressure








To conclude, the WPS500X pressure transducer offers multiple applications that can be incorporated with existing fluid pressure testing kits that are already in use today. The distinct advantage is that we can measure fluid pressure against time with the PicoScope. Fluid pressure measurements can be documented, saved, shared and compared, for proof of ‘good practice’ and development of product knowledge to a level that has never been possible before.



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Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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