Pressure Senders 20 December 2005

The typical pressures you will need to monitor are:

• Fuel pressure
• Supercharger / Turbocharger / Nitrous Oxide pressure
• Scoop pressure / Sump vacuum / Plenumn vacuum
• Engine oil pressure
• Auto transmission oil pressure

FXL DATA Pty Ltd supplies three types of pressure senders:

1. Stainless steel senders made by FXL DATA
2. Stainless steel 4-20 mA senders bought in from a supplier
3. VDO marine two wire oil pressure senders

1: FXL DATA Pressure Senders.

The stainless steel pressure senders made by FXL DATA have a bought in transducer and have their own amplifier board and box. These senders produce 0 - 4 volts calibrated, and from 4 - 5 volts uncalibrated.

No other manufacturer makes senders like these, that's why we make our own.

The idea behind them is the transducers have 3 main pressure ranges:

• 50 psi
• 100 psi
• 200 psi

You must appreciate some maths here. The loggers have "8 bit Analog to Digital Converters". An 8 bit system will give numbers between 0 and 255, representing 0-5 volts. This means that if the ADC saw 1 volt exactly it would give the number 51. If it saw 2.7 volts, it would give the number (2.7 divided by 5) multiplied by 255 which would equal 138. 4.0 volts would give the number 204.

Because the bought in transducers have the ranges above, (50, 100 and 200 psi) then the amplifiers are calibrated to take the transducer voltages over their range and make 0-4 volts.

• A 50 psi transducer will give the number 200 at 50 psi
• A 100 psi transducer will give the number 200 at 100 psi
• A 200 psi transducer will give the number 200 at 200 psi

The 50 psi transducer number is multipled by 0.25 and has 0.25 psi accuracy
The 100 psi transducer number is multiplied by 0.5 and has 0.5 psi accuracy
The 200 psi transducer uses the raw number and has 1 psi accuracy

The transducers can go slightly over pressure without damage, (according to the manufacturer) so the amplifiers will still give a reading. eg:

• A 50 psi sender will read 0 - 50 psi over 0 - 4 volts and overread to about 63 psi or so
• A 100 psi sender will read 0 - 100 psi over 0 - 4 volts and overread to about 127 psi or so
• A 200 psi sender will read 0 - 200 psi over 0 - 4 volts and will overread up to about 250 psi but be uncalibrated. If you are running a mechanical fuel injection and normally operate below 200 psi but the pressure goes way over that when you shut the throttle going through the traps, you will see if your pump savers work. The reading will be below what it actually is, but you will know if your pump savers work

The advantages of the FXL DATA pressure senders are:

• 0 - 4 volts calibrated range and 4 - 5 volts uncalibrated
• They will over read so you can see what happens if you go slightly over pressure
• They MAY be repairable (not always)
• All the older design FXL DATA loggers were designed for this type of sender

The disadvantages are:

• They take almost 8 hours to assemble and calibrate each sender
• They need a braided hose to connect them to the point to be monitored
• The braided line can leak at the 370-3 fitting
• They can leak internally because the transducer is fitted in the box to a Speedflow 370-3 fitting
• They are difficult to assemble (at the factory)
• They need a zip tie to hold the connector on because
• The cable connectors sometimes disconnect themselves under vibration
• Must not be mounted so the fitting takes the weight. (MUST have the braided hose)
• All stainless steel pressure senders are expensive irrespective of manufacturer
• This type of pressure sender will continue to be available, however, all new designs will incorporate the 4-20 mA type sender. The only thing the 4-20 mA senders can't do that the FXL senders can do is they wont overread

Be aware pressure senders have no warranty.

2: 4 - 20 mA Stainless Steel Pressure Senders.

All future FXL DATA logger designs are going to use 4 - 20 mA senders. The latest thermocouple amplifier design for the FXL22 already incorporates the 4 - 20 mA sender design. Reasons are:

Advantages:

• They are a bought in item, (no assembly or calibration required)
• They wont leak internally, (unless they are destroyed)
• Highly accurate, calibrated by the manufacturer
• More compact than the FXL DATA senders
• No adaptor fittings to leak
• 1/8 NPT 27 tpi standard thread
• Sender can be screwed directly to manifold without hoses
• Very durable cable connector (according to the manufacturer)
• Cable connector wont disconnect itself under vibration (according to the manufacturer)

Disadvantages:

• VERY EXPENSIVE, this is unavoidable
• Cannot over read. It is IMPOSSIBLE to get an overreading
• Unrepairable. If they fail, they are a throw away item at $450 each
• Not always in stock, and can have long lead times from the supplier
• Need an internal voltage doubler to drive them as they are typically a 28 volt item

The advantages far outweigh the disadvantages.

The new logger designs will have a voltage doubler built in to provide the correct supply voltage.

Pressure ranges are:

• 0 - 15 psi Not stocked
• 0 - 25 psi Not stocked
• 0 - 50 psi Intend to hold in stock
• 0 - 100 psi Intend to hold in stock
• 0 - 200 psi Intend to hold in stock
• Other psi ranges up to 5000 psi (Not stocked)

Metric ranges available, but not stocked

• 0 - 6 Bar
• 0 - 10 Bar
• 0 - 16 Bar
• 0 - 25 Bar
• 0 - 40 Bar
• 0 - 60 Bar
• 0 - 100 Bar (1400 psi for Nitrous systems) Special order, long delays
• 0 - 160 Bar

Be aware pressure senders have no warranty.

3: VDO Marine Oil Pressure Senders.

These are simple pressure senders that will give satisfactory results at a low cost. They are typically $115 or $125 depening on the pressure rating.

They do not read out directly in psi. You have to use a conversion sheet to calculate the actual pressure. Considering the cost, this is reasonable. Stainless steel pressure senders cost $450 each, VDO marine oil pressure senders cost $125 or less.

They are only designed for use as an oil pressure sender. They CANNOT be used with methanol as a fuel or boost pressure sender. They will sieze up internally in less than 12 months, and have to be thrown away.

Typical pressure ranges are: (Metric)

• 2 Bar 28 psi P/No 360.723 Rarely used, not held in stock
• 5 Bar 72 psi P/No 360.720 Rarely used, not held in stock
• 10 Bar 140 psi P/No 360081032014C or 360.721 Normally used for engine oil pressure
• 25 Bar 360 psi P/No 360081038003C or 360.722 Normally used for auto trans pressure

The pressure rating is visible on the big nut at the threaded end. The thread is 1/8 NPT 27 tpi. It will simply say 0-10 for a 10 Bar sender or 0 - 25 for a 25 Bar sender.

Advantages:

• Low cost
• Over the counter item
• Amost always available (only once in 9 years has the supplier not been able to supply)
• Standard 1/8 NPT 27 tpi thread
• Works well as an oil pressure sender (That's what it was designed for)

Disadvantages:

• Not super accurate (No fault of VDO),
• Don't read out on the graph in psi (No fault of VDO),
• Needs a conversion sheet to calculate the actual pressure (No fault of VDO),
• CANNOT be used as a methanol fuel pressure sender (No fault of VDO).

Available from SGESCO in Archerfield Queensland and elsewhere.
Details in the Buying Spare Parts page.

Be aware pressure senders have no warranty.