Frequently Asked Questions

Temperature Instrument Questions

Q: When my sensor exceeds 7500 Ω, the display shows S. Over
A:

Many of the temperature instruments manufactured by Lake Shore have a 7500 Ω input resistance limit. Once the sensor exceeds this value, the S. Over error will be displayed.

Q: I calibrated a Cernox™ sensor and entered the curve into my instrument; however, I do not see it as a curve selection when configuring the input.

A:	Each sensor type requires a specific curve format. In the case of a Cernox™ sensor, the curve format must be in LogΩ (Base 10) vs. K. If you entered the curve in the resistance vs. temperature format, it will not appear as a selection.

Q: I am using a silicon diode and have the standard curve assigned to the input; however, my room temperature reading is off by 20 K.

A:	You most likely have selected the wrong standard curve. Most of the Lake Shore temperature instruments have both the DT-470 (Curve 10) and the DT-670 standard curves included. At room temperature, the sensor value will show an approximate 20 K difference if the wrong curve is selected.

Q: Should I use 2 or 4 wires to attach my sensor to the instrument?

A:	We always recommend using 4 wires to attach a sensor to any temperature instrument. If you use 2-wire leads, the resistance of the leads adds to the actual sensor value and induces errors into the measurement. The use of 4 wires eliminates any error induced from the resistance of the leads.

Q: Why does my temperature instrument display S. Over, T. Over, S. Under or T. Under?

These messages indicate that the instrument cannot display a valid temperature because the sensor value exhibits one of these conditions

S. Over: the sensor value is greater than the physical limit of the input

S. Under: the sensor value is at zero or is negative

T. Over: the sensor value is beyond the highest temperature point on the curve assigned to the input

For NTC devices, this means the sensor value is lower than the value shown in the highest curve breakpoint
For PTC devices, this means the sensor value is higher than the value shown in the highest curve breakpoint

T. Under: the sensor value is beyond the lowest temperature point on the curve assigned to the input

For NTC devices, this means the sensor value is higher than the value shown in the highest curve breakpoint
For PTC devices, this means the sensor value is lower than the value shown in the highest curve breakpoint

Water Cooled Power Supply Questions

Q: Model 662, 665 or 668 Fault + Failure Condition

The Fault + Failure condition is an indication that one of the transistor banks has a defective transistor. There are two failure LEDs. The first one indicates a failure on the positive banks. The second indicates a failure on the negative banks.

To clear the Fault + Failure condition:

Disconnect the load from the power supply.
Disconnect the control wires from the banks.

For the positive side, the control wire is B20
For the negative side, the control wire is B200.

For each bank, measure the impedance (with a multimeter) between the gate and the drain of one of the transistors of the bank (the gate is the left pin, and the drain is the middle one).
An impedance measurement of anything more than 20 kΩ reveals that the transistor bank is good.
An impedance measurement of approximately 2 kΩ reveals that the bank is damaged, but the transistor is OK.

In this case, check all the transistors of the bank until you find a transistor with low impedance (usually less than 200 Ω).
If you do not want to remove it immediately mechanically, cut the three pins of this transistor to eliminate it.
Re-test the transistor bank to be sure that the bank is now more than 20 kΩ.

If you did not fully test the transistor banks, or if you still have one lit failure LED, you need to perform an additional test of polarity by polarity.

Q: Model 662, 665 or 668 Fault + Bank OVh Condition

The Fault + Bank OVh condition indicates that one or more of the 2, 4 or 8 transistor banks that provide the output current is overheating. Inside the power supply are banks of transistors that are cooled by water flow through a copper block that the banks are mounted on. Each of the banks is equipped with a temperature sensor mounted on the copper block that is wired in series with the other banks. If the temperature of any bank exceeds 60 °C, the sensor opens the circuit that produces the OVh alarm. This overheating is usually the result of restricted water flow caused by a buildup in the plumbing that connects the banks to each other.

To troubleshoot this issue:

Disconnect the power supply from the AC circuit.
Disconnect the water output.
Connect a short hose to the hose barb on the water output.
Run water into a bucket to confirm that you have the proper flow through the power supply.

The power supplies require a continuous water flow with a temperature between +15 °C and +25 °C as follows:

Model 662: 1.5 gal/min (5.7 L/min)

Model 665: 2.2 gal/min (8.0 L/min)

Model 668: 2.2 gal/min (8.0 L/min)

If you find that the flow rate is below the listed minimum, you need to verify that the source is flowing at that value. If the source is flowing at that value, you can assume that you have blockage within the power supply.

To clear a blockage in the power supply:

Remove the covers so you have access to the banks in question. On each bank mounting are two hose fittings that connect the plastic hose between the water source and each bank. The most common place where the blockage occurs is in the 90° elbows.
Remove each of the fittings and clean them out with a wire brush.
If the buildup is excessive, the internal walls of the copper mounting adapters might also need to be cleaned in the same manner as the fittings to obtain proper thermal transfer.

Note: Mark and label all wiring that you remove in this process. This will help you to ensure proper reconnection to the banks once you have cleared the blockage.